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Seasonal variation in home-range size, and habitat area requirement of the lesser spotted woodpecker (Dendrocopos minor) in southern Sweden
Biological Conservation 100 (2001) 387–395 www.elsevier.com/locate/biocon
Seasonal variation in home-range size, and habitat area requirement of the lesser spotted woodpecker (Dendrocopos minor) in southern Sweden Ulf Wiktander, Ola Olsson *, Sven G. Nilsson Department of Ecology, Animal Ecology, Lund University, Ecology Building, SE-223 62 Lund, Sweden Received 8 May 2000; received in revised form 11 January 2001; accepted 12 January 2001
Abstract Seasonal variation in home-range size and habitat area requirement of lesser spotted woodpeckers (Dendrocopos minor) were studied by radio-tracking in southern Sweden for 6 years. Home-range size did not vary between age-groups or sexes, but varied with season and decreased successively from 742 ha in winter (n=10), 355 ha in early spring (n=15), 103 ha in late spring (n=22) to 43 ha during nesting (n=10). The home-range in late spring (i.e. the 3–5 weeks preceeding egg-laying) represents the defended breeding territory. This included on average 39 ha of forest utilised for foraging (range 31–46 ha, n=15). Since food availability in late spring has a significant influence on reproductive success, and mortality is highest in this period, we regard this as an estimate of the habitat area requirement. This estimate is valid primarily for birds in southern Sweden, but circumstantial evidence indicate that the area requirement may not be grossly different in other areas with different forest types. For conservation of lesser spotted woodpeckers, management should focus on a minimum of 40 ha of forest dominated by deciduous trees, which may be fragmented over a maximum of 200 ha. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Habitat requirement; Habitat fragmentation; Territoriality; Territory size; Telemetry; Piciformes
1. Introduction This study concerns seasonal variation in home-range size and area requirement of foraging habitat of the lesser spotted woodpecker (Dendrocopos minor). A detailed knowledge about these factors, and of foraging preferences and habitat use, is essential for our ability to take appropriate measures for the conservation of threatened species. Like several woodpeckers it is affected by modern forestry and has declined in numbers in both Sweden (Nilsson et al., 1992) and Finland (Tiainen, 1985) at rates similar to that of the now endangered white-backed woodpecker (Dendrocopos leucotos). It is also scarce or decreasing in many other European countries (Mikusinski and Angelstam, 1993). In spite of a still wide distribution in the Palearctic region it has been surprisingly poorly known, which makes the study of habitat requirement urgent.
* Corresponding author. Fax: +46-46-2224716. E-mail address: [email protected] (Ola Olsson).
This work is part of a long-term study of the ecology of this smallest European woodpecker (body-mass 24 g) in a population in southern Sweden. It is highly dependent upon deciduous forests (Cramp, 1985; Olsson, 1998). From autumn to late spring it is a specialised forager, feeding almost exclusively on beetle larvae from dead branches in living, mainly deciduous, trees (Olsson, 1998). During spring in years when birch (Betula pendula, B. pubescens) and/or common alder (Alnus glutinosa) bloom, larvae of the moth Argyresthia goedarthella become an important food source (Olsson, 1998). After bud burst, its diet changes to include surface-living insects, which is also the main food for the nestlings (To¨ro¨k, 1990; Wiktander et al., 1994; Olsson, 1998). A new nest is excavated each year, mostly in the trunk of a dead deciduous tree, occasionally in the dead limb of a living tree (Cramp, 1985; Stenberg, 1996; Wiktander, 1998). Both parents participate in all stages of parental care. During our study, 16% of the breeding attempts involved a polyandrous female, which was simultaneously paired with two or three males in a season. In these cases, each male held a separate territory
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with a nest, the female laying a clutch in each male’s nest in rapid succession and then dividing her parental care among all nests (Wiktander et al., 2000). The prelaying season, i.e. from late March to egglaying in mid-May, stands out as a critical period of the year for the lesser spotted woodpecker. Mortality rate during this period is significantly higher than in winter (Wiktander, 1998). Moreover, foraging and food availability in the prelaying season has a significant influence on the start of egg-laying and is also positively correlated with number of fledglings (Olsson et al., 1999). It therefore appears that the availability of prey in dead wood well in advance of breeding may be more decisive for overall reproductive success than the availability of surface-living prey during breeding. Because of these relationships we regard the prelaying season to be the relevant period to focus upon when estimating habitat area requirement.
2. Methods Field work was conducted from March 1991 to June 1996 in a 125 km2 study area adjacent to lake Mo¨ckeln, situated in the boreo–nemoral region in southern Sweden (56 400 N, 14 100 E). About 40 km2 of the study area was covered by water. The land was mainly forested, but with smaller areas (about 10%) of agricultural land interspersed. Most of the forest was coniferous, mainly Norway spruce (Picea abies) but also some Scotch pine (Pinus silvestris), and subject to more or less intense commercial management. About 20% of the land was covered by patches of deciduous and mixed deciduous/coniferous forest (Fig. 1), which were subject to less intense management. These forests were of two major types. Most were naturally regenerated on former meadows and pastures since the beginning and middle of the 20th century. Hence, these areas have a 50–80 year history as closed forests, but with individual trees well over 100 years frequently included. These forests were dominated by oak (Quercus robur, Q. petraea), birch and lime (Tilia cordata), with an understorey of hazel (Corylus avellana). In some areas hornbeam (Carpinus betulus) and beech (Fagus sylvatica) were prominent. Regular but more sparse were aspen (Populus tremula), elm (Ulmus glabra), maple (Acer platanoides), ash (Fraxinus excelsior) and rowan (Sorbus aucuparia). The second forest type occurred on wetland and along the shores of the lake and rivers. These forests were dominated by birch and common alder, with regular occurrence of aspen, sallow (Salix caprea) and willows (Salix spp.). The deciduous forests along the shores of the lake have regenerated naturally on land that appeared after a lowering of the lake’s water-level by 1.5 m around 1850. The woodpeckers were captured at the roosting or nesting hole by means of a bag net or a mist net. Cap-
tured birds were indivdually colour-ringed and aged as yearling or older on the basis of moult of secondaries and greater upper wing-coverts (Baker, 1993; Wiktander, 1998). The radio-transmitters we used were model BD-2 (Holohil Ltd, Canada), weighing 1.15 g (4–5% of the body-mass). Battery life was 9 weeks and the range varied with topography from 100 m up to 1.5 km under optimal conditions. The transmitter was tied at the base of two central tail-feathers (e.g. left R1 and right R2) and the string secured to the feather-shafts with a drop of cyanoacrylic glue. By cutting the string between the transmitter and the feathers, transmitters were easily replaced or removed. If the battery ran out and we lost contact with the bird before we could remove the transmitter, the bird lost the transmitter when moulting. The tail-mount however also meant that radio-tracking was not possible during moult, i.e. July–September. It appeared to us that the birds adapted rapidly to the transmitter, and in our eyes the birds carrying a transmitter behaved just like those that did not. Moreover, we found no negative effects on either survival or reproductive success from the use of transmitters. Survival to 21 March the following season (see Wiktander et al., 2001 ) did not differ between birds with and without transmitter (males: 21=0.18, P=0.67, n=63; females: 21=1.48, P=0.22, n=58). Neither did the start of egg-laying differ (males: ANOVA F1,55=0.017, P=0.90; females: F1,54=1.35, P=0.25), nor the number of young fledged by males (F1,55=0.14, P=0.71), whereas females with a transmitter actually raised significantly more fledglings than females without (F1,51=12.30, P=0.001). Consequently, we feel confident that the transmitters did not hamper the birds or affect them or their behaviour negatively. Home-range areas were measured with the minimum convex polygon method, including 100% of locations. This method was chosen because, in contrast to other methods, it is not based on any assumptions about the size of grid cells or the percentage of locations to be included, which makes it straightforward to interpret and the only method that is strictly comparable between studies (Harris et al., 1990). Moreover, area estimates based on the minimum convex polygon method are unaffected by possible autocorrelation between locations (Harris et al., 1990). Our principal method of data collection was ‘radioassisted surveillance’ (Harris et al., 1990), that is, the radio equipment was primarily used to find the birds in the field, and once located they were observed directly. Thus, the vast majority of the locations are based on direct observations, rather than bearings or triangulation, and possible errors due to inaccurate positioning of locations (Harris et al., 1990) ought to be negligible. Radio-equipped birds were located one to three times weekly, and once located followed for 30 min up to several hours. The location of the bird was recorded at
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Fig. 1. Map of the study area. Hatched areas represent forest dominated by deciduous trees 550 years old. Water is shaded grey. White areas on land represent coniferous forest or open agricultural land. The polygons show the winter home-ranges of two males in the same year (broken lines) surrounding their late spring breeding territory (continuous lines), and the late spring breeding territory of one female. Location map of Sweden shows study area as a filled square.
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15 min intervals, which we regarded as a time sufficient to allow a lesser spotted woodpecker to move anywhere within its home-range. On average, the birds moved 79 m between successive 15 min locations, but with a large variation (S.D.=160), and movements were regularly 1 km or more (Olsson, 1998). In many cases the movements of the birds were such that we could not relocate them within 15 min, and often the time between locations was much longer. To investigate the seasonal change in home-range size, we divided the year into four periods, which gave large enough sample sizes and also had ecological significance. (1) Winter, from October to 20 March, when the birds spent most time singly. (2) Early spring, 21 March to 20 April, start of territorial displays, pair formation and establishment of breeding territories in our study area. (3) Late spring, 21 April to laying of the first egg (i.e. the 3–5 weeks preceeding egg-laying), a period when we assume breeding territories were established and activities occurred mainly within these. (4) Nesting, the day of the first egg until the young fledged. To investigate whether the number of locations was enough for home-range area to reach an asymptote, we made incremental area plots by adding successive locations for all tracked individuals in each period. Only individuals for which the home-range reached an asymptote are included in analyses. On average 96 locations (over 10 days) were necessary to reach a stable size in winter, 46 (seven) in early spring, 45 (five) in late spring and 31 (four) in the nesting period. For individuals tracked during the same period on the same territory for more than one year, we used the average of the area estimates of the years (n=3 individuals). As independent observations we treated the same individual on different territories (n=4; early spring one, late spring two, nesting one) and the same territory with different individuals (n=4; late spring two, nesting two). With these selections, 10 individuals were tracked in winter (four yearlings, six older: six males, four females), 15 in early spring (six yearlings, nine older: 13 males, two females), 22 in late spring (six yearlings, 16 older: 14 males, eight females) and 10 during nesting (three yearlings, seven older: six males, 4 females). Basic data for these individuals, including number of tracking-days and locations, are given in the Appendix A.
To estimate habitat area requirement, we summed the area of the forest stands utilised for foraging in the late spring period (see also Olsson, 1998). For this analysis we only included those 15 individuals (two yearlings, 13 older: eight males, seven females) for which we had direct observations of their activity in all visited stands, thus allowing us to discriminate whether stands were used for foraging or not. Forests were classified into stands according to tree species composition and age. In most areas this was made by the Regional Forestry Board for forest management purposes, independent of our study. In three areas we made the classification ourselves. Average size of forest stands was 4.7 ha (S.D.=3.6). Area estimates were normalised by log-transformation before use in ANOVA.
3. Results A three-way ANOVA showed that home-range size of the lesser spotted woodpeckers declined significantly from winter to nesting (F3,51=30.88, P<0.001; Table 1, Fig. 2), but did not vary between age-classes (F1,51=1.52, P=0.22) or sexes (F1,51=0.51, P=0.48). Moreover, in all but one of the individuals tracked over consecutive periods within a year, the home-range decreased from the earlier to the later period (Fig. 2). Polyandrous females are not included in the above analysis. The home-range of one polyandrous female tracked in late spring was 349 ha, and that of three females tracked during nesting was 169, 214 and 236 ha, i.e. considerably larger than that of monogamous birds (Table 1). The average of the summed area of forest stands used for foraging in the late spring period, i.e. our estimate of habitat area requirement, was 38.8 ha (n=15, S.D.=4.17, range 31.2–46.0 ha). The average of the 100% polygons of these 15 individuals was 104 ha (S.D.=38.7, range 47–168 ha), i.e. very similar to that for all tracked individuals (Table 1). On average, 44% (range 23–79%) of the polygon home-range consisted of forest stands utilised for foraging. A striking result of this exclusion of unused areas is the drastic decrease in variation and the similarity of resulting areas (Fig. 3).
Table 1 Home range size of lesser spotted woodpeckers during winter, spring and nestinga
Mean area (ha) Range (ha) S.D. CV% n a
Winter (Oct/Feb–20 March)
Early spring (21 March–20 April)
Late spring (21 April–1st egg)
Nesting (1st egg–fledging)
742 234–1654 423.0 57.0 10
355 36–1587 408.1 115.0 15
103 26–174 43.0 41.7 22
43 20–101 25.6 59.5 10
Ranges were calculated by minimum convex polygons, including 100% of locations.
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Fig. 3. Area of polygons (&) and the sum of forest stands utilised for foraging (*) of 15 individuals in late spring.
Fig. 2. Seasonal variation in home-range size of lesser spotted woodpeckers. * represent old birds, & yearlings. Lines connect individuals followed over consecutive periods in a year.
The relative variation in size of foraging areas (CV=10.75%) was significantly smaller than that of polygons (CV=37.21%; t14=2.74, P=0.005). The proportion of the polygons used for foraging may be regarded as an estimate of the level of fragmentation of habitat, which could influence travel costs and foraging economy, and hence reproduction. There was, however, no correlation between this measure of fragmentation and start of egg-laying (Spearman rank correlation, rs=0.09, n=11, P>0.50) or number of young fledged (rs=0.23, n=12, P>0.20).
4. Discussion 4.1. Home-range and territory size Compared with other species inhabiting similar habitats, home-ranges of the lesser spotted woodpecker appear unusually large for a bird of this size. In a radiotracking study of middle spotted woodpeckers (Dendrocopos medius; body-mass 58 g), Pasinelli (1999) found home-ranges in oak–hornbeam forest in Switzerland of 18.8 ha in winter, 11.6 ha in spring and 8.8 ha during nesting. In deciduous forest in our study area, the nuthatch (Sitta europea), similar in size to the lesser spotted woodpecker, held territories of 3.2 ha in winter
and 2.0 ha in spring (Nilsson, 1976) and breeding season density of great spotted woodpeckers (Dendrocopos major; body-mass 90 g) was 15.9 pairs/100 ha (6.3 ha/ pair; Nilsson, 1979). One reason for these differences is probably to be found in specific differences of foraging behaviour and distribution of food items (cf Cramp, 1985; Cramp and Perrins, 1993). The size of home-ranges and the seasonal variation found in our study correspond well with the results from radio-tracking of lesser spotted woodpeckers in Hessen, Germany by Hoentsch (1996). With periods delimited approximately as in our study, the homerange of one individual in winter was 540 ha, of two individuals in early spring 120 and 297 ha (mean=209 ha), and of three individuals during nesting 22, 33 and 57 ha (mean=37 ha). Apart from this we know only of studies reporting densities, which may not be directly comparable with measuring individual home-range size. In unfragmented habitat in Bialowieza forest in Poland, breeding season density of lesser spotted woodpeckers was 0.3 pairs/10 ha (33 ha/pair) and 0.2 pairs/10 ha (50 ha/pair) in ash–alder and oak–hornbeam forest, respectively (Wesolowski and Tomialojc, 1986). Spitznagel (1990) reported densities of 0.16 pairs/10 ha forested land (63 ha/pair) in willow, alder and oak-elm woods in south-west Germany. The reasons for the seasonal variation in home-range size may be interpreted from territorial behaviour and the trade-off between energy demand and predation risk. One reason for the large winter home-ranges is that the birds utilised somewhat different habitats in winter compared with spring, spending more time in spruce forest foraging from spruce (O. Olsson and U. Wiktander, unpublished data). This may in turn be a consequence of the fact that energy demand appeared to be lower in winter (Wiktander, 1998), allowing the birds to
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follow a strategy of minimizing predation risk, and of selecting habitats for higher safety rather than energetic profitability (Olsson, 1998). The common picture in winter was for the birds to roost in a hole located in a larger area of deciduous forest, which later became the breeding territory. Regularly, however, the birds left the deciduous forest to spend the winter days in the surrounding spruce forests, leading to the large winter home-range surrounding the breeding territory, exemplified in Fig. 1. The spruce forest was utilised even though spruce is less profitable than, e.g. lime (Olsson, 1998), but possibly the spruce forest is a safer place in winter than a defoliated deciduous forest. Commonly, the woodpeckers also followed mixed species flocks of tits (Parus spp.), nuthatches, treecreepers (Certhia familiaris) and goldcrests (Regulus regulus; O. Olsson and U. Wiktander, unpublished data), something which probably enhances protection from predation (Pulliam and Caraco, 1984). Moreover, the birds apparently spent the winter solitarily, with no territorial or social activites (Wiktander, 1998), the lack of territoriality possibly giving opportunity to travel over large areas with relatively low expense. Regularly, winter homeranges of different individuals were partly over-lapping, as depicted in Fig. 1. Home-range size in the early spring period is more difficult to interpret. The period we have delimited includes the transition from the undefended winter home-range to the defended territory in late spring. This transition could occur abruptly over a short period, e.g. in response to some phenological cue such as temperature. It could also be a gradual process, which could come about, e.g. if energy demands increase gradually and force the birds to gradually select the more profitable habitats more strongly. However, our data does not allow us to evaluate which process may be valid. The higher energy demand in late spring may force the birds to a strategy contrary to that in winter, with utilisation of the most profitable habitats, even if this occurs at the expense of a higher predation risk. This, and possibly also the need for mate and nest-hole guarding, would then restrict activity to a limited area of defendable size. The relatively small home-ranges during nesting may be explained by a high energy demand due to high travel costs during nestling feeding, coupled with a high food availability of surface-living insects. 4.2. Habitat area requirement The home-ranges in late spring included about 40 ha of forest utilised for foraging. At this time of year (3–5 weeks before egg-laying) foraging occurs more or less exclusively within the defended territories. This period is also part of the prelaying season, during which foraging, food availability and habitat availability has a profound
influence on reproductive success (Olsson, 1998; Olsson et al., 1999) and when mortality is highest (Wiktander, 1998). Hence, we regard this as an estimate of habitat area requirement for successfully reproducing lesser spotted woodpeckers. Primarily, the estimate is valid for birds inhabiting forests in southern Sweden, with a high or dominating inclusion of nemoral deciduous trees (i.e. oak, lime, hornbeam, beech, elm, ash and maple, in contrast to birch, alder, aspen, and sallow). It is difficult to evaluate how general our results are since this is the only study of its kind known to us, and since area requirements are likely to be influenced by structure and composition of forests, which varies between areas. The value 40 ha, however, corresponds with the densities from homogenous habitat in Poland and Germany reported by Wesolowski and Tomialojc (1986) and Spitznagel (1990), mentioned above. Wiktander et al. (1992) investigated the occurrence of lesser spotted woodpeckers in relation to area of deciduous forest within 200 ha plots. Lesser spotted woodpeckers occurred in areas with as little as 17 ha of deciduous forest, although at a low probability (25%). Probability of occurrence increased with area of deciduous forest, to reach 80% in areas with more than 38 ha. That study only recorded occurrence of the birds, however, and not if they were reproducing. Whether the value 40 ha is also valid for boreal regions, where nemoral trees are lacking, is difficult to evaluate since data on habitat use in the boreal region are very scarce. However, probability of occurrence was found to be unaffected by latitude in the study by Wiktander et al. (1992) which covered south and central Sweden north to 62 N, i.e. well into the boreal region. The home-range of one male radio-tracked in late April in the Swedish boreal region (60 400 N) was 104 ha, and included 51 ha of deciduous forest [mainly birch and grey alder (Alnus incana) but also sallow and aspen] utilised for foraging (U. Wiktander, unpublished data). Hence these studies indicate that habitat requirements may not be grossly different in the boreal region, but further study is required before firm conclusions can be drawn. A previous estimate of habitat area requirement, based on the size of islands with confirmed breeding, suggested 10–15 ha of deciduous forest (Ahle´n and Nilsson, 1982). According to our results, this is probably an under-estimate. As mentioned earlier, we believe it is the prelaying season that should be decisive for estimates of area requirement, rather than the nesting season. On nine occasions during our study we found nests on islands, 0.3–2.5 ha in size and isolated by 100– 200 m of water. These stretches of water were frequently crossed for collecting nestling food on other islands or the mainland, and during radio-tracking we regularly observed individuals flying over 500 m of open water. Hence, open water is apparently no major obstacle and nesting on an island does not necessarily mean that all
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prelaying foraging occurred on that island. The estimate in Ahle´n and Nilsson (1982) may be valid for the nesting season when, as shown in this study, home-ranges are significantly smaller than in the prebreeding season. 4.3. Fragmentation of habitat The summed area of foraging stands in late spring showed that the home-ranges based on 100% minimum convex polygons included large unused areas. Hence, it may be asked what is the ecological significance of the 100% polygons. In the late spring period, foraging stands were patchily distributed over the home-range, separated by up to 500 m of unused areas which consisted of avoided forest stands, open fields and water. The unused areas were not included in the polygon as a consequence of occasional excursions to peripheral parts outside some core area of foraging activity (cf Harris et al., 1990), but were part of the home-range and regularly traversed to reach foraging stands. Thus, the difference between polygon area and area of foraging stands reflects fragmentation of habitat. The polygon home-ranges as such, estimated in this study, shows sizes of defendable territories of successfully reproducing lesser spotted woodpeckers in a landscape with fragmented habitat. The polygon home-range also shows the very large mobility of these birds, which has practical implications when inventorying lesser spotted woodpeckers and is necessary to keep in mind to avoid the obvious risk of over-estimating numbers. An unresolved issue, but which is of great importance for conservation, is how large the fragmentation of habitat may be before an area becomes uneconomic to explore and defend. The most fragmented home-ranges in our study included only 23–30% habitat used for foraging, which apparently was an acceptable degree of fragmentation as judged by the reproductive performance. Tjernberg et al. (1993) found that black woodpeckers (Dryocopus martius) accepted a similar degree of fragmentation; the reproduction of birds in homogenous forest was not different from that of birds in an area with only 26% forest. Wiktander et al. (1992) found that lesser spotted woodpeckers occurred in areas with habitat fragmented over 200 ha, although it is not known if they were successfully reproducing. From the evidence at hand, we suggest that 20% of deciduous forest in the landscape should be regarded as the limit to the level of fragmentation. 4.4. Conservation guidelines Because of its dependence upon deciduous forests, and its specialised foraging for prey in dead wood of deciduous trees, the lesser spotted woodpecker is vulnerable to changes of this habitat. An important action for conservation of the species is therefore to consider
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the content and management of deciduous forest in the landscape. According to the results of this study, we give the recommendation that forest management should focus on a minimum of 40 ha of foraging habitat, i.e. forest dominated by deciduous trees, which may be distributed over a maximum of 200 ha. We believe this estimate to be a useful and relevant rule of thumb, since it is based on the woodpeckers’ own selection of foraging habitat and their reproductive performance. The definition of foraging habitat used is qualitatively simplified, however. Previous work shows that it is necessary to consider habitat selection on a finer scale, e.g. tree species preferences, selection of foraging patches and their characteristics, to complete the qualitative picture of habitat requirement. (Olsson, 1998; Olsson et al., 2001). One relationship that appears from those studies should be mentioned here, since it is of direct importance to the management of forests. Within territories, forests stands were used in relation to the amount of dead wood of preferred tree species. Since prey density in different tree species varies between years, so does the woodpeckers’ preference for tree species vary, and consequently the preference for forest stands may vary between years. Hence, a variation in tree species content (and not just area of deciduous forest) is likely to be a decisive factor for the long-term suitability of an area, and something which is important to consider in conservation work. In addition to characteristics of forests related to foraging and food availability, Smith (1997) and Pasinelli (2000) stressed the importance of a supply of suitable trees for nest excavation (i.e. dead trees or potential cavity trees) for great spotted woodpeckers and middle spotted woodpeckers. The same certainly applies to the lesser spotted woodpecker, which excavates a new nest each year and which appears to be obligately dependent upon dead wood for nest excavation, with dead deciduous trees being highly preferred (Wesolowski and Tomialojc, 1986; Ha˚gvar et al., 1990; Stenberg, 1996; Wiktander, 1998). Hence, a further important aspect of the management of forests for the lesser spotted woodpecker is to allow a continuous development of dead deciduous trees.
Acknowledgements We wish express our gratitude to Anders Stagen, Krister Wahlstro¨m, Fredrik O¨strand, Fredrik Haas, Leif Appelgren, Martin Stjernman and Ramo´n Martı´nez for many hours of help in the field, to Shahrzad Bakthiar for help with entering field data into the computer, and to all land-owners who let us work freely in their forests. Fredrik Haas also helped with preparing Fig. 1. The Regional Forestry Boards in A¨lmhult and
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Ljungby kindly let us use their forest data. The study was made possible by financial support from Lunds Djurskyddsfond, WWF-Sweden, Stiftelsen Oscar & Lili Lamms Minne, and Mistra/SUFOR.
Appendix A. Data for all individuals used in analyses of home-ranges in the four periods. F, female, M, male. Y, yearling, O, older bird. The area is that of the minimum convex polygon. The last two columns refers to the number of days and locations tracked. Individuals marked with an asterisk in the late spring period are those used for the analysis of habitat area requirement. Individual
Year Sex Age Area (ha) Days Locations
Winter BGG BOG BYL GVG RGB RGR RLB ROO YGB YRY
1993 1994 1995 1994 1991 1995 1992 1993 1993 1993
F F F F M M M M M M
Y O O Y O O O O Y Y
875 234 410 750 281 1060 568 943 650 1654
11 12 11 4 7 10 11 36 17 10
71 87 109 24 78 77 185 283 148 73
RGY* RLB* RLB* RLL* ROG* ROG* ROG* ROO RRV RRY* RYG* RYM YRY
1996 1991 1992 1992 1992 1995 1996 1993 1996 1996 1995 1995 1993
M M M F M M M M M M M M M
O O O O O O O O Y O Y Y Y
61 62 140 131 168 79 47 87 26 108 127 99 86
6 8 13 12 7 8 5 5 7 5 7 5 9
47 80 100 132 98 49 24 36 45 36 40 26 43
Nesting BMV BMY BMY BOO RGY RLB ROO RRY RVG RYM
1996 1995 1996 1994 1996 1992 1992 1996 1996 1996
F F F F M M M M M M
Y O O O O O Y O Y O
20 54 29 34 20 101 48 68 23 36
5 7 4 6 5 9 6 6 5 3
55 36 38 30 38 47 34 38 33 29
References
Early spring BOG GLB RGO RGR RGY RLB RMV ROG ROO RRB RYG YGB YOS YRY YRY
1996 1996 1993 1996 1996 1992 1996 1996 1993 1996 1995 1993 1996 1993 1996
F M M M M M M M M M M M F M M
O O Y O O O Y O O O Y Y Y Y O
36 82 564 173 70 360 98 235 866 123 312 458 253 1587 108
17 8 6 7 5 12 6 11 8 7 5 9 6 10 5
69 52 55 50 31 122 51 55 53 48 32 69 37 72 44
Late spring BMY* BOG* BOG* BOO* BYG* BYL BYO* RGO RGR
1996 1994 1996 1994 1995 1995 1996 1993 1995
F F F F F F F M M
O O O O Y O O Y O
71 103 58 109 156 174 133 63 172
4 6 5 5 9 5 6 4 4
32 42 37 48 89 25 64 35 32
Ahle´n, I., Nilsson, S.G., 1982. Species richness and area requirements of forest bird species on islands with natural forests in Lake Ma¨laren and Hja¨lmaren. Va˚r Fa˚gelva¨rld 41, 161–184 (In Swedish with English summary).. Baker, K., 1993. Identification Guide to European Non-Passerines: BTO Guide 24. British Trust for Ornithology, Thetford, UK. Cramp, S., 1985. The Birds of the Western Palearctic, Vol. 4. Oxford University Press, Oxford. Cramp, S., Perrins, C.M., 1993. The Birds of the Western Palearctic, Vol. 7. Oxford University Press, Oxford. Ha˚gvar, S., Ha˚gvar, G., Mønness, E., 1990. Nest site selection in Norwegian woodpeckers. Holarctic Ecology 13, 156–165. Harris, S., Cresswell, W.J., Forde, P.G., Trewhella, W.J., Woollard, T., Wray, S., 1990. Home-range analysis using radio-tracking data — a review of problems and techniques particularly as applied to the study of mammals. Mammal review 20, 97–123. Hoentsch, K., 1996. Radiotelemetrische Untersuchungen zur RaumZeit-Nutzung des Kleinspechts Picoides minor. Diplomarbeit, Technische Hochschule Darmstadt, Darmstadt (In German).. Mikusinski, G., Angelstam, P., 1993. European woodpeckers and anthropogenic habitat change: a review. Vogelwelt 118, 277–283. Nilsson, S.G., 1976. Habitat, territory size, and reproductive successs in the Nuthatch Sitta europea. Ornis Scandinavica 7, 179–184. Nilsson, S.G., 1979. Density and species richness of some forest bird communities in South Sweden. Oikos 33, 392–401. Nilsson, S.G., Olsson, O., Svensson, S., Wiktander, U., 1992. Population trends and fluctuations in Swedish woodpeckers. Ornis Svecica 2, 13–21. Olsson, O., 1998. Through the eyes of a woodpecker: understanding habitat selection, territory quality and reproductive decisions from individual behaviour. PhD thesis, Lund University.
U. Wiktander et al. / Biological Conservation 100 (2001) 387–395 Olsson, O., Wiktander, U., Holmgren, N.M.A., Nilsson, S.G., 1999. Gaining ecological information about Bayesian foragers through their behaviour. II. A field test with woodpeckers. Oikos 87, 264–276. Olsson, O., Wiktander, U., Malmqvist, A., Nilsson, S.G., 2001. Variability of patch type preferences in relation to resource availability and breeding success in a bird. Oecologia, in press. Pasinelli, G., 1999. Relations between habitat structure, space use and breeding success of the Middle Spotted Woodpecker Dendrocopos medius. PhD thesis, Zu¨rich University. Pasinelli, G., 2000. Oaks (Quercus sp.) and only oaks? Relations between habitat structure and home range size of the middle spotted woodpecker (Dendrocopos medius). Biological Conservation 93, 227–235. Pulliam, H.R., Caraco, T., 1984. Living in groups: Is there an optimal group size?. In: Krebs, J.R., Davies, N.B. (Eds.), Behavioural Ecology. An Evolutionary Approach, 2nd Edition. Blackwell Scientific Publications, Oxford, pp. 122–147. Smith, K.W., 1997. Nest site selection of the great spotted woodpecker (Dendrocopos major) in two oak woods in southern England and its implications for woodland management. Biological Conservation 80, 283–288. Spitznagel, A., 1990. The influence of forest management on woodpecker density and habitat use of the Upper Rhine valley. In: Carlson, A., Aule´n, G. (Eds.), Conservation and Mangement of Woodpecker Populations. Report 17. Swedish University of Agricultural Sciences, Uppsala. Stenberg, I., 1996. Nest site selection in six woodpecker species. Cinclus 19, 21–38.
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Tiainen, J., 1985. Monitoring bird populations in Finland. Ornis Fennica 62, 80-89. Tjernberg, M., Johansson, K., Nilsson, S.G., 1993. Density variation and breeding success of the Black Woodpecker Dryocopus martius in relation to forest fragmentation. Ornis Fennica 70, 155–162. To¨ro¨k, J., 1990. Resource partitioning among three woodpecker species Dendrocopos spp. during the breeding season. Holarctic Ecology 13, 257–264. Wesolowski, T., Tomialojc, L., 1986. The breeding ecology of woodpeckers in a temperate primaeval forest — preliminary data. Acta Ornithologica 22, 1–21. Wiktander, U., 1998. Reproduction and survival in the lesser spotted woodpecker. Effects of life history, mating system and age. PhD thesis, Lund University. Wiktander, U., Nilsson, I.N., Nilsson, S.G., Olsson, O., Pettersson, B., Stagen, A., 1992. Occurrence of the lesser spotted woodpecker Dendrocopos minor in relation to area of deciduous forest. Ornis Fennica 69, 113–118. Wiktander, U., Nilsson, S.G., Olsson, O., Stagen, A., 1994. Breeding success of a Lesser Spotted Woodpecker Dendrocopos minor population. Ibis 136, 318–322. Wiktander, U., Olsson, O., Nilsson, S.G., 2000. Parental care and social mating system in the Lesser Spotted Woodpecker Dendrocopos minor. Journal of Avian Biology 31, 447–456. Wiktander, U., Olsson, O., Nilsson, S.G., 2001. Age and reproduction in the Lesser Spotted Woodpecker (Dendrocopos minor). Auk, in press.
Seasonal variation in home-range size, and habitat area requirement of the lesser spotted woodpecker (Dendrocopos minor) in southern Sweden Ulf Wiktander, Ola Olsson *, Sven G. Nilsson Department of Ecology, Animal Ecology, Lund University, Ecology Building, SE-223 62 Lund, Sweden Received 8 May 2000; received in revised form 11 January 2001; accepted 12 January 2001
Abstract Seasonal variation in home-range size and habitat area requirement of lesser spotted woodpeckers (Dendrocopos minor) were studied by radio-tracking in southern Sweden for 6 years. Home-range size did not vary between age-groups or sexes, but varied with season and decreased successively from 742 ha in winter (n=10), 355 ha in early spring (n=15), 103 ha in late spring (n=22) to 43 ha during nesting (n=10). The home-range in late spring (i.e. the 3–5 weeks preceeding egg-laying) represents the defended breeding territory. This included on average 39 ha of forest utilised for foraging (range 31–46 ha, n=15). Since food availability in late spring has a significant influence on reproductive success, and mortality is highest in this period, we regard this as an estimate of the habitat area requirement. This estimate is valid primarily for birds in southern Sweden, but circumstantial evidence indicate that the area requirement may not be grossly different in other areas with different forest types. For conservation of lesser spotted woodpeckers, management should focus on a minimum of 40 ha of forest dominated by deciduous trees, which may be fragmented over a maximum of 200 ha. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Habitat requirement; Habitat fragmentation; Territoriality; Territory size; Telemetry; Piciformes
1. Introduction This study concerns seasonal variation in home-range size and area requirement of foraging habitat of the lesser spotted woodpecker (Dendrocopos minor). A detailed knowledge about these factors, and of foraging preferences and habitat use, is essential for our ability to take appropriate measures for the conservation of threatened species. Like several woodpeckers it is affected by modern forestry and has declined in numbers in both Sweden (Nilsson et al., 1992) and Finland (Tiainen, 1985) at rates similar to that of the now endangered white-backed woodpecker (Dendrocopos leucotos). It is also scarce or decreasing in many other European countries (Mikusinski and Angelstam, 1993). In spite of a still wide distribution in the Palearctic region it has been surprisingly poorly known, which makes the study of habitat requirement urgent.
* Corresponding author. Fax: +46-46-2224716. E-mail address: [email protected] (Ola Olsson).
This work is part of a long-term study of the ecology of this smallest European woodpecker (body-mass 24 g) in a population in southern Sweden. It is highly dependent upon deciduous forests (Cramp, 1985; Olsson, 1998). From autumn to late spring it is a specialised forager, feeding almost exclusively on beetle larvae from dead branches in living, mainly deciduous, trees (Olsson, 1998). During spring in years when birch (Betula pendula, B. pubescens) and/or common alder (Alnus glutinosa) bloom, larvae of the moth Argyresthia goedarthella become an important food source (Olsson, 1998). After bud burst, its diet changes to include surface-living insects, which is also the main food for the nestlings (To¨ro¨k, 1990; Wiktander et al., 1994; Olsson, 1998). A new nest is excavated each year, mostly in the trunk of a dead deciduous tree, occasionally in the dead limb of a living tree (Cramp, 1985; Stenberg, 1996; Wiktander, 1998). Both parents participate in all stages of parental care. During our study, 16% of the breeding attempts involved a polyandrous female, which was simultaneously paired with two or three males in a season. In these cases, each male held a separate territory
0006-3207/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0006-3207(01)00045-3
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with a nest, the female laying a clutch in each male’s nest in rapid succession and then dividing her parental care among all nests (Wiktander et al., 2000). The prelaying season, i.e. from late March to egglaying in mid-May, stands out as a critical period of the year for the lesser spotted woodpecker. Mortality rate during this period is significantly higher than in winter (Wiktander, 1998). Moreover, foraging and food availability in the prelaying season has a significant influence on the start of egg-laying and is also positively correlated with number of fledglings (Olsson et al., 1999). It therefore appears that the availability of prey in dead wood well in advance of breeding may be more decisive for overall reproductive success than the availability of surface-living prey during breeding. Because of these relationships we regard the prelaying season to be the relevant period to focus upon when estimating habitat area requirement.
2. Methods Field work was conducted from March 1991 to June 1996 in a 125 km2 study area adjacent to lake Mo¨ckeln, situated in the boreo–nemoral region in southern Sweden (56 400 N, 14 100 E). About 40 km2 of the study area was covered by water. The land was mainly forested, but with smaller areas (about 10%) of agricultural land interspersed. Most of the forest was coniferous, mainly Norway spruce (Picea abies) but also some Scotch pine (Pinus silvestris), and subject to more or less intense commercial management. About 20% of the land was covered by patches of deciduous and mixed deciduous/coniferous forest (Fig. 1), which were subject to less intense management. These forests were of two major types. Most were naturally regenerated on former meadows and pastures since the beginning and middle of the 20th century. Hence, these areas have a 50–80 year history as closed forests, but with individual trees well over 100 years frequently included. These forests were dominated by oak (Quercus robur, Q. petraea), birch and lime (Tilia cordata), with an understorey of hazel (Corylus avellana). In some areas hornbeam (Carpinus betulus) and beech (Fagus sylvatica) were prominent. Regular but more sparse were aspen (Populus tremula), elm (Ulmus glabra), maple (Acer platanoides), ash (Fraxinus excelsior) and rowan (Sorbus aucuparia). The second forest type occurred on wetland and along the shores of the lake and rivers. These forests were dominated by birch and common alder, with regular occurrence of aspen, sallow (Salix caprea) and willows (Salix spp.). The deciduous forests along the shores of the lake have regenerated naturally on land that appeared after a lowering of the lake’s water-level by 1.5 m around 1850. The woodpeckers were captured at the roosting or nesting hole by means of a bag net or a mist net. Cap-
tured birds were indivdually colour-ringed and aged as yearling or older on the basis of moult of secondaries and greater upper wing-coverts (Baker, 1993; Wiktander, 1998). The radio-transmitters we used were model BD-2 (Holohil Ltd, Canada), weighing 1.15 g (4–5% of the body-mass). Battery life was 9 weeks and the range varied with topography from 100 m up to 1.5 km under optimal conditions. The transmitter was tied at the base of two central tail-feathers (e.g. left R1 and right R2) and the string secured to the feather-shafts with a drop of cyanoacrylic glue. By cutting the string between the transmitter and the feathers, transmitters were easily replaced or removed. If the battery ran out and we lost contact with the bird before we could remove the transmitter, the bird lost the transmitter when moulting. The tail-mount however also meant that radio-tracking was not possible during moult, i.e. July–September. It appeared to us that the birds adapted rapidly to the transmitter, and in our eyes the birds carrying a transmitter behaved just like those that did not. Moreover, we found no negative effects on either survival or reproductive success from the use of transmitters. Survival to 21 March the following season (see Wiktander et al., 2001 ) did not differ between birds with and without transmitter (males: 21=0.18, P=0.67, n=63; females: 21=1.48, P=0.22, n=58). Neither did the start of egg-laying differ (males: ANOVA F1,55=0.017, P=0.90; females: F1,54=1.35, P=0.25), nor the number of young fledged by males (F1,55=0.14, P=0.71), whereas females with a transmitter actually raised significantly more fledglings than females without (F1,51=12.30, P=0.001). Consequently, we feel confident that the transmitters did not hamper the birds or affect them or their behaviour negatively. Home-range areas were measured with the minimum convex polygon method, including 100% of locations. This method was chosen because, in contrast to other methods, it is not based on any assumptions about the size of grid cells or the percentage of locations to be included, which makes it straightforward to interpret and the only method that is strictly comparable between studies (Harris et al., 1990). Moreover, area estimates based on the minimum convex polygon method are unaffected by possible autocorrelation between locations (Harris et al., 1990). Our principal method of data collection was ‘radioassisted surveillance’ (Harris et al., 1990), that is, the radio equipment was primarily used to find the birds in the field, and once located they were observed directly. Thus, the vast majority of the locations are based on direct observations, rather than bearings or triangulation, and possible errors due to inaccurate positioning of locations (Harris et al., 1990) ought to be negligible. Radio-equipped birds were located one to three times weekly, and once located followed for 30 min up to several hours. The location of the bird was recorded at
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Fig. 1. Map of the study area. Hatched areas represent forest dominated by deciduous trees 550 years old. Water is shaded grey. White areas on land represent coniferous forest or open agricultural land. The polygons show the winter home-ranges of two males in the same year (broken lines) surrounding their late spring breeding territory (continuous lines), and the late spring breeding territory of one female. Location map of Sweden shows study area as a filled square.
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15 min intervals, which we regarded as a time sufficient to allow a lesser spotted woodpecker to move anywhere within its home-range. On average, the birds moved 79 m between successive 15 min locations, but with a large variation (S.D.=160), and movements were regularly 1 km or more (Olsson, 1998). In many cases the movements of the birds were such that we could not relocate them within 15 min, and often the time between locations was much longer. To investigate the seasonal change in home-range size, we divided the year into four periods, which gave large enough sample sizes and also had ecological significance. (1) Winter, from October to 20 March, when the birds spent most time singly. (2) Early spring, 21 March to 20 April, start of territorial displays, pair formation and establishment of breeding territories in our study area. (3) Late spring, 21 April to laying of the first egg (i.e. the 3–5 weeks preceeding egg-laying), a period when we assume breeding territories were established and activities occurred mainly within these. (4) Nesting, the day of the first egg until the young fledged. To investigate whether the number of locations was enough for home-range area to reach an asymptote, we made incremental area plots by adding successive locations for all tracked individuals in each period. Only individuals for which the home-range reached an asymptote are included in analyses. On average 96 locations (over 10 days) were necessary to reach a stable size in winter, 46 (seven) in early spring, 45 (five) in late spring and 31 (four) in the nesting period. For individuals tracked during the same period on the same territory for more than one year, we used the average of the area estimates of the years (n=3 individuals). As independent observations we treated the same individual on different territories (n=4; early spring one, late spring two, nesting one) and the same territory with different individuals (n=4; late spring two, nesting two). With these selections, 10 individuals were tracked in winter (four yearlings, six older: six males, four females), 15 in early spring (six yearlings, nine older: 13 males, two females), 22 in late spring (six yearlings, 16 older: 14 males, eight females) and 10 during nesting (three yearlings, seven older: six males, 4 females). Basic data for these individuals, including number of tracking-days and locations, are given in the Appendix A.
To estimate habitat area requirement, we summed the area of the forest stands utilised for foraging in the late spring period (see also Olsson, 1998). For this analysis we only included those 15 individuals (two yearlings, 13 older: eight males, seven females) for which we had direct observations of their activity in all visited stands, thus allowing us to discriminate whether stands were used for foraging or not. Forests were classified into stands according to tree species composition and age. In most areas this was made by the Regional Forestry Board for forest management purposes, independent of our study. In three areas we made the classification ourselves. Average size of forest stands was 4.7 ha (S.D.=3.6). Area estimates were normalised by log-transformation before use in ANOVA.
3. Results A three-way ANOVA showed that home-range size of the lesser spotted woodpeckers declined significantly from winter to nesting (F3,51=30.88, P<0.001; Table 1, Fig. 2), but did not vary between age-classes (F1,51=1.52, P=0.22) or sexes (F1,51=0.51, P=0.48). Moreover, in all but one of the individuals tracked over consecutive periods within a year, the home-range decreased from the earlier to the later period (Fig. 2). Polyandrous females are not included in the above analysis. The home-range of one polyandrous female tracked in late spring was 349 ha, and that of three females tracked during nesting was 169, 214 and 236 ha, i.e. considerably larger than that of monogamous birds (Table 1). The average of the summed area of forest stands used for foraging in the late spring period, i.e. our estimate of habitat area requirement, was 38.8 ha (n=15, S.D.=4.17, range 31.2–46.0 ha). The average of the 100% polygons of these 15 individuals was 104 ha (S.D.=38.7, range 47–168 ha), i.e. very similar to that for all tracked individuals (Table 1). On average, 44% (range 23–79%) of the polygon home-range consisted of forest stands utilised for foraging. A striking result of this exclusion of unused areas is the drastic decrease in variation and the similarity of resulting areas (Fig. 3).
Table 1 Home range size of lesser spotted woodpeckers during winter, spring and nestinga
Mean area (ha) Range (ha) S.D. CV% n a
Winter (Oct/Feb–20 March)
Early spring (21 March–20 April)
Late spring (21 April–1st egg)
Nesting (1st egg–fledging)
742 234–1654 423.0 57.0 10
355 36–1587 408.1 115.0 15
103 26–174 43.0 41.7 22
43 20–101 25.6 59.5 10
Ranges were calculated by minimum convex polygons, including 100% of locations.
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Fig. 3. Area of polygons (&) and the sum of forest stands utilised for foraging (*) of 15 individuals in late spring.
Fig. 2. Seasonal variation in home-range size of lesser spotted woodpeckers. * represent old birds, & yearlings. Lines connect individuals followed over consecutive periods in a year.
The relative variation in size of foraging areas (CV=10.75%) was significantly smaller than that of polygons (CV=37.21%; t14=2.74, P=0.005). The proportion of the polygons used for foraging may be regarded as an estimate of the level of fragmentation of habitat, which could influence travel costs and foraging economy, and hence reproduction. There was, however, no correlation between this measure of fragmentation and start of egg-laying (Spearman rank correlation, rs=0.09, n=11, P>0.50) or number of young fledged (rs=0.23, n=12, P>0.20).
4. Discussion 4.1. Home-range and territory size Compared with other species inhabiting similar habitats, home-ranges of the lesser spotted woodpecker appear unusually large for a bird of this size. In a radiotracking study of middle spotted woodpeckers (Dendrocopos medius; body-mass 58 g), Pasinelli (1999) found home-ranges in oak–hornbeam forest in Switzerland of 18.8 ha in winter, 11.6 ha in spring and 8.8 ha during nesting. In deciduous forest in our study area, the nuthatch (Sitta europea), similar in size to the lesser spotted woodpecker, held territories of 3.2 ha in winter
and 2.0 ha in spring (Nilsson, 1976) and breeding season density of great spotted woodpeckers (Dendrocopos major; body-mass 90 g) was 15.9 pairs/100 ha (6.3 ha/ pair; Nilsson, 1979). One reason for these differences is probably to be found in specific differences of foraging behaviour and distribution of food items (cf Cramp, 1985; Cramp and Perrins, 1993). The size of home-ranges and the seasonal variation found in our study correspond well with the results from radio-tracking of lesser spotted woodpeckers in Hessen, Germany by Hoentsch (1996). With periods delimited approximately as in our study, the homerange of one individual in winter was 540 ha, of two individuals in early spring 120 and 297 ha (mean=209 ha), and of three individuals during nesting 22, 33 and 57 ha (mean=37 ha). Apart from this we know only of studies reporting densities, which may not be directly comparable with measuring individual home-range size. In unfragmented habitat in Bialowieza forest in Poland, breeding season density of lesser spotted woodpeckers was 0.3 pairs/10 ha (33 ha/pair) and 0.2 pairs/10 ha (50 ha/pair) in ash–alder and oak–hornbeam forest, respectively (Wesolowski and Tomialojc, 1986). Spitznagel (1990) reported densities of 0.16 pairs/10 ha forested land (63 ha/pair) in willow, alder and oak-elm woods in south-west Germany. The reasons for the seasonal variation in home-range size may be interpreted from territorial behaviour and the trade-off between energy demand and predation risk. One reason for the large winter home-ranges is that the birds utilised somewhat different habitats in winter compared with spring, spending more time in spruce forest foraging from spruce (O. Olsson and U. Wiktander, unpublished data). This may in turn be a consequence of the fact that energy demand appeared to be lower in winter (Wiktander, 1998), allowing the birds to
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follow a strategy of minimizing predation risk, and of selecting habitats for higher safety rather than energetic profitability (Olsson, 1998). The common picture in winter was for the birds to roost in a hole located in a larger area of deciduous forest, which later became the breeding territory. Regularly, however, the birds left the deciduous forest to spend the winter days in the surrounding spruce forests, leading to the large winter home-range surrounding the breeding territory, exemplified in Fig. 1. The spruce forest was utilised even though spruce is less profitable than, e.g. lime (Olsson, 1998), but possibly the spruce forest is a safer place in winter than a defoliated deciduous forest. Commonly, the woodpeckers also followed mixed species flocks of tits (Parus spp.), nuthatches, treecreepers (Certhia familiaris) and goldcrests (Regulus regulus; O. Olsson and U. Wiktander, unpublished data), something which probably enhances protection from predation (Pulliam and Caraco, 1984). Moreover, the birds apparently spent the winter solitarily, with no territorial or social activites (Wiktander, 1998), the lack of territoriality possibly giving opportunity to travel over large areas with relatively low expense. Regularly, winter homeranges of different individuals were partly over-lapping, as depicted in Fig. 1. Home-range size in the early spring period is more difficult to interpret. The period we have delimited includes the transition from the undefended winter home-range to the defended territory in late spring. This transition could occur abruptly over a short period, e.g. in response to some phenological cue such as temperature. It could also be a gradual process, which could come about, e.g. if energy demands increase gradually and force the birds to gradually select the more profitable habitats more strongly. However, our data does not allow us to evaluate which process may be valid. The higher energy demand in late spring may force the birds to a strategy contrary to that in winter, with utilisation of the most profitable habitats, even if this occurs at the expense of a higher predation risk. This, and possibly also the need for mate and nest-hole guarding, would then restrict activity to a limited area of defendable size. The relatively small home-ranges during nesting may be explained by a high energy demand due to high travel costs during nestling feeding, coupled with a high food availability of surface-living insects. 4.2. Habitat area requirement The home-ranges in late spring included about 40 ha of forest utilised for foraging. At this time of year (3–5 weeks before egg-laying) foraging occurs more or less exclusively within the defended territories. This period is also part of the prelaying season, during which foraging, food availability and habitat availability has a profound
influence on reproductive success (Olsson, 1998; Olsson et al., 1999) and when mortality is highest (Wiktander, 1998). Hence, we regard this as an estimate of habitat area requirement for successfully reproducing lesser spotted woodpeckers. Primarily, the estimate is valid for birds inhabiting forests in southern Sweden, with a high or dominating inclusion of nemoral deciduous trees (i.e. oak, lime, hornbeam, beech, elm, ash and maple, in contrast to birch, alder, aspen, and sallow). It is difficult to evaluate how general our results are since this is the only study of its kind known to us, and since area requirements are likely to be influenced by structure and composition of forests, which varies between areas. The value 40 ha, however, corresponds with the densities from homogenous habitat in Poland and Germany reported by Wesolowski and Tomialojc (1986) and Spitznagel (1990), mentioned above. Wiktander et al. (1992) investigated the occurrence of lesser spotted woodpeckers in relation to area of deciduous forest within 200 ha plots. Lesser spotted woodpeckers occurred in areas with as little as 17 ha of deciduous forest, although at a low probability (25%). Probability of occurrence increased with area of deciduous forest, to reach 80% in areas with more than 38 ha. That study only recorded occurrence of the birds, however, and not if they were reproducing. Whether the value 40 ha is also valid for boreal regions, where nemoral trees are lacking, is difficult to evaluate since data on habitat use in the boreal region are very scarce. However, probability of occurrence was found to be unaffected by latitude in the study by Wiktander et al. (1992) which covered south and central Sweden north to 62 N, i.e. well into the boreal region. The home-range of one male radio-tracked in late April in the Swedish boreal region (60 400 N) was 104 ha, and included 51 ha of deciduous forest [mainly birch and grey alder (Alnus incana) but also sallow and aspen] utilised for foraging (U. Wiktander, unpublished data). Hence these studies indicate that habitat requirements may not be grossly different in the boreal region, but further study is required before firm conclusions can be drawn. A previous estimate of habitat area requirement, based on the size of islands with confirmed breeding, suggested 10–15 ha of deciduous forest (Ahle´n and Nilsson, 1982). According to our results, this is probably an under-estimate. As mentioned earlier, we believe it is the prelaying season that should be decisive for estimates of area requirement, rather than the nesting season. On nine occasions during our study we found nests on islands, 0.3–2.5 ha in size and isolated by 100– 200 m of water. These stretches of water were frequently crossed for collecting nestling food on other islands or the mainland, and during radio-tracking we regularly observed individuals flying over 500 m of open water. Hence, open water is apparently no major obstacle and nesting on an island does not necessarily mean that all
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prelaying foraging occurred on that island. The estimate in Ahle´n and Nilsson (1982) may be valid for the nesting season when, as shown in this study, home-ranges are significantly smaller than in the prebreeding season. 4.3. Fragmentation of habitat The summed area of foraging stands in late spring showed that the home-ranges based on 100% minimum convex polygons included large unused areas. Hence, it may be asked what is the ecological significance of the 100% polygons. In the late spring period, foraging stands were patchily distributed over the home-range, separated by up to 500 m of unused areas which consisted of avoided forest stands, open fields and water. The unused areas were not included in the polygon as a consequence of occasional excursions to peripheral parts outside some core area of foraging activity (cf Harris et al., 1990), but were part of the home-range and regularly traversed to reach foraging stands. Thus, the difference between polygon area and area of foraging stands reflects fragmentation of habitat. The polygon home-ranges as such, estimated in this study, shows sizes of defendable territories of successfully reproducing lesser spotted woodpeckers in a landscape with fragmented habitat. The polygon home-range also shows the very large mobility of these birds, which has practical implications when inventorying lesser spotted woodpeckers and is necessary to keep in mind to avoid the obvious risk of over-estimating numbers. An unresolved issue, but which is of great importance for conservation, is how large the fragmentation of habitat may be before an area becomes uneconomic to explore and defend. The most fragmented home-ranges in our study included only 23–30% habitat used for foraging, which apparently was an acceptable degree of fragmentation as judged by the reproductive performance. Tjernberg et al. (1993) found that black woodpeckers (Dryocopus martius) accepted a similar degree of fragmentation; the reproduction of birds in homogenous forest was not different from that of birds in an area with only 26% forest. Wiktander et al. (1992) found that lesser spotted woodpeckers occurred in areas with habitat fragmented over 200 ha, although it is not known if they were successfully reproducing. From the evidence at hand, we suggest that 20% of deciduous forest in the landscape should be regarded as the limit to the level of fragmentation. 4.4. Conservation guidelines Because of its dependence upon deciduous forests, and its specialised foraging for prey in dead wood of deciduous trees, the lesser spotted woodpecker is vulnerable to changes of this habitat. An important action for conservation of the species is therefore to consider
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the content and management of deciduous forest in the landscape. According to the results of this study, we give the recommendation that forest management should focus on a minimum of 40 ha of foraging habitat, i.e. forest dominated by deciduous trees, which may be distributed over a maximum of 200 ha. We believe this estimate to be a useful and relevant rule of thumb, since it is based on the woodpeckers’ own selection of foraging habitat and their reproductive performance. The definition of foraging habitat used is qualitatively simplified, however. Previous work shows that it is necessary to consider habitat selection on a finer scale, e.g. tree species preferences, selection of foraging patches and their characteristics, to complete the qualitative picture of habitat requirement. (Olsson, 1998; Olsson et al., 2001). One relationship that appears from those studies should be mentioned here, since it is of direct importance to the management of forests. Within territories, forests stands were used in relation to the amount of dead wood of preferred tree species. Since prey density in different tree species varies between years, so does the woodpeckers’ preference for tree species vary, and consequently the preference for forest stands may vary between years. Hence, a variation in tree species content (and not just area of deciduous forest) is likely to be a decisive factor for the long-term suitability of an area, and something which is important to consider in conservation work. In addition to characteristics of forests related to foraging and food availability, Smith (1997) and Pasinelli (2000) stressed the importance of a supply of suitable trees for nest excavation (i.e. dead trees or potential cavity trees) for great spotted woodpeckers and middle spotted woodpeckers. The same certainly applies to the lesser spotted woodpecker, which excavates a new nest each year and which appears to be obligately dependent upon dead wood for nest excavation, with dead deciduous trees being highly preferred (Wesolowski and Tomialojc, 1986; Ha˚gvar et al., 1990; Stenberg, 1996; Wiktander, 1998). Hence, a further important aspect of the management of forests for the lesser spotted woodpecker is to allow a continuous development of dead deciduous trees.
Acknowledgements We wish express our gratitude to Anders Stagen, Krister Wahlstro¨m, Fredrik O¨strand, Fredrik Haas, Leif Appelgren, Martin Stjernman and Ramo´n Martı´nez for many hours of help in the field, to Shahrzad Bakthiar for help with entering field data into the computer, and to all land-owners who let us work freely in their forests. Fredrik Haas also helped with preparing Fig. 1. The Regional Forestry Boards in A¨lmhult and
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Ljungby kindly let us use their forest data. The study was made possible by financial support from Lunds Djurskyddsfond, WWF-Sweden, Stiftelsen Oscar & Lili Lamms Minne, and Mistra/SUFOR.
Appendix A. Data for all individuals used in analyses of home-ranges in the four periods. F, female, M, male. Y, yearling, O, older bird. The area is that of the minimum convex polygon. The last two columns refers to the number of days and locations tracked. Individuals marked with an asterisk in the late spring period are those used for the analysis of habitat area requirement. Individual
Year Sex Age Area (ha) Days Locations
Winter BGG BOG BYL GVG RGB RGR RLB ROO YGB YRY
1993 1994 1995 1994 1991 1995 1992 1993 1993 1993
F F F F M M M M M M
Y O O Y O O O O Y Y
875 234 410 750 281 1060 568 943 650 1654
11 12 11 4 7 10 11 36 17 10
71 87 109 24 78 77 185 283 148 73
RGY* RLB* RLB* RLL* ROG* ROG* ROG* ROO RRV RRY* RYG* RYM YRY
1996 1991 1992 1992 1992 1995 1996 1993 1996 1996 1995 1995 1993
M M M F M M M M M M M M M
O O O O O O O O Y O Y Y Y
61 62 140 131 168 79 47 87 26 108 127 99 86
6 8 13 12 7 8 5 5 7 5 7 5 9
47 80 100 132 98 49 24 36 45 36 40 26 43
Nesting BMV BMY BMY BOO RGY RLB ROO RRY RVG RYM
1996 1995 1996 1994 1996 1992 1992 1996 1996 1996
F F F F M M M M M M
Y O O O O O Y O Y O
20 54 29 34 20 101 48 68 23 36
5 7 4 6 5 9 6 6 5 3
55 36 38 30 38 47 34 38 33 29
References
Early spring BOG GLB RGO RGR RGY RLB RMV ROG ROO RRB RYG YGB YOS YRY YRY
1996 1996 1993 1996 1996 1992 1996 1996 1993 1996 1995 1993 1996 1993 1996
F M M M M M M M M M M M F M M
O O Y O O O Y O O O Y Y Y Y O
36 82 564 173 70 360 98 235 866 123 312 458 253 1587 108
17 8 6 7 5 12 6 11 8 7 5 9 6 10 5
69 52 55 50 31 122 51 55 53 48 32 69 37 72 44
Late spring BMY* BOG* BOG* BOO* BYG* BYL BYO* RGO RGR
1996 1994 1996 1994 1995 1995 1996 1993 1995
F F F F F F F M M
O O O O Y O O Y O
71 103 58 109 156 174 133 63 172
4 6 5 5 9 5 6 4 4
32 42 37 48 89 25 64 35 32
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