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Territory quality and reproductive success in the Dartford warbler Sylvia undata in Dorset, England
Biological Conservation 1992, 61,209-215
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Territory quality and reproductive success in the Dartford warbler Sylvia undata in Dorset, England Clive K. Catchpole & James F. Phillips Department of Biology, Royal Holloway & Bedford New College, University of London, Egham, Surrey, UK, TW20 OEX (Received 24 June 1991; revised version received 11 September 1991; accepted 30 September 1991)
The relationship between territory quality and reproductive success was investigated in a population of the Dartford warbler Sylvia undata, breeding at Holt Heath National Nature Reserve, Dorset, UK. Reproductive success, as measured by the number of young produced, was high, with most pairs raising two broods of young. Nine separate measurements of territory quality revealed considerable variation and larger territories contained more heather Calluna vulgaris, but not more gorse Ulex europaeus. There was no initial relationship between measures of territory quality and reproductive success, but when adult losses were removed from the analysis, territories containing more gorse produced more young. Territories containing more gorse were also closer to the road where most adult losses occurred, thus initially masking the positive relationship between the amount of gorse and reproductive success. The study suggests that the amount of gorse within a territory, and distance to a road, are the two most important territory quality variables affecting reproductive success in this population. These findings have important implications for the future conservation of Dartford warblers, and in particular for the effective management of nature reserves on the remaining lowland heaths of southern England.
INTRODUCTION
bodies involved in their conservation in southern England (Bibby, 1978; Westerhoff & Tubbs, 1991). The breeding biology, feeding ecology and population dynamics of the Dartford warbler have been studied in some detail by Bibby (1979a,b,c). He found that their heathland habitat had a low floristic diversity, being dominated by heather Calluna vulgaris associated with gorse Ulex europaeus. Bibby (1979b) investigated the relative food value of the two plants, and found that gorse contained a much higher biomass of invertebrates. Gorse was also favoured for feeding, is traditionally associated with the presence of Dartford warbiers on heathland, and its presence and quality play an important role in the management of nature reserves (Bibby, 1978; Westerhoff & Tubbs, 1991). Previous studies which have attempted to investigate the importance of gorse have classified territories where it is dominant, present or absent (Bibby & Tubbs, 1975), or divided territories into those where gorse is abundant or not (Bibby,
The Dartford warbler Sylvia undata is a characteristic species of the maquis of southwestern Europe, extending north onto the heathlands of western France and southern England. In the British Isles it is at the very edge of its range, and is now confined to the lowland heaths of southern England (Bibby & Tubbs, 1975). The Dartford warbler is also one of the few resident insectivorous passerines, and subject to heavy losses in severe winters. Lowland heath is fast disappearing from southern England (Moore, 1962; Webb & Haskins, 1980), and detailed surveys have revealed that Dartford warblers are critically dependent upon this declining resource (Bibby & Tubbs, 1975; Robins & Bibby, 1985; Westerhoff & Tubbs, 1991). Their distribution is becoming increasingly fragmented, often confined to nature reserves, and they remain a continuing concern to the many Biological Conservation 0006-3207/92/$05.00 © 1992 Elsevier Science Publishers Ltd, England. Printed in Great Britain 209
210
Clive K. Catchpole, James F. Phillips
1979a). Measurements of reproductive success were either not attempted, or restricted in some way. This study attempts to extend previous work by a more detailed quantitative study of both territory quality and reproductive success in a population of Dartford warblers. In particular, we set out to test the prediction made by Bibby (1979a) that reproductive success in any one year might be related to the amount of gorse contained within each territory.
METHODS
survey. The following variables were extracted from the final maps, and precise areas determined using a Quantimet Image Analyser (Cambridge Instruments, Cambridge, UK). The Quantimet is capable of measuring irregular shapes, and can be programmed to measure the precise area of each vegetation type in a territory, as well as the territory perimeter and total area. The following variables were selected for measurement: (1) (2) (3)
The study site (4) The study site was Holt Heath National Nature Reserve, Dorset, a 400-ha area of lowland heath in southern England, dominated by heather Calluna vulgaris and gorse Ulex europaeus. The reserve is situated some 6 km from the town of Wimborne, and is traversed by two minor roads. The floristic simplicity of this type of heathland (Bibby, 1979a) makes it ideal material for measuring the precise area of the dominant plants. Heathland is also subject to periodic burning followed by the gradual recovery of the vegetation, and the last major burning at Holt Heath was in 1976. Thus the age and quality of gorse and heather at Holt Heath tend to be fairly uniform, and the main variable is the amount present within each territory. Accurate scale maps divided into 100-m grids were used to map vegetation and for plotting the positions of birds and nests. The project started in January 1989, and the heath was then visited on an almost daily basis for two years. Although a few adults were caught and colour-ringed for individual identification, the species is highly sedentary (Bibby, 1979c) and the main method relied upon intensive daily observations of territorial birds throughout the year.
Territory quality Territory boundaries were obtained by plotting the positions of singing and fighting males onto the scale maps, and by including observations from the daily visits. By the end of the season, there was very little doubt about the precise location of territory boundaries. The vegetation in each territory was then mapped using a ground
(5) (6) (7)
(8)
(9)
Territory perimeter (m): the total length of the territory boundary; Territory size (ha): the total area of the territory; Area of gorse (ha): the total area of gorse within the territory; Area of heather (ha): the total area of heather within the territory; Area remaining (ha): the total area of all other vegetation within the territory; Height of gorse (m): the average height of gorse within the territory; Number of neighbours: the number of surrounding territories whose boundaries are shared at some point; Nearest neighbour (m): the distance from the centre of a territory to the nearest centre of a neighbouring territory; and Distance from road (m): the distance from the centre of a territory to the nearest road.
Reproductive success Dartford warblers are usually double-brooded and enjoy relatively high nesting success (Bibby, 1979a), possibly because the nests are placed in dense vegetation and are difficult to locate. Although breeding activity in each territory was continually monitored, relatively few nests with eggs were found during incubation. Most nests were found when young were being fed in the nest, or soon after fledging when the young could be counted while still in the territory. In 1989, we were not able to cover every territory as thoroughly as we would have liked, and there were several gaps in our reproductive success data. However, in 1990, with extra effort and the benefit of experience, we were confident that the number of young produced in each territory was known, and this is our measure of breeding success. The following analysis is therefore restricted to the complete 1990 data set, consisting of a population •of 37 breeding pairs.
Reproductive success in Dartford warbler
211
Table 2. Success of first and second broods
Statistical analysis
Non-parametric statistics (Siegel, 1956) have been used throughout. Unless stated otherwise, correlations were obtained using the Spearman rank correlation coefficient r s and pairwise comparisons using the Mann-Whitney U-test. A two-tailed region of rejection was set at the 5% level of significance.
RESULTS Reproductive success
The reproductive success of the Dartford warbler at Holt Heath during 1990 is set out in Table 1. The 37 territory-holding pairs raised a total of 187 young, giving an overall figure of 5-05 young per pair. If pairs who raised no young at all are excluded, the figure rises to 6.03 per pair. Most o f the pairs (26) raised two broods, and only five raised just a single brood. O f these, two were late breeders who did not fledge their first brood until July. Both these pairs occupied peripheral territories in less favoured areas of the reserve. The other three were males who lost their female while raising a first brood. Although they raised their first brood successfully, and stayed on territory, they failed to attract another female. Clearly, the number of broods a pair can raise is one o f the main factors determining reproductive success. When broods are examined separately (Table 2), it can be seen that the overall mean brood size was 3-28. This compares with a figure o f 3.66 given by Bibby (1979a) although his Dorset data included two years and were for young still in the nest eight days after hatching. In this study, there was no significant difference between the size o f first and second broods (Table 2). Six pairs raised no young at all (Table 1). This was not due to nest predation, but to the disappearance o f one or both o f the adults during the breeding cycle. The Table 1. Reproductive success of Dartford warblers at Holt Heath
Number of pairs
Number of young
Young per pair
Two broods One brood No broods
26 5 6
172 15 --
6.62 3.00 --
Total
37
187
5.05
First brood
Secondbrood
31 101 3.26
26 86 3.31
Number of broods Total young Young per brood
Total 57 187 3.28
only case of known nest predation occurred in a territory where the male disappeared during late May. The female continued to feed the young, but the nest was predated on 28 May. In another case, a female disappeared quite early in the season on 22 April, and the male stayed alone in territory until 5 June. In the four other cases, two pairs disappeared in late April, one in early May, and one in mid-June. The possible causes o f adults disappearing from the population will be discussed later. The important point at this stage is that reproductive success seemed relatively high, most adults managed to raise two broods, and that the most significant factor affecting reproductive success was apparently adult mortality. T e r r i t o r y quality
The nine territory quality variables investigated are set out in Table 3. The mean territory size of 2.13 ha compares with the estimate of 2.38 ha obtained by Bibby and Tubbs (1975), also working on Dorset heathland. Although there was considerable variation in territory size, all the territories contained at least some gorse and heather, and the relationship between these three variables is of interest. For example, there is no correlation between territory size and area of gorse (rs -- 0.238, n -- 37, p = 0-157). However, there is a highly significant correlation between territory size and area o f heather (r s = 0.752, n -- 37, p = 0-000). So, as territories get bigger, they contain more heather, but not more gorse (Fig. l(a) and (b)). The remaining area of vegetation is insignificant, and Table 3. Categories and measurements of territory quality
Territory quality Territory perimeter (m) Territory size (ha) Area of gorse (ha) Area of heather (ha) Area remaining (ha) Height of gorse (m) Number of neighbours Nearest neighbour (m) Distance from road (m)
Mean 646.49 2.13 0-60 1.45 0.07 1-57 2.11 180.14 409.19
SD
Range
117.52 0.61 0.37 0.65 0.13 0.22 1.52 82.70 438.88
422-936 0.96-3-51 0.05-1-62 0.38-3.20 0.004).50 0.95-2.00 0.00-6.00 105-600 20-1 770
Clive K. Catchpole, James F. Phillips
212
(a)
_
Table 4. Spearman rank correlation coefficients (rs) between aspects of territory quality and the number of young produced Territory quality
All data (n = 37)
J:=
Excluding losses (n = 27)
(3;
rs
p
rs
-0.146 -0-140 -0-157 -0.036 -0.127 -0.036 -0.061 -0.108 0-153
0-390 0.407 0.354 0.832 0.451 0.832 0.721 0.523 0-367
-0-256 0.035 0.411 -0.191 -0-290 -0-019 0-107 -0-236 -0.459
tO
tO
0
•
00 O0 •
• OO0
i
4.
2
Territory size (hal (b) e-
. .../.
¢-
¢,'W,.0
CO
p
1
II
i.
=-:.-'t" °
f...
i
2 Territory size (ha)
i
t.
Fig. 1. The relationship between territory size and (a) the area of gorse within each territory (r s -- 0.238, n = 37, p -- 0.157), and (b) the area of heather within each territory (rs = 0.752, n = 37, p = 0.000).
small amounts of bracken or grass only occcured in 12 of the 37 territories. The mean height of gorse was just over 1.5 m, compared to a median height o f just over 1 m found by Bibby and Tubbs (1975). The number of neighbouring territories was usually two, and the mean nearest neighbour distance of 180 m compares well with the figure o f 166 m obtained by Bibby and Tubbs (1975). The remaining variable, distance from road, shows a strong inverse correlation with the a m o u n t of gorse (rs = -0"535, n = 37, p = 0.001), but not the a m o u n t of heather (r~ = 0.165, n = 37, p = 0.329). Territories which contain more gorse are also situated closer to the road.
Territory quality and reproductive success Our working hypothesis is that territory quality has some effect upon reproductive success in Dartford warblers. However, an initial correlation
Territory perimeter Territory size Area of gorse Area of heather Area remaining Height of gorse N u m b e r of neighbours Nearest neighbour Distance from road
0.198 0.861 0-033* 0.340 0.141 0.924 0.596 0-236 0.016"
* Indicates where p < 0.05.
analysis found no relationship between any of the nine territory quality variables and the number of young (Table 4). Another technique is to split the territory quality variables into high and low categories, and then test for differences in reproductive success. To do this, the mean values in Table 3 were used to split the data into two groups for each variable. However, no significant differences in the number of young produced were obtained by the Mann-Whitney U-test. In studies of reproductive success, subtle differences can be masked by major factors such as predation, and we have already seen that the loss o f adults has a significant effect in this population. The correlation analysis was repeated, this time excluding the ten territories where adult losses occurred. In this analysis, a significant positive correlation between the a m o u n t of gorse and n u m b e r of young was obtained (r s = 0.411, n = 27, p = 0.033) (Fig. 2(a)). The only other significant result to emerge was a strong negative correlation between distance from the road and number of young (r s = -0.459, n = 27, p = 0.016) (Fig. 2(b)). The split analysis was also repeated excluding adult losses, and this time one significant difference emerged. As shown in Fig. 3, more young were produced from territories which contained above-average quantities of gorse (~= 6.90 + 0.18) than from those with below average quantities (~ = 6-06 + 0.34) ( Z = -1.953, n = 27, p = 0.05). It seems that territories with more gorse do produce more young, but this only appears when adult losses are removed from the analysis. The most likely explanation is that some territories with high amounts o f gorse are also suffering adult losses. We have already seen that there is a highly significant inverse correlation between the
Reproductive success in Dartford warbler
213
(a)
10
0
,
8
0 0
0
0 0
0 0
•
0
0
6
0~'"~0
t--
.
t-
~>" 6
Z
0
.
T
II low
.C3
E 4 -1
0
0
i
i
i
0"5 1.0 Area of gorse (ha)
1.5
10
B
high
gorse gorse
z
g
"1"
0
(b)
0
O0
Fig. 3. The number of young produced (mean and standard error) in territories containing either below- or above-average quantities of gorse. The difference between the two groups is significant with the Mann-Whitney U-test ( Z -- -1.953, n = 27, p = 0.05).
contain more gorse (Z -- - 3.130, n = 37, p = 0-002) but were also much nearer the road ( Z = - 3 - 0 1 0 , n = 37, p = 0-003).
o
DISCUSSION 6
z
4 O
O
i
1000 2000 Disfance from road (m) Fig. 2. The relationship between the number of young produced and (a) the area of gorse in each territory (r~ -- 0.411, n = 27, p = 0.033), and (b) distance to the nearest road (r s = -0.459, n = 27, p = 0.016). Filled circles indicate more than one data point.
area of gorse and the distance to the road, so that territories containing more gorse are often located near the road. Visual inspection of territory maps shows that territories near the road are often those where adults disappeared. Several dead adults at Holt Heath have been picked up from the road, presumably hit by passing cars. If this interpretation is correct, then we can predict that territories where adult losses occurred do have high amounts of gorse, but are also situated closer to the road. To test this, the data were split into two final groups, the ten territories where total losses occurred, and the remaining 27. As predicted, only two variables showed a significant difference with the Mann-Whitney U-test. Territories where adult losses occurred did indeed
The present study has confirmed the suggestion by Bibby (1979a) that reproductive success in Dartford warblers can be related to the amount of gorse within their territories. He also found that mean brood size was significantly higher in territories which were classified as having abundant gorse. A later study by Robins and Bibby (1985) established an overall correlation between the distribution of Dartford warblers in England and regions which contain extensive areas of gorse. In a detailed study of feeding ecology, Bibby (1979b) showed that the biomass of invertebrate food on gorse far exceeded that of heather, and even when gorse accounted for only 2% of cover, it accounted for 68% of total foraging. Adults also flew extensive distances over heather to forage in stands of gorse. Although the importance of gorse has been clearly demonstrated, the relationship between gorse and heather in Dartford warbler territories is less well understood. In the present study, a strong correlation was obtained between territory size and area of heather (but not with gorse), and neither of these were correlated with reproductive success. If we assume that territory size is adaptive, there are several possible interpretations. One, proposed by Verner (1977), suggests that birds defend territories much larger than they really need (superterritories) to prevent neighbours using the resource. It may also be that, in areas
214
Clive K. Catchpole, James F. Phillips
less rich in gorse, overall territory size increases in an attempt to contain more gorse. Another interpretation is that heather is more important to Dartford warblers than has been suspected, and indeed there is some evidence to support this view. Although gorse is preferred for feeding, Bibby (1979a) found that heather was the preferred nest site, even in territories where there was plenty of gorse. Furthermore, surveys of Dartford warblers in England (Bibby & Tubbs, 1975; Robins & Bibby, 1985; Westerhoff & Tubbs, 1991) recorded several territories without gorse present, but none where heather or ericaceous plants were totally absent. The present study also found that reproductive success was high, with most pairs raising two broods successfully. Bibby (1979a) had earlier established that nesting success in Dartford warblers (70-80%) was considerably higher than estimates for other Sylvia warblers (3545%). The main factor affecting reproductive success in our study was the loss of adults during the breeding season. Unfortunately, Holt Heath is traversed by two roads, and in both years our analysis revealed that adults disappeared from territories adjacent to or near the roads. There was also an inverse correlation between area of gorse and distance to the road; attractive territories with high amounts of gorse were often situated nearer to the road. Gorse often occurs where there has been soil disturbance, such as along boundary banks by roads. During the study two dead adults were picked up off the road, and Bibby (1979a) also discovered a dead adult on the road. The Dartford warbler has a slow, low, dipping flight path, and seems more likely than many birds to be hit by passing road traffic. Not all nature reserves have roads, but those that do might well be incurring higher adult mortality from this source than previously suspected. Any management or development plans involving the siting of roads and access in relation to gorse should certainly take this factor into account. The increased mortality of adults on high gorse territories near the road clearly has a masking effect, and explains the absence of an overall correlation between the amount of gorse and reproductive success. That an eventual positive correlation (confirmed by a split analysis) was obtained, even after excluding the high gorse territories near the road, suggests that there is indeed a potentially stronger underlying relationship. Although we can never know the real number of young these adults may have produced, extrapolation from the existing correlation suggests they would
have been high rather than low. The only way to test this would be to close the roads across Holt Heath, or repeat the study in a similar area with no roads. The study certainly suggests that road traffic has more impact on adult mortality and thus reproductive success than has previously been thought. Several studies have shown that the Dartford warbler habitat of lowland heath is gradually disappearing from southern England (Moore, 1962; Webb & Haskins, 1980). There is also increasing fragmentation with isolated populations of Dartford warblers becoming confined to nature reserves. The conservation and management of such areas may be vital if the species is to continue breeding in southern England. Existing techniques (Bibby, 1978; Westerhoff & Tubbs, 1991) may well involve maintaining, but also manipulating, the gorse/heather ratio and quality to achieve maxim u m effect. Whilst this study confirms the importance of gorse in maximising reproductive success, more research is still needed on the importance of territory quality to adult mortality, offspring survival and lifetime reproductive success. The most recent survey by Westerhoff & Tubbs (1991) is encouraging, as it shows that the Dartford warbler population is maintaining and even increasing its tenuous foothold in southern England. However, they also point out that shortterm increases may well be due to the recent spell of mild winters. Clearly we should not be complacent, and the fact remains that their habitat is still under tremendous pressure from a variety of sources, including increasing development. We must endeavour to manage what remains with the utmost care.
ACKOWLEDGEMENTS This study was supported by a grant from the World Wide F u n d for Nature United Kingdom. We would also like to thank the Nature Conservancy Council for permission to work at Holt Heath National Nature Reserve.
REFERENCES
Bibby, C. J. (1978). Conservation of the Dartford warbler on English lowland heaths: a review. Biol. Conserv., 13, 299-307. Bibby, C. J. (1979a). Breeding biology of the Dartford warbler Sylvia undata in England. Ibis, 121, 41-52.
Reproductive success in Dartford warbler Bibby, C. J. (1979b). Foods of the Dartford warbler Sylvia undata on southern English heathland. J. Zool., 188, 557-76. Bibby, C. J. (1979c). Mortality and movements of Dartford warblers in England. Brit. Birds, 72, 10-22. Bibby, C. J. & Tubbs, C. R. (1975). Status, habitats and conservation of the Dartford warbler in England. Brit. Birds, 68, 177-95. Moore, N. W. (1962). The heaths of Dorset and their conservation. J. Ecol., 50, 369-91. Robins, M. & Bibby, C. J. (1985). Dartford warblers in 1984 Britain. Brit. Birds, 78, 269-80.
215
Siegel, S. (1956). Nonparametric Statistics. McGraw-Hill, New York. Verner, J. (1977). On the adaptive significance of territoriality. Amer. Nat., 111,769-75. Webb, N. R. & Haskins, L. E. (1980). An ecological survey of heathlands in the Poole Basin, Dorset, England in 1978. Biol. Conserv., 17, 281-96. Westerhoff, D. & Tubbs, C. R. (1991). Dartford warblers Sylvia undata, their habitat and conservation in the New Forest, Hampshire, England in 1988. Biol. Conserv., 56, 89-100.
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Territory quality and reproductive success in the Dartford warbler Sylvia undata in Dorset, England Clive K. Catchpole & James F. Phillips Department of Biology, Royal Holloway & Bedford New College, University of London, Egham, Surrey, UK, TW20 OEX (Received 24 June 1991; revised version received 11 September 1991; accepted 30 September 1991)
The relationship between territory quality and reproductive success was investigated in a population of the Dartford warbler Sylvia undata, breeding at Holt Heath National Nature Reserve, Dorset, UK. Reproductive success, as measured by the number of young produced, was high, with most pairs raising two broods of young. Nine separate measurements of territory quality revealed considerable variation and larger territories contained more heather Calluna vulgaris, but not more gorse Ulex europaeus. There was no initial relationship between measures of territory quality and reproductive success, but when adult losses were removed from the analysis, territories containing more gorse produced more young. Territories containing more gorse were also closer to the road where most adult losses occurred, thus initially masking the positive relationship between the amount of gorse and reproductive success. The study suggests that the amount of gorse within a territory, and distance to a road, are the two most important territory quality variables affecting reproductive success in this population. These findings have important implications for the future conservation of Dartford warblers, and in particular for the effective management of nature reserves on the remaining lowland heaths of southern England.
INTRODUCTION
bodies involved in their conservation in southern England (Bibby, 1978; Westerhoff & Tubbs, 1991). The breeding biology, feeding ecology and population dynamics of the Dartford warbler have been studied in some detail by Bibby (1979a,b,c). He found that their heathland habitat had a low floristic diversity, being dominated by heather Calluna vulgaris associated with gorse Ulex europaeus. Bibby (1979b) investigated the relative food value of the two plants, and found that gorse contained a much higher biomass of invertebrates. Gorse was also favoured for feeding, is traditionally associated with the presence of Dartford warbiers on heathland, and its presence and quality play an important role in the management of nature reserves (Bibby, 1978; Westerhoff & Tubbs, 1991). Previous studies which have attempted to investigate the importance of gorse have classified territories where it is dominant, present or absent (Bibby & Tubbs, 1975), or divided territories into those where gorse is abundant or not (Bibby,
The Dartford warbler Sylvia undata is a characteristic species of the maquis of southwestern Europe, extending north onto the heathlands of western France and southern England. In the British Isles it is at the very edge of its range, and is now confined to the lowland heaths of southern England (Bibby & Tubbs, 1975). The Dartford warbler is also one of the few resident insectivorous passerines, and subject to heavy losses in severe winters. Lowland heath is fast disappearing from southern England (Moore, 1962; Webb & Haskins, 1980), and detailed surveys have revealed that Dartford warblers are critically dependent upon this declining resource (Bibby & Tubbs, 1975; Robins & Bibby, 1985; Westerhoff & Tubbs, 1991). Their distribution is becoming increasingly fragmented, often confined to nature reserves, and they remain a continuing concern to the many Biological Conservation 0006-3207/92/$05.00 © 1992 Elsevier Science Publishers Ltd, England. Printed in Great Britain 209
210
Clive K. Catchpole, James F. Phillips
1979a). Measurements of reproductive success were either not attempted, or restricted in some way. This study attempts to extend previous work by a more detailed quantitative study of both territory quality and reproductive success in a population of Dartford warblers. In particular, we set out to test the prediction made by Bibby (1979a) that reproductive success in any one year might be related to the amount of gorse contained within each territory.
METHODS
survey. The following variables were extracted from the final maps, and precise areas determined using a Quantimet Image Analyser (Cambridge Instruments, Cambridge, UK). The Quantimet is capable of measuring irregular shapes, and can be programmed to measure the precise area of each vegetation type in a territory, as well as the territory perimeter and total area. The following variables were selected for measurement: (1) (2) (3)
The study site (4) The study site was Holt Heath National Nature Reserve, Dorset, a 400-ha area of lowland heath in southern England, dominated by heather Calluna vulgaris and gorse Ulex europaeus. The reserve is situated some 6 km from the town of Wimborne, and is traversed by two minor roads. The floristic simplicity of this type of heathland (Bibby, 1979a) makes it ideal material for measuring the precise area of the dominant plants. Heathland is also subject to periodic burning followed by the gradual recovery of the vegetation, and the last major burning at Holt Heath was in 1976. Thus the age and quality of gorse and heather at Holt Heath tend to be fairly uniform, and the main variable is the amount present within each territory. Accurate scale maps divided into 100-m grids were used to map vegetation and for plotting the positions of birds and nests. The project started in January 1989, and the heath was then visited on an almost daily basis for two years. Although a few adults were caught and colour-ringed for individual identification, the species is highly sedentary (Bibby, 1979c) and the main method relied upon intensive daily observations of territorial birds throughout the year.
Territory quality Territory boundaries were obtained by plotting the positions of singing and fighting males onto the scale maps, and by including observations from the daily visits. By the end of the season, there was very little doubt about the precise location of territory boundaries. The vegetation in each territory was then mapped using a ground
(5) (6) (7)
(8)
(9)
Territory perimeter (m): the total length of the territory boundary; Territory size (ha): the total area of the territory; Area of gorse (ha): the total area of gorse within the territory; Area of heather (ha): the total area of heather within the territory; Area remaining (ha): the total area of all other vegetation within the territory; Height of gorse (m): the average height of gorse within the territory; Number of neighbours: the number of surrounding territories whose boundaries are shared at some point; Nearest neighbour (m): the distance from the centre of a territory to the nearest centre of a neighbouring territory; and Distance from road (m): the distance from the centre of a territory to the nearest road.
Reproductive success Dartford warblers are usually double-brooded and enjoy relatively high nesting success (Bibby, 1979a), possibly because the nests are placed in dense vegetation and are difficult to locate. Although breeding activity in each territory was continually monitored, relatively few nests with eggs were found during incubation. Most nests were found when young were being fed in the nest, or soon after fledging when the young could be counted while still in the territory. In 1989, we were not able to cover every territory as thoroughly as we would have liked, and there were several gaps in our reproductive success data. However, in 1990, with extra effort and the benefit of experience, we were confident that the number of young produced in each territory was known, and this is our measure of breeding success. The following analysis is therefore restricted to the complete 1990 data set, consisting of a population •of 37 breeding pairs.
Reproductive success in Dartford warbler
211
Table 2. Success of first and second broods
Statistical analysis
Non-parametric statistics (Siegel, 1956) have been used throughout. Unless stated otherwise, correlations were obtained using the Spearman rank correlation coefficient r s and pairwise comparisons using the Mann-Whitney U-test. A two-tailed region of rejection was set at the 5% level of significance.
RESULTS Reproductive success
The reproductive success of the Dartford warbler at Holt Heath during 1990 is set out in Table 1. The 37 territory-holding pairs raised a total of 187 young, giving an overall figure of 5-05 young per pair. If pairs who raised no young at all are excluded, the figure rises to 6.03 per pair. Most o f the pairs (26) raised two broods, and only five raised just a single brood. O f these, two were late breeders who did not fledge their first brood until July. Both these pairs occupied peripheral territories in less favoured areas of the reserve. The other three were males who lost their female while raising a first brood. Although they raised their first brood successfully, and stayed on territory, they failed to attract another female. Clearly, the number of broods a pair can raise is one o f the main factors determining reproductive success. When broods are examined separately (Table 2), it can be seen that the overall mean brood size was 3-28. This compares with a figure o f 3.66 given by Bibby (1979a) although his Dorset data included two years and were for young still in the nest eight days after hatching. In this study, there was no significant difference between the size o f first and second broods (Table 2). Six pairs raised no young at all (Table 1). This was not due to nest predation, but to the disappearance o f one or both o f the adults during the breeding cycle. The Table 1. Reproductive success of Dartford warblers at Holt Heath
Number of pairs
Number of young
Young per pair
Two broods One brood No broods
26 5 6
172 15 --
6.62 3.00 --
Total
37
187
5.05
First brood
Secondbrood
31 101 3.26
26 86 3.31
Number of broods Total young Young per brood
Total 57 187 3.28
only case of known nest predation occurred in a territory where the male disappeared during late May. The female continued to feed the young, but the nest was predated on 28 May. In another case, a female disappeared quite early in the season on 22 April, and the male stayed alone in territory until 5 June. In the four other cases, two pairs disappeared in late April, one in early May, and one in mid-June. The possible causes o f adults disappearing from the population will be discussed later. The important point at this stage is that reproductive success seemed relatively high, most adults managed to raise two broods, and that the most significant factor affecting reproductive success was apparently adult mortality. T e r r i t o r y quality
The nine territory quality variables investigated are set out in Table 3. The mean territory size of 2.13 ha compares with the estimate of 2.38 ha obtained by Bibby and Tubbs (1975), also working on Dorset heathland. Although there was considerable variation in territory size, all the territories contained at least some gorse and heather, and the relationship between these three variables is of interest. For example, there is no correlation between territory size and area of gorse (rs -- 0.238, n -- 37, p = 0-157). However, there is a highly significant correlation between territory size and area o f heather (r s = 0.752, n -- 37, p = 0-000). So, as territories get bigger, they contain more heather, but not more gorse (Fig. l(a) and (b)). The remaining area of vegetation is insignificant, and Table 3. Categories and measurements of territory quality
Territory quality Territory perimeter (m) Territory size (ha) Area of gorse (ha) Area of heather (ha) Area remaining (ha) Height of gorse (m) Number of neighbours Nearest neighbour (m) Distance from road (m)
Mean 646.49 2.13 0-60 1.45 0.07 1-57 2.11 180.14 409.19
SD
Range
117.52 0.61 0.37 0.65 0.13 0.22 1.52 82.70 438.88
422-936 0.96-3-51 0.05-1-62 0.38-3.20 0.004).50 0.95-2.00 0.00-6.00 105-600 20-1 770
Clive K. Catchpole, James F. Phillips
212
(a)
_
Table 4. Spearman rank correlation coefficients (rs) between aspects of territory quality and the number of young produced Territory quality
All data (n = 37)
J:=
Excluding losses (n = 27)
(3;
rs
p
rs
-0.146 -0-140 -0-157 -0.036 -0.127 -0.036 -0.061 -0.108 0-153
0-390 0.407 0.354 0.832 0.451 0.832 0.721 0.523 0-367
-0-256 0.035 0.411 -0.191 -0-290 -0-019 0-107 -0-236 -0.459
tO
tO
0
•
00 O0 •
• OO0
i
4.
2
Territory size (hal (b) e-
. .../.
¢-
¢,'W,.0
CO
p
1
II
i.
=-:.-'t" °
f...
i
2 Territory size (ha)
i
t.
Fig. 1. The relationship between territory size and (a) the area of gorse within each territory (r s -- 0.238, n = 37, p -- 0.157), and (b) the area of heather within each territory (rs = 0.752, n = 37, p = 0.000).
small amounts of bracken or grass only occcured in 12 of the 37 territories. The mean height of gorse was just over 1.5 m, compared to a median height o f just over 1 m found by Bibby and Tubbs (1975). The number of neighbouring territories was usually two, and the mean nearest neighbour distance of 180 m compares well with the figure o f 166 m obtained by Bibby and Tubbs (1975). The remaining variable, distance from road, shows a strong inverse correlation with the a m o u n t of gorse (rs = -0"535, n = 37, p = 0.001), but not the a m o u n t of heather (r~ = 0.165, n = 37, p = 0.329). Territories which contain more gorse are also situated closer to the road.
Territory quality and reproductive success Our working hypothesis is that territory quality has some effect upon reproductive success in Dartford warblers. However, an initial correlation
Territory perimeter Territory size Area of gorse Area of heather Area remaining Height of gorse N u m b e r of neighbours Nearest neighbour Distance from road
0.198 0.861 0-033* 0.340 0.141 0.924 0.596 0-236 0.016"
* Indicates where p < 0.05.
analysis found no relationship between any of the nine territory quality variables and the number of young (Table 4). Another technique is to split the territory quality variables into high and low categories, and then test for differences in reproductive success. To do this, the mean values in Table 3 were used to split the data into two groups for each variable. However, no significant differences in the number of young produced were obtained by the Mann-Whitney U-test. In studies of reproductive success, subtle differences can be masked by major factors such as predation, and we have already seen that the loss o f adults has a significant effect in this population. The correlation analysis was repeated, this time excluding the ten territories where adult losses occurred. In this analysis, a significant positive correlation between the a m o u n t of gorse and n u m b e r of young was obtained (r s = 0.411, n = 27, p = 0.033) (Fig. 2(a)). The only other significant result to emerge was a strong negative correlation between distance from the road and number of young (r s = -0.459, n = 27, p = 0.016) (Fig. 2(b)). The split analysis was also repeated excluding adult losses, and this time one significant difference emerged. As shown in Fig. 3, more young were produced from territories which contained above-average quantities of gorse (~= 6.90 + 0.18) than from those with below average quantities (~ = 6-06 + 0.34) ( Z = -1.953, n = 27, p = 0.05). It seems that territories with more gorse do produce more young, but this only appears when adult losses are removed from the analysis. The most likely explanation is that some territories with high amounts o f gorse are also suffering adult losses. We have already seen that there is a highly significant inverse correlation between the
Reproductive success in Dartford warbler
213
(a)
10
0
,
8
0 0
0
0 0
0 0
•
0
0
6
0~'"~0
t--
.
t-
~>" 6
Z
0
.
T
II low
.C3
E 4 -1
0
0
i
i
i
0"5 1.0 Area of gorse (ha)
1.5
10
B
high
gorse gorse
z
g
"1"
0
(b)
0
O0
Fig. 3. The number of young produced (mean and standard error) in territories containing either below- or above-average quantities of gorse. The difference between the two groups is significant with the Mann-Whitney U-test ( Z -- -1.953, n = 27, p = 0.05).
contain more gorse (Z -- - 3.130, n = 37, p = 0-002) but were also much nearer the road ( Z = - 3 - 0 1 0 , n = 37, p = 0-003).
o
DISCUSSION 6
z
4 O
O
i
1000 2000 Disfance from road (m) Fig. 2. The relationship between the number of young produced and (a) the area of gorse in each territory (r~ -- 0.411, n = 27, p = 0.033), and (b) distance to the nearest road (r s = -0.459, n = 27, p = 0.016). Filled circles indicate more than one data point.
area of gorse and the distance to the road, so that territories containing more gorse are often located near the road. Visual inspection of territory maps shows that territories near the road are often those where adults disappeared. Several dead adults at Holt Heath have been picked up from the road, presumably hit by passing cars. If this interpretation is correct, then we can predict that territories where adult losses occurred do have high amounts of gorse, but are also situated closer to the road. To test this, the data were split into two final groups, the ten territories where total losses occurred, and the remaining 27. As predicted, only two variables showed a significant difference with the Mann-Whitney U-test. Territories where adult losses occurred did indeed
The present study has confirmed the suggestion by Bibby (1979a) that reproductive success in Dartford warblers can be related to the amount of gorse within their territories. He also found that mean brood size was significantly higher in territories which were classified as having abundant gorse. A later study by Robins and Bibby (1985) established an overall correlation between the distribution of Dartford warblers in England and regions which contain extensive areas of gorse. In a detailed study of feeding ecology, Bibby (1979b) showed that the biomass of invertebrate food on gorse far exceeded that of heather, and even when gorse accounted for only 2% of cover, it accounted for 68% of total foraging. Adults also flew extensive distances over heather to forage in stands of gorse. Although the importance of gorse has been clearly demonstrated, the relationship between gorse and heather in Dartford warbler territories is less well understood. In the present study, a strong correlation was obtained between territory size and area of heather (but not with gorse), and neither of these were correlated with reproductive success. If we assume that territory size is adaptive, there are several possible interpretations. One, proposed by Verner (1977), suggests that birds defend territories much larger than they really need (superterritories) to prevent neighbours using the resource. It may also be that, in areas
214
Clive K. Catchpole, James F. Phillips
less rich in gorse, overall territory size increases in an attempt to contain more gorse. Another interpretation is that heather is more important to Dartford warblers than has been suspected, and indeed there is some evidence to support this view. Although gorse is preferred for feeding, Bibby (1979a) found that heather was the preferred nest site, even in territories where there was plenty of gorse. Furthermore, surveys of Dartford warblers in England (Bibby & Tubbs, 1975; Robins & Bibby, 1985; Westerhoff & Tubbs, 1991) recorded several territories without gorse present, but none where heather or ericaceous plants were totally absent. The present study also found that reproductive success was high, with most pairs raising two broods successfully. Bibby (1979a) had earlier established that nesting success in Dartford warblers (70-80%) was considerably higher than estimates for other Sylvia warblers (3545%). The main factor affecting reproductive success in our study was the loss of adults during the breeding season. Unfortunately, Holt Heath is traversed by two roads, and in both years our analysis revealed that adults disappeared from territories adjacent to or near the roads. There was also an inverse correlation between area of gorse and distance to the road; attractive territories with high amounts of gorse were often situated nearer to the road. Gorse often occurs where there has been soil disturbance, such as along boundary banks by roads. During the study two dead adults were picked up off the road, and Bibby (1979a) also discovered a dead adult on the road. The Dartford warbler has a slow, low, dipping flight path, and seems more likely than many birds to be hit by passing road traffic. Not all nature reserves have roads, but those that do might well be incurring higher adult mortality from this source than previously suspected. Any management or development plans involving the siting of roads and access in relation to gorse should certainly take this factor into account. The increased mortality of adults on high gorse territories near the road clearly has a masking effect, and explains the absence of an overall correlation between the amount of gorse and reproductive success. That an eventual positive correlation (confirmed by a split analysis) was obtained, even after excluding the high gorse territories near the road, suggests that there is indeed a potentially stronger underlying relationship. Although we can never know the real number of young these adults may have produced, extrapolation from the existing correlation suggests they would
have been high rather than low. The only way to test this would be to close the roads across Holt Heath, or repeat the study in a similar area with no roads. The study certainly suggests that road traffic has more impact on adult mortality and thus reproductive success than has previously been thought. Several studies have shown that the Dartford warbler habitat of lowland heath is gradually disappearing from southern England (Moore, 1962; Webb & Haskins, 1980). There is also increasing fragmentation with isolated populations of Dartford warblers becoming confined to nature reserves. The conservation and management of such areas may be vital if the species is to continue breeding in southern England. Existing techniques (Bibby, 1978; Westerhoff & Tubbs, 1991) may well involve maintaining, but also manipulating, the gorse/heather ratio and quality to achieve maxim u m effect. Whilst this study confirms the importance of gorse in maximising reproductive success, more research is still needed on the importance of territory quality to adult mortality, offspring survival and lifetime reproductive success. The most recent survey by Westerhoff & Tubbs (1991) is encouraging, as it shows that the Dartford warbler population is maintaining and even increasing its tenuous foothold in southern England. However, they also point out that shortterm increases may well be due to the recent spell of mild winters. Clearly we should not be complacent, and the fact remains that their habitat is still under tremendous pressure from a variety of sources, including increasing development. We must endeavour to manage what remains with the utmost care.
ACKOWLEDGEMENTS This study was supported by a grant from the World Wide F u n d for Nature United Kingdom. We would also like to thank the Nature Conservancy Council for permission to work at Holt Heath National Nature Reserve.
REFERENCES
Bibby, C. J. (1978). Conservation of the Dartford warbler on English lowland heaths: a review. Biol. Conserv., 13, 299-307. Bibby, C. J. (1979a). Breeding biology of the Dartford warbler Sylvia undata in England. Ibis, 121, 41-52.
Reproductive success in Dartford warbler Bibby, C. J. (1979b). Foods of the Dartford warbler Sylvia undata on southern English heathland. J. Zool., 188, 557-76. Bibby, C. J. (1979c). Mortality and movements of Dartford warblers in England. Brit. Birds, 72, 10-22. Bibby, C. J. & Tubbs, C. R. (1975). Status, habitats and conservation of the Dartford warbler in England. Brit. Birds, 68, 177-95. Moore, N. W. (1962). The heaths of Dorset and their conservation. J. Ecol., 50, 369-91. Robins, M. & Bibby, C. J. (1985). Dartford warblers in 1984 Britain. Brit. Birds, 78, 269-80.
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Siegel, S. (1956). Nonparametric Statistics. McGraw-Hill, New York. Verner, J. (1977). On the adaptive significance of territoriality. Amer. Nat., 111,769-75. Webb, N. R. & Haskins, L. E. (1980). An ecological survey of heathlands in the Poole Basin, Dorset, England in 1978. Biol. Conserv., 17, 281-96. Westerhoff, D. & Tubbs, C. R. (1991). Dartford warblers Sylvia undata, their habitat and conservation in the New Forest, Hampshire, England in 1988. Biol. Conserv., 56, 89-100.