The breeding behaviour of the pink-footed goose: Behavioural correlates of nesting success

Anim. Behav., 1977, 25, 747-764

THE

BREEDING BEHAVIOUR

OF THE

PINK-FOOTED

GOOSE

BEHAVIOURAL CORRELATES OF NESTING SUCCESS BY I . R . INGLIS*

Psychology Department, University of Bristol, Bristol BS8 lHH Abstract . This paper reports a study of the nesting behaviour of the pink-footed goose (Anser brachyrhynchus) on its Icelandic breeding grounds . Behaviour over the incubation period was examined to determine any behavioural correlates of nesting success . Most differences occurred in the females' behaviour prior to completion of the clutch . The results are discussed in relation to the need for early hatching and the probable influence of prior breeding experience . and a small stream . Figure 1 shows the major vegetation types, the nests and the marker stones used to grid the area . Twenty-two nesting pairs established themselves in the study area . In two cases a nest was used by more than one pair hence only 20 nest sites were utilized . Of the 20 nests, four were in positions where it was difficult to observe the female's behaviour when she was incubating hence the behavioural data presented in this paper are derived from 18 nesting pairs .

This paper reports a study of the nesting behaviour of the pink-footed goose (Anser brachyrhynchus) at Thjdrsarver, its major Icelandic breeding ground . This study was conducted in the spring of 1973 beginning with the arrival of the geese on the nesting grounds and terminating at the time of hatching . Its aim was to investigate possible behavioural correlates of nesting success by monitoring the changes in the behaviour of the geese over the incubation period . Therefore the results presented here do not of themselves demonstrate causal relationships . It was hoped that they would suggest future experimental manipulations that could elucidate such causal linkages .

Methods Observations were made from a hut on the crest of the moraine overlooking the study area . This area had been subdivided into 30 . 5-m squares before the arrival of geese . The corners of the squares were marked with large, yellow painted boulders which stayed on the surface as the snow melted, were always visible, and did not appear to disturb the geese . Following `gridding' the area was not disturbed ; the hut could be entered unseen by any goose in the vast majority of the study area. Observations were made using telescopes (x20 to x50) although binoculars were used to scan the area for predators . Observations were restricted to between the hours of 05 .00 and 23 .00 when there was sufficient light, although strong winds and blizzards occasionally precluded observations . A 10-h observation day was worked staggering the hours so that over a 7-day period the whole day was monitored although hours in the middle of the day contributed more data than did hours at the extreme ends of the day . No data were gathered on 7 days from the total of 53 days of the nesting period . Data were collected in two ways . Every 30 min a scan was made of the study area noting the position (relative to markers and nests) and

Study Area Thjorsarver is an oasis of vegetation in the central highlands (64° 35' N ., 18° 40' W .) surrounded on three sides by sandur desert and on the fourth by the Hofsjokull glacier . It is about 21 km long and 11 km wide, at a height of approximately 600 m . Apart from the numerous moraines, the area is flat with many ponds, streams and rivers . The vegetation can be divided into areas of swamp (mainly species of Carex and Eriophorum), mire (mainly Sphagnum teres and Drepanocladus uncinatus) and the drier heathlands (mainly Racomitrium canescens, D . uncinatus and species of Salix; see Johansson, Kristinsson & P'alsson 1974) . The heathlands form the nesting areas of the geese and the swamps and mires, the feeding grounds . The study area was in part of a traditional nesting area in the south of the oasis which was overlooked by a large gravel moraine from which observations could be made . The area was mainly flat heathland dissected by a river *Present address : Pest Infestation Control Laboratory, Ministry of Agriculture, Fisheries & Food, Tangley Place, Worplesdon, Surrey GU3 3LQ . 747

748

ANIMAL BEHAVIOUR, 25, 3

10

Fig . 1 . Map of study area . H, observation hut . N, nest . M, marker stone, the shortest distance between two markers is 30 .5 m. 1 . Gravel hill . 2. Dry, flat heathland composed mainly of Racomitrium canescens, Drepanocladus uncinatus and Empetrum hermafroditum with patches of Carex bigelowii . 3 . Relatively dry tussocks with C. bigelowii in hollows and R, canescens on tops . 4. Wet gullies containing lush areas of C . bigelowil. 5 . Clumps of Salix glauca or Salix phylicifolia . 6. Flat, low lying area waterlogged after heavy rain, but quick to dry out, containing sparse covering of C. bigelowii and Carex rariflora . 7. Area which was pond for most of the study period, subsequently covered by species of Drepanocladus and C. bigelowii. 8 . Stream banks composed of a complex mixture of liverworts . 9 . Steep banks at edge of heathland, on the slopes below the dwarf willow band were dense areas of Geranium sylvaticum and Alchemilla vulgaris . 10 . Damp gully covered solely by species of Drenpanocladus. behaviour of every bird. The mean number of scans per day over the 46 days of observation, was 22 . 0 (SE = 0 .95) . Additionally, eight of the more easily observed nests were studied in detail . Every hour six nests were each observed for 10 min ; the order of nest observation was kept constant (eight nests could be monitored as two of the initial six nests were predated very early in the season and this allowed two other nests that were established after these predations to be monitored in their place) . A nest was always observed for its allotted time even if both birds were absent. Observations of the

number of predators over the area and of agonistic encounters between geese were gathered opportunistically . A total of 930 man-hours of observation were made . Body size was used to sex members of a pair since male pinkfeet are heavier than females of similar age (Elder 1955) . Individuals were identified by their nests, which were numbered in the order in which they were established . If a bird was within 9 . 2 m of a nest then it was assigned to that nest although this seemed a conservative distance because males usually defended a larger area than this

INGLIS : NESTING BEHAVIOUR OF THE PINK-FOOTED GOOSE

Head on back

749

Head low

Fig. 2 . Diagrams of the common postures .

was incubating on nest 6, for example, she was female 6, and if no other bird was within 9 .2 m, male 6 was noted as absent . If there was a bird at, for example, 12 m from the nest then that bird was recorded as an unknown, even though it was likely (as shown by the detailed observations), to have been male 6 . Goose behaviour was subdivided into activities and postures . Activities were locomotory states, i.e . flying, swimming (both of these were very rare and could for the purpose of analysis be ignored), sitting (or incubating if sitting on a nest), standing, walking and running . Postures comprised a more diverse collection of categories all involving the posture of the head and neck . The simplest categories were head on back, head low, head up, and extreme head up (Fig . 2), possibly reflecting an increasing scale of vigilance . For example, head on back was the sleep posture with the eyes usually closed whilst extreme head up was associated with the presence of predators or intruding conspecifics . More obvious in function were feeding, preening, drinking, threatening, and nest adjusting . (The complex of behaviours which is included within each of

these `postures' will be described elsewhere ; Shephard, Inglis & Lazarus in preparation) . The scan data were analysed using SPSS (Statistical Package for the Social Sciences ; Nie et al . 1975) . No diurnal rhythms were found between the 05 .00 and 23 .00 hours, (the few observations conducted outside these hours suggested that females were invariably on their nests and that from 24 .00 until about 03 .00 hours males were sitting close to the nest) . Consequently, a day was chosen as the basic time unit of analysis. For each day the percentage of scans that contained a particular behavioural category was calculated for each goose for each of the 17 categories (including the absent category). The mean temperature, humidity, wind strength and type of precipitation for the day were also coded . The date, the number of days from the start of nest building activities, nest number, sex and whether the bird was from a pair which successfully hatched goslings or not, made up the remaining variables . Subsequently, new variables were created by transformation and amalgamation of these basic categories. The transcribed continuous records

750

ANIMAL BEHAVIOUR, 25, 3

have been used in the present paper mainly for details concerning territorial defence. Weather recordings were collected at the main base camp approximately 8 km to the north of the study area. Temperature, humidity, and maximum windspeed were recorded automatically while rainfall, cloud cover, and visibility were noted at 09 .00 and 21 .00 hours. Results Data from all geese were combined by choosing as a common starting point the day on which the female of the pair began nest building activities . All tests are two-tailed unless otherwise stated . Weather The overall mean temperature throughout the nest period was 3 C with - 12 C being the lowest and 10 C the highest recorded temperatures . The last blizzard was on 15 June . The weather was predominantly dry on 15 days (31-9%), raining on 17 days (36 . 2 %), sleeting on 9 days (19 .1 %) and snowing on the remaining 6 days (12-8%), with a mean overall humidity of 69%. The wind direction was very variable with the wind speed generally lying between 4. 6 and 10 . 7 m per s. The weather had little effect on the birds' behaviour. For each of the 18 males and females the mean daily values of the particular weather variable were correlated with the particula: behavioural scores . Windspeed was positively correlated with the incubation scores of nine of the females (Spearman rank correlations, 0. 01 < P < 0 . 05 for all birds) . Down and moss would presumably be more likely to be blown away from a vacated nest in strong winds, thereby exposing the eggs to predation and chilling, although even in an undisturbed nest chilling must presumably be greater the stronger the wind . Windspeed was also negatively correlated with head up and extreme head up postures for 10 of the males (Spearman rank correlations, 0 .001 < P < 0 .05 for all birds) . It was thought that fewer predators might be flying in strong winds and that this could account for the decrease in these possibly vigilant postures . However, there were no significant correlations between gull or skua numbers and windspeed . This negative correlation could be simply a consequence of the change in the females' behaviour with increase in windspeed for, as will be discussed below, males frequently

assume head up and extreme head up postures when females leave their nests . Breeding Biology Figure 3 shows the mean number of geese on the study area per scan for each day of the nesting period. The first nest was started on 14 May, although the peak of nest building was between 19 and 24 May. The last nest was begun on 26 May . The first family left the area on 17 June and the last family on 27 June. The study nests were not examined until all the geese had left the area ; hence no reliable data could be obtained concerning clutch size or hatching success ; however, extensive studies of the breeding biology of the geese in Thdrsarver have been made by Sigurdsson (1974) and Gardarsson (1976) . The data for the region of the oasis containing the present study area gives the mean clutch size immediately after the laying period as 4 . 57 . The mean clutch size before the beginning of the hatching period was 3.84 and the mean number of young hatched per clutch was 2 .24 . Thus the proportional hatching success was 49 % . Over the whole oasis, Sigurdsson (1974) estimated that 30 of eggs were either infertile or died at various stages of development. Of the 22 nests in the study area, 13 hatched young. Of these successful pairs, two hatched five goslings, two hatched four, six hatched three and three hatched two, giving a mean of 3 .23 young per successful pair . In two of these successful nests a single, addled, infertile egg was found, a skua was observed feeding on an egg in a third nest after the departure of the pair with their young and from a fourth nest a dead gosling was pulled by a gull shortly after that pair and their young had left. The most frequent and easily observed reason for nest failure was predation following absence by the female . Nesting Sites The nests of the pink-footed goose are usually built upon traditional nest mounds . The vegetation types of these mounds change with time as a result of the gradual accumulation of droppings, and Sigurdsson (1974) has devised a simple age classification based upon the relative dominance of various nest site plants. Grade 1 contains nests which may have been used before but are relatively young, having no special nest site flora. Grade 2 nests are older showing the beginnings of the vegetation change and in



INGLIS : NESTING BEHAVIOUR OF THE PINK-FOOTED GOOSE

751

23

t

3 I t I 1 1 I I 1 1 i i I 19 21 23 25 27 29 31 2 4 8 10 12 14 16 1 8 20 22 24 26 28 May June

i

9

11 13 15 1 7

Fig. 3. Mean number of geese on study area per 30-min scan (Bar length = SE ; number = sample size.)

particular having no loose material present. The oldest nest mounds, grade 3, are covered with a dense growth of grasses and other nest site plants . The region of Thjorsarver containing the study area had overall 56 % of grade 3 nests, 20 grade 2 nests and 24 % grade 1 nests (Sigurdsson 1974) . Of the study nests six (27 %) were of grade 3, nine (41 %) were of grade 2 and seven (32 %) were of grade 1 . The females seemed to be responsible for nest-site selection and they apparently preferred older nest sites . Upon dividing the pairs more or less equally into early (14 to 20 May) and late (21 to 26 May) arrivals, it was found that early birds had a higher `grade score' than did late birds (Median test, P = 0.024) . All the subsequently utilized nest sites were above the snow and therefore potentially available when the first geese arrived . The small sample sizes within each grade preclude any analysis of proportional nesting success between the grades. It is noteworth however that the data of Sigurdsson (1974) reveals that the regions of the oasis containing the greatest proportion of old nests had the highest proportional hatching success . Most nest sites colonized between 14 and 20 May were at the edges of the heathland near patches of dwarf willow (Salix sp .) . All but one

of the grade 3 nests (i .e . 83 %), four of the grade 2 nests (i .e. 45 %) and three of the grade 1 nests (i .e. 43 %) were of this type. Only one nest was not established on heathland. This was a grade 2 nest built on a flattened tussock . It was occupied relatively late in the season, on 25 May . There was one unoccupied nest site . It was a grade 2 site within 1 .5 m of a grade 3 site that was occupied early on, and whose male defended an area which contained the grade 2 site . The mean nearest-neighbour, inter-nest distance was 15 .4 m with a range of 7 .0 m to 27 .2 m . There was no significant difference between successful and unsuccessful nests in this measure . Predation The most important predators were the arctic Skua (Stercorarius parasiticus), the greater blackbacked gull (Larus marinus), the raven (Corvus corax) and the arctic fox (Alopex lagopus) . Other predators known to be present were the gyrfalcon (Falco rusticolus), the snowy owl (Nyctea scandiaca) and the great skua (Stercorarius skua). Figure 4 shows the relative abundance of the major predators over the study area throughout the incubation period . Skuas were the most successful predators for they were relatively inconspicuous, elicited no response from nesting geese they flew over, and



752

ANIMAL BEHAVIOUR, 25,

3

May June . Number of predators over the study period. (Closed circles, mean Fig. 4 number of greater black-backed gulls seen per hour of observation ; open circles, mean number of arctic skuas seen per hour of observation ; closed triangle, one raven seen on that day ; open star, one arctic fox seen on that day .)

several could feed peaceably from the same egg. Similar observations have been reported by Gardarsson & Sigurdsson (1972) . Six skuas were caught and colour ringed near the study area. They were regularly seen and were responsible for the three observed skua predations in or near the area . They were never seen 8 km away at base camp, however, and similarly skuas caught and ringed at base were never seen over the study area . Skuas may therefore have a relatively circumscribed area over which they hunt, possibly centred around their nests. Gulls were relatively unsuccessful predators, for the response of any goose to the presence of a gull was a vigorous and noisy attack and this, together with the habit of gulls to fight noisily amongst themselves over any eggs, tended to make their predation attempts very conspicuous . On three occasions gulls were observed on the study area scavenging upon infertile eggs and a dead gosling left in nests that had already been abandoned by pairs with young. Ravens were rarely seen and only one raven predation was observed . Unlike the skua the raven carried the eggs one by one away from the nest and ate them under cover of a clump of dwarf willow 150 m from the nest . Like the skua it elicited little response from the geese it flew over . As long as the female was incubating the eggs were safe from the avian predators . The arctic fox however is capable of displacing an incubating goose (Gardarsson personal communication) . One of the study nests was predated between 23 .00 hours 16 May and

05 .00 hours 17 May, hours when almost certainly the female would have been incubating . The nest had been partially dug up, with down and moss scattered over a wide area suggesting fox predation . Geese were certainly eaten by foxes since both geese and eggs were found in the den entrance of the one active fox earth in Thj6rs'arver . Sigurdsson (1974) reported a lowered hatching success in nests within an area of about 0. 7 km2 around the den compared with another similar nesting area (1 .54 hatched per clutch around the den compared with 2 .24) . Behaviour at the Nest Upon arrival at the nesting grounds a pair would typically walk around feeding and inspecting the potential nest sites . It appeared that the female was solely responsible for nestsite selection . The male simply followed his mate around, occasionally threatening nearby pairs . Once the female had selected a site, nest building would begin at once and the pair would not fly out of the study area at dusk as did the pairs that had not begun nest building. If a pair settled down for the night then inevitably that spot became their nest site . Most nest building took place up to day 6 although it was occasionally seen throughout the entire incubation period. The male did not assist in the building of the nest . Assuming that pinkfeet resemble other geese in that they begin to lay eggs immediately after nest-site selection at a rate of 1 .0 to 1 .5 per day, (occasionally missing a day with large clutches, e .g. Maclnnes 1962 ; Barry 1967 ; Ryder 1971), then it can be seen that nest-building activity declined

INGLIS : NESTING BEHAVIOUR OF THE PINK-FOOTED GOOSE

753

10

7-

6 12 1

12

12

5

12 13

14

4-

10 6

/0

11 14 12

15

3-

1•

1

13 13

11

13

11

2-

13 11

10

1

I1 5i~

00 2 4 6 8

10 12

14 16 Days

18 20 22 24 26 28 30 32 34

Fig . 5 . Mean percentage of day that females spent in nest maintenance, days being numbered from the onset of nest building . (Bar length = SE, number = sample size .)

around the time of clutch completion, i .e . on average day 5 . Down-plucking was rare before day 3 or 4 and afterwards occasionally occurred throughout the entire incubation period . Similarly egg rolling was an occasional event over the whole period . No differences were detected between successful and unsuccessful females in any measure of these behaviours . These behaviours were then amalgamated into one category, nest maintenance, but again no differences were found between successful and unsuccessful females in this measure . Figure 5 shows the changes in the proportion of time spent in nest maintenance. The incubation scores show a sharp rise around the time of clutch completion (Fig. 6) . All females left their nests for short periods throughout the incubation period (see below) . However, unsuccessful females had a significanty higher mean daily absent score (.f successful ? = 3 .8 %, z unsuccessful ? = 12 . 9 %, P < 0 .02 Mann-Whitney U-test) and lower incubation

score successful ?=91-3%, .9 unsuccessful ? = 82 . 8 %, P < 0 . 05 Mann-Whitney U-test) than had successful females. As in general the eggs were safe from predation as long as the female was incubating then obviously the greater the time the female was off the nest the greater was the risk of predation . By examining individual records it was possible to assess whether females abandoned their nests only after egg predation or whether a nest was abandoned prior to predation . Of the six unsuccessful nests that were predated after clutch completion and whose females were easily visible on their nests, three had incubation scores that decreased steadily prior to predation, suggesting that these females may have abandoned their nests even if predation had not occurred . The scores of the other three females did not differ from the scores of successful birds, suggesting that the former were simply `unlucky' . Table I gives examples of these types . The data from one of the unsuccessful pairs showed (x



754

ANIMAL BEHAVIOUR, 25, 3

100 .

J

90-

88 1

0

60-

1

30

' 1\

508

8

i3

40 0'

2

A

6

~

1b

1'2 1'4 . 1'6 1'8 210 2 L2 2~4 2'6 2~8 30 3~

314

Days Fig. 6 . Mean percentage of day spent incubating by successful and unsuccessful females, days being numbered from the onset of nest building . (Closed circles, successful Y ; open circles, unsuccessful Y ; bar length = SE ; number = sample size.) conclusively that desertion is not always caused by predation . The female began increasingly to leave the vicinity of her nest for longer periods . Her mate however stayed at the nest at such times . Eventually the female never returned but the male stayed near the nest until all other geese had left the area (Fig . 7) . The eggs of this nest had dead, well-formed embryos . The females incubated with their heads in the head low or head on back posture . There were no differences between successful and unsuccessful females in the scores for the daily percentage of incubation time spent in one or other of these postures or in a measure of their relative predominance over the incubation period . The entire clutch as far as could be judged, hatched within 24 hr of each other . The night after the first goslings had hatched was invariably spent on the nest and it is likely that imprinting occurs during this early period . Goslings seldom left the nest for the first 12 h after hatching . During the second day after the first hatchings the family moved around the nest, returning to the latter at intervals . The female fed continuously but the goslings were seldom seen to feed . The family would

100 80-

601

0

2

6 8

r 10 72 14 16 18 20 22 2 4 26 230334 36 38 40

Fig. 7. Mean percentage of day spent more than 9 m from nest by members of pair 10, days being numbered from the onset of nest building . (Closed circle a ; open circle?.) usually have abandoned their nest and moved into the feeding areas by the second evening after the first hatchings . Three families stayed for 3 days after the first hatchings . An infertile egg was subsequently found in one of these nests, a skua was observed eating an egg from a second of these nests after the family had finally departed, while a gull was seen pulling a dead gosling from the third nest after the family had departed. Successful and unsuccessful males were not found to differ in their behaviour at the nest for



INGLIS : NESTING

BEHAVIOUR OF

Table I . Examples of Changes in Percentage of Time Females Spent Incubating Over the Nesting Period

Days after start of nest building

Successful Nest

7

Unsuccessful Y

Nest

15

Nest

I

Nest 20

1

42

42

33

100

2

48

-

72

96

3

50

-

57

87

4

80

100

83

100

5

-

96

-

72 96

100

6

96

96

7

100

83

81

96

8

95

100

95

100

9

91

96

96

10

90

96

-

-

11

86

100

-

100

12

100

100

96

94

13

100

-

96

100

14

96

-

90

92

15

100

100

100

100

16

-

100

100

100

17

-

98

100

-

18

100

100

100

-

19 20

100 100

100 100

98

96

100

87

21

96

-

-

100

22

100

-

-

100

23

100

100

100

100

24

-

91

94

87

25

-

98

100

90

26 27

100

100

100

83

100

100

100

59

28

100

100

98

57

29

100

100

-

Predated

30

100

100

-

31

95

100

96

32

95

100

Predated

33

Goslings

Goslings

as long as comparisons could be made (i .e. up to day 30) . When the female left the nest the male would stand in the head up or extreme head up posture very close to her . Two females did not bother to cover the eggs with down and moss before leaving and their mates were observed doing this . These males were also seen to crouch over the eggs for up to 3 min during the absence of their females . Most males went with their females when the latter left the nest but two successful males and one unsuccessful

THE PINK-FOOTED GOOSE

755

male stayed at the nest . All males appeared to be `agitated' whenever the female was off the nest or even when she was adjusting her nest . In such instances the males showed components of threat and pre-flight behaviour (Shephard, Inglis & Lazarus in preparation) . Figure 8 showed changes in the males' head up and extreme head up behaviour . Before clutch completion the incidence of these behaviours is probably high as a consequence of the female's lower incubation scores during this period, i .e . the male assumed these postures when the female was off her nest . The abrupt changes from day 30 onwards are derived solely from successful male data . The successful male at this time stood very close to the nest . It is thought that the 'pipping' of the eggs triggered these abrupt changes, since it is known that 46 to 52 h may elapse in greylag geese (Anser anser, Boyd & Ogilvie 1972) and 36 to 48 h in Canada geese (Branta canadensis, MacInnes 1962) from first 'pipping' to hatching. Further work on the behaviour of geese on the feeding grounds (Lazarus & Inglis in preparation) has shown that of these two postures (i .e. head up and extreme head up) extreme head up is the vigilant posture associated with parental care . Feeding Behaviour Snow cover was extensive at the beginning of the nesting period, only the higher windswept heathlands being relatively free of snow . At this time the geese fed mainly on rootstocks, e .g. Polygonum viviparum (Gardarsson & Sigurdsson 1972) . Upon first arrival, females were frequently observed to eat something from the bottoms of unoccupied nest mounds . Possibly they were eating fragments of egg shell as a means of obtaining extra calcium (Gardarsson 1976). From about the beginning of June the geese fed upon the buds and leaves of the various species of dwarf willow (e .g. Salix glauca) which were common around the nest sites . Finally as the snow left the lower lying hollows, sedges (Carex sp .) grew and the geese fed increasingly in such areas . Females spent a much greater proportion of their day feeding around (i .e . within 9 .2 m) the nest early in the nesting period than they did later on, (Fig . 9). During the period prior to clutch completion successful females spent more time feeding around the nest than did unsuccessful females (z successful 9 = 7 .7 %, z unsuccessful ? = 1 . 6 %, P < 0 .05 Mann-Whitney U-test) . Continuous monitoring revealed that when



756

ANIMAL BEHAVIOUR, 25, 3 6

35-

3012 10

10 13

14 11

1

i

15-

11

10-

50

I 10 12 14 16 18 20 22 24 26 28 30 32 34 Days Fig. 8 . Mean percentage of day that males spent in head up and extreme head up postures, days being numbered from the onset of nest building . (Circles, head up ; squares, extreme head up ; bar length = se ; number = sample size .)

0

4

6 8

females were absent (i .e . > 9 .2 m from the nest) they spent the whole time feeding . Successful and unsuccessful females did not differ in their absent scores within the pre-clutch completion period . Figure 10 shows total feeding activity (i .e . feed + absent scores) for all females over the incubation period. During the pre-clutch incubation period successful females spent a greater proportion of their total feeding activity around the nest (i.e. < 9 . 2 m) than did unsuccessful females (5 successful Y = 23 . 5 %, z unsuccessful Y = 10 . 5 %, P < 0 . 05, Mann-Whitney U-test) . Early nesting females fed more than did late nesting females (z early Y = 29 . 5 %, z late = 12 . 7 %, P < 0 Mann-Whitney U-test) . Females could obtain water without leaving the nest. In snow and rain the bird would wipe the flat of her lower mandible along her flanks and drink the water thus gathered . No differences were found between successful and

unsuccessful females in the incidence of this behaviour . Presumably the breakdown of fat deposits also supplies much of the water needed by the bird . Over the period in which comparisons could be made between the feeding activity of successful and unsuccessful males (i .e . up to day 30) no significant differences were found . It can be seen (Fig . 11) that the daily percentage of time spent feeding around the nest (i .e . within 9 . 2 m) by the males increased steadily up to about day 20 and then declined . From continuous monitoring it was found that this decline was caused by the males feeding increasingly in the Carex-rich hollows that were becoming abundant farther from their nests in the previously snow-covered areas . In order to obtain an estimate of total feeding time the male absent scores were added to their feeding scores (Fig . 12) . The daily total feeding time percentage increases up to day 30. The subsequent sharp



757

INGLIS : NESTING BEHAVIOUR OF THE PINK-FOOTED GOOSE

2

4

6

8

10 12 14 16 18 20 22 24 26 28 30 32 34 Days

Fig. 9. Mean percentage of day spent feeding within 9 m of nest by successful and unsuccessful females, days being numbered from the onset of nest building. (Closed circles, successful ~ ; open circles, unsuccessful 9 ; bar length = SE ; sample sizes are as in Fig . 6.)

around the nest during this period was high . This percentage did not differ between successful and unsuccessful males . Agonistic Activity Figure 13 shows the time spent by males in agonistic activity over the study period as derived from the scan data (details of the types 0 2 4 6 6 10 12 1

16 18 20 22 24 26 28 30 32 34 Dry,

Fig . 10. Mean percentage of day that females spent feeding, days being numbered from the onset of nest building. (Bar length = SE ; sample sizes are as in Fig . 5 .)

decline results solely from the data of successful males and is associated with the switching to head up and extreme head up postures triggered by the pipping of the eggs, as discussed above . The early peak seen in Fig . 12 during the days prior to clutch completion does not accurately reflect total feeding activity since continuous monitoring revealed that throughout this period males away from the nest were almost invariably with their mates and fed very little, spending the majority of the time in the head up and extreme head up postures . Thus the percentage of their total feeding time spent

28^ 24-

4

6

i0

1< 1.4 15 I4 L"0 12 24 7b .^8 ;i0 D,,n

3~

;4

Fig. 11 . Mean percentage of day that males spent feeding within 9 m of nest, days being numbered from onset of nest building . (Bar length = SE ; samples sizes are as in Fig . 8 .)



ANIMAL BEHAVIOUR, 25, 3

758

Fig. 12. Mean percentage of day that males spent feeding, days being numbered from the onset of nest building . (Bar length = SE ; sample sizes are as in Fig . 8 .)

of displays used will be presented elsewhere, Shephard, Inglis & Lazarus in preparation) . It can be seen, with reference to Fig . 3 that the daily proportion of time spent in agonistic encounters was greatest throughout the period of nest establishment . Agonistic activity was not a simple function of goose numbers however, for the latter was relatively constant from 26 May until 14 June, although the number of encounters fell . Indeed on the majority of days

after the nest establishment period no threats of any kind were recorded in the scan data . (The peak on 23 June occurred as a number of pairs with young moved through the study area on their way to the feeding grounds, and it was the males of these pairs that initiated the agonistic encounters .) Figure 14 expresses (scan data) the agonistic activity in relation to the days from the start of nest building . Agonistic activity was most prevalent when the female was most frequently off her nest . Successful males did not differ from unsuccessful males in the pattern of their agonistic activity over days, but did have overall the higher mean daily percentage score (x successful d = 0-244%, x unsuccessful d = 0. 057 % P < 0 .05, Mann-Whitney U-test) . Prior to completion of the clutch females were, very rarely, seen assisting their mates in defending the nest . It is interesting that only successful females were observed defending their nests (P = 0 .068, Fisher Exact Probability Test) . Plate V, Fig. 15 shows the territories of the eight ganders whose behaviour was monitored in detail (territory is here taken to mean a defended area) . The lines demarcating the territories are those joining the furthest positions from the nest of intruding geese when threatened by the

1 2 .8

2.4

1.2 6 9

0.8

0. 4

I6

16

~,15

14

16

18

20 22 24 26 28 May

30

3 5 7 9 11 13 June

F

15

17

19

21

Fig. 13 . Mean percentage of day that males spent in agonistic activity over study period. (Bar length = SE ; number = sample size .)

23 25 27

INGLIS : NESTING BEHAVIOUR OF THE PINK-FOOTED GOOSE

PLATE V

Fig . 15 . Map of territories . (Closed circles are nests ; dotted lines join the furthest positions from the nest at which birds were threatened by the male of the nest enclosed by the lines ; darkest shading is the extent of the exposed land on May 14 ; lighter shading is the extent of additional land exposed by May 28 ; H is the observation hut .)

Inglis, Anim. Behav, 25, 3



INGLIS : NESTING BEHAVIOUR OF THE PINK-FOOTED GOOSE 151 I

I

Fig. agonistic Mean percentage of day that males spent in agomstic activity, days being numbered from the onset of nest building . (Bar length = sE ; sample sizes are as in Fig, 8 .)

resident male . Figure 15 also shows the areas of land exposed from the snow by 14 May and by 28 May (i .e. over the period of maximum agonistic activity) . It can be seen that these boundaries can to a large degree explain the asymmetric shapes of various territories . Geese were seldom seen walking for any length of time in snow and this probably accounts for the boundary affect. Another possibility is that snow-covered areas were in some degree neutral ground . The data on this point is too sparse to allow any firm conclusions but only once was a resident male seen attacking intruders that were walking on snow. Indeed, on three occasions, intruding pairs walking on snow passed unmolested within 9 in of nests whose resident males had been seen on the same day to chase away pairs who were over 15 m from the nest, but who were grazing on the exposed land . As already discussed, the period of maximum agonistic activity was relatively short and the territories were therefore transient . Even within the period of maximum agonistic activity, however, intruders found within the territory were not invariably threatened . Resident males seemed to be more tolerant of solitary birds than of pairs . As there were more pairs of geese looking for nest sites on the study area throughout the period of maximum agonistic activity than single birds, it is not surprising that 69% of the threats by the eight ganders were against pairs (Table 11) . However, the mean distance at which pairs were threatened was greater than that at which single birds were threatened (Table 11, P < 0. 05 Wilcoxon test). Most

759

solitary intruders were the males from neighbouring nests . Males could apparently recognize neighbours and tolerated them more than strangers. For example, consider the extreme case of male 8 (Table II) who was the exception to the rule that single birds were tolerated more than pairs . Of the six threats observed by him against single birds, two were against the neighbouring male . However, on one of these occasions the neighbour immediately returned to within 10 m of male 8's nest and was allowed to feed there without being threatened . Further, on six other occasions the neighbouring male was seen feeding at a mean distance of I I in from male 8's nest without being threatened in any way. Tolerance to neighbours was also indicated by the form of threat used against them . The resident would only very rarely move towards the neighbour but rather stood in the threat posture . The neighbour usually made little response but continued to feed and then slowly turned back toward his own nest . The few threats that occurred outside the period of maximum agonistic activity were generally of this mild form. As we have seen, outside this period males were increasingly seen feeding well away from the nest, and thus obviously territorial defence would fall simply as a consequence of this . However, no neighbour was ever observed within 9 m of a nest even in the absence of the resident male, and no harassment of incubating females by neighbouring males was observed (see Ryder 1975) . Successful and unsuccessful males did not differ in the size of their territories or in the mean distance from the nest at which intruders were threatened . Discussion In 1970 approximately 11000 breeding pairs nested in Tbj6rsfirver (Kerbes, Ogilvie & Boyd 1971) . The great importance of this area as a breeding ground is due to the fact that, more than any other part of Iceland, it is (a) free from disturbance by man, (b) can support only a few arctic foxes and (c) has both suitable nesting terrain and feeding grounds . However, the time available for breeding at this relatively high altitude is short and, further, it appears that in years of high goose numbers the feeding grounds become overgrazed, creating a food shortage towards the end of the breeding period (Gardarsson & Sigurdsson 1972). It is therefore advantageous for a pair to reach the feeding grounds with their young as early as possible . The complementary behaviour of male and



760

ANIMAL BEHAVIOUR, 25, 3 Table H. Summary of Agonistic Encounters Involving Ganders of the Detailed Monitored Nests metres when No. of threats against

Nest no .

Single bi

Single

s

Pairs

Overall mean distance in metres when threatened

5 .5

7.3

7 .0

10

14 .4

24 . 5

23 .6

3

9

13 . 8

6

7

24 . 5

1

1

5

6

7 8 9

24 . 2

2

7 .7

10

4

12

19.0

22 .6

21 .7

15

1

4

9. 2

24 .5

21 .4

16

5

5

11 .3

38 .5

24 .8

x=21 . 8

x = 18 .9

female can at least partly understood as functioning to form an efficie breeding to meet this need for early young . After discussing the basic pattern of behaviour over the incubation period, it may be seen that the behavioural differences found between successful and unsuccessful pairs are quantitative rather than qualitative and possibly result from differences in the pairs' prior breeding experience as pairs. The geese arrived on the study area having already paired and, as only one copulation was observed, probably having already copulated . Nest building, usually on a traditional nest mound, began almost immediately . Similar findings have been reported for other species of arctic nesting geese (e .g. Mickelson 1975 ; Kistchinski 1971 ; Barry 1967) . It is very likely that the pinkfoot resembles larger geese and lays on average one egg per day, skipping a day with other clutches (e .g. Maclnnes 1962 ; Ryder 1971 ; Barry 1956) . This being the case it is advantageous if the development of the earlierlaid eggs can be halted so that the entire clutch hatches together. The young goslings were not observed to feed to any large extent on the nesting area and it has been found that they require easily digestible and nutrient-rich food which is present in limited quantities in Thjdrsarver and then not on the nesting grounds (Gardarsson 1976) . Having both unhatched ether with goslings for any length of e herefore be distinctly disadvan-

s, although the goslings require some est in order to move food reserves o liver (Gardarsson personal communication) . If incubation is prolonged the goslings will grow weaker, yet if the family moves into the feeding areas the eggs will be predated. This conflict of interests was apparent in the delayed departure from their nests of the three pairs that were in just such a situation . Ryder (1972) has observed in Rose's goose (Anser rosii) that adults deserted unhatched f goslings . However, if no eggs e can incubate at least 50 atc e longer than normal (e .g. Skutch 1962 ; Brakhage 1965). Maintaining eggs for moderate periods below the range 25 to 27 C is compatible with later development (see White & Kinney 1974) . The females allowed the temperature of the earlier laid eggs to approach equilibrium with the environment by (a) delaying intensive incubation until clutch completion and (b) by building up a down layer to the nest only after the laying of the third or fourth egg . The development of the earlier eggs was therefore halted . The female was solely responsible for incubation, and it was clearly advantageous for her to remain on the eggs for as long as possible in order to prevent chilling and predation . When females left their eggs it was to feed, suggesting that the level of food reserves was a crucial factor in determining their ability to incubate . Although the geese spent some time feeding in e

INGLIS : NESTING BEHAVIOUR OF THE PINK-FOOTED GOOSE

the lower highland regions of Iceland before reaching Thj6rsArver, any extra feeding that can be done on the nesting grounds is probably of advantage . The best time for any additional feeding is prior to clutch completion since the female does not have to incubate intensively throughout this period, the lack of down in the nest makes it less conspicuous and geese can apparently replace the odd predated egg if predation occurs prior to clutch completion (e .g . Maclnnes 1962 ; Weigland, Pollock & Petrides 1968) although this would delay the onset of incubation and hence of hatching. All females vacated their nests for short periods to a greater or lesser extent after intensive incubation had begun . Of the two dangers to the eggs at such times, predation pressure would appear to be relatively constant over the incubation period (Fig. 4) . The risk of chilling should decline over the same period, not only because of the increase in air temperature but also because the developing embryo becomes less dependent upon extraneous heat sources (Drent 1970) . The covering behaviour of the geese when leaving the nest could clearly help to prevent chilling, predation, and possibly dehydration . The territorial behaviour of the male would appear to function to safeguard an area around the nest in which his mate can feed, particularly before her clutch has been completed . If the female feeds within her male's territory she is free from any harassment or competition from other geese . If however she had to travel far from the nest, not only would she use up more energy but also it is likely that she would intrude into other pairs' territories and be chased off. If the male accompanies his female he can to some extent deter vigorous attacks by other geese upon her, but their nest now becomes vulnerable to predation . It would therefore seem to be advantageous for the male to secure a food reserve around the nest for the female . The data already described is congruent with this functional explanation of the territory . Thus, the peak of agonistic activity is during the nest extablishment phase and falls off sharply once the area has been settled with nests and the females have begun intensive incubation. Males were more aggressive towards pairs than towards single birds because the former were likely to be nest prospecting, and the establishment of a new nest within the territory would reduce the food available for his mate . The territorial phase corresponded to the peak of the females'

761

feeding activity while the males' feeding increased after this phase when they spent increasing amounts of time feeding outside the old territory boundaries in the newly exposed patches of Carex. Ryder (1975) has argued that Arctic goose territories are formed as a result of a gander's defence of his incubating female from attacks by neighbouring males . As has been argued elsewhere (Inglis 1976) this hypothesis seems inappropriate for the pinkfoot . It should be stressed however that the function of territory proposed here is not intended to apply to other goose species . After the territorial phase the males' activity is predominantly concerned with feeding . (A male loses approximately 200 g over the territorial phase, Gardarsson 1976 .) However, a core area persisted around the nest into which other geese were never seen to venture, whether or not the resident male was present . Similar observations have been made for other goose species (e.g . Mickelson 1975 ; Balham 1954) . After hatching, the female has to regain her lost weight (approximately 800 g, Gardarsson 1976) and has little time available for vigilance . The male therefore bears the major responsibility for the protection of the young (Lazarus & Inglis in preparation) and it is obviously advantageous if his food reserves can be as high as possible at hatching ; hence the intensive feeding shown throughout the latter half of the incubation period . Early nesters had a lower clutch failure rate (25%) than had late nesters (50%) (X 2 = 12 .5, df = 1, P < 0 .001) . Newton & Kerbes (1975) found a similar effect in greylag (A . anser) . The disadvantages of early nesting as compared with late are first that the weather is colder and therefore more energy is needed simply to maintain body temperature, secondly that snow cover is slightly more extensive, making foraging more difficulty and thirdly, that the nest is more likely to be singled out by predators when fewer nests are available . The advantages of early nesting, however, probably outweigh the disadvantages. First, as already stated food resources on the feeding grounds seem to be limited and thus the earlier a family can reach the feeding areas the better are the chances that their goslings can obtain sufficient food for rapid growth . Secondly, the early pairs can select the older nest sites which are well drained and surrounded to some extent by dwarf willow . The willow is an important factor for it provides a valuable source of food within the territory,

762

ANIMAL BEHAVIOUR, 25, 3

and also shelter for the incubating female from wind . Although the small sample sizes of the different nest types within the present study preclude any analysis of proportional nesting success between these types, as already mentioned the data of Sigurdsson (1974) reveals that the areas containing the greatest proportion of old nests had the highest hatching success . Ryder (1972) in a study of Ross's goose (Anser rosii) and Munro (1960) investigating Canada geese (B. canadensis) found that the nearest neighbour inter-nest distance was less for successful nests than for unsuccessful nests . No such differences were found in the present study . Inter-nest distance in the present context would appear to be too gross a measure of territory quality for it does not take into account the nature of the terrain ; for example, nests could be very close providing they were visually isolated from each other. Successful males spent more time defending their territories than did unsuccessful males, but no significant differences were found in territory size or the mean distance from the nest at which geese were threatened . Territory size per se may not be important to the female, rather the food reserves contained within it are the crucial factor . The importance of the pinkfoot territory as a source of food for the female is again suggested by the finding that successful females spent a greater proportion of their feeding time near the nest than did unsuccessful females . Most of the differences between the behaviour of successful and unsuccessful geese were found in the period prior to clutch completion . The exception is that after clutch completion the mean daily incubation scores for unsuccessful females were lower than for successful ones . Individual variation in the levels of food reserves could partly account for this finding . Examination of individual records however, suggests that another factor is involved . The temporal pattern of feeding periods of some unsuccessful females did not differ markedly from that of successful females . In these cases the length of the feeding periods were all of the same order and spaced over days, as would be expected if hunger was a major causal factor . The pattern of other unsuccessful females (e.g. female 10, Fig . 7) and and female 20, Table I) was different in that a feeding period seemed to increase the tendency to vacate the nest again, and that the duration of each successive period grew greater, suggesting that although hunger could have been the main factor underlying the initial periods it was not

the sole factor underlying subsequent breaks in incubation. Prior breeding experience may be the crucial factor governing the strength of nestsite attachment (e.g . Geis 1956). As a solitary healthy female is unlikely to be predated, and as she can graze without interference in the newly exposed feeding areas, guarding the nest during the feeding excursions of his mate would seem to be the only useful function a male can perform to increase hatching success once the female has begun intensive incubation. However, all but three males went with their mates . Why does the male not desert his mate entirely once she has begun intensive incubation? As Dawkins & Carlisle (1976) have suggested, it is somewhat fallacious to argue that the prior investment of the male (i .e . in this case the time and energy spent in courtship and territorial defence) `binds' him to the female . Rather, would any alternative strategy (i .e. in this case desertion) be more profitable in terms of the male's genetic contribution to the next generation? Desertion would not seem to be a profitable alternative . The male most probably would not subsequently mate with other females, since the latter will have copulated before reaching Thjbrs'arver and most will be already laying and incubating . Further, although the deserted female might be capable of hatching the clutch it is doubtful whether in her exhausted state she would be capable of protecting the young on the feeding grounds . Staying with the same mate is also likely to lead to a long-term increase in breeding efficiency It has been found in kittiwakes (Rissa tridactyla ; Coulson 1966) and arctic skuas (Stercorarius parasiticus ; Davis 1976) that long-established pairs have a greater breeding success than have younger and inexperienced pairs . In geese, young and therefore inexperienced Canada geese (B. canadensis) desert the nest more readily (Geis 1956), lay smaller clutches, and hatch a smaller proportion of the eggs (e .g. Brakhage 1965) than do older birds. Further, young captive Hawaiian geese (Branta sandvicensis) have a higher rate of infertility than do `middle aged' birds (Kear personal communication) . In a longlived monogamous species, however, it would be advantageous if following unsuccessful attempts at breeding, members of the pair could `divorce' and re-mate with other individuals . `Divorce' has been found in kittiwakes (R . tridactyla) to be less likely the older the birds and more likely following unsuccessul breeding (Coulson 1972) . The data from pair 10 (Fig . 7)



INGLIS : NESTING BEHAVIOUR OF THE PINK-FOOTED GOOSE

suggests one way in whch `divorce' could occur in geese. If the strength of attachment to undamaged eggs (pinkfeet showed no hesitation in abandoning predated eggs) differs greatly between the members of a pair, then that pair is unlikely to breed successfully and is likely to split up . Obviously it would be very valuable to conduct long-term behavioural studies on a marked wild goose population where the age and breeding experience of the birds was known Acknowledgments I wish to thank the Wildfowlers' Association of Great Britain and Ireland, and the Museum of Natural History, Reykjavik, for financing the expedition . I owe a tremendous debt to the other members of the expedition . Barry Shephard shared the observations with me and subsequently assisted in decoding some of the data . Erling Olafsson supplied the botanical and meteorological data as well as assisting in a multitude of other ways . Without them this study would not have been possible . I also wish to thank Professor Matthews and the staff of the Wildfowl Trust, and Professor Gardarsson for much helpful advice and for all the work they put into organizing the expedition . Finally, I am grateful to the following for constructive criticisms of earler drafts of this paper, A . Gardarsson, J . Goss-Custard, J. Lazarus, J . Kear and M . Owen, and to J . Lazarus for assistance with the running of the SPSS programme and R . Torrance for drawing Fig. 2 . REFERENCES Balham, R . W. 1954 . The behaviour of Canada geese in Manitoba. Unpub . Ph.D . thesis, University of Missouri. Barry, T . W. 1956. Observations of a nesting colony of American Brant . Auk, 73,193-202. Barry, T . W. 1967 . The geese of the Anderson River Delta, N .W.T. Unpub . Ph .D. thesis, University of Alberta, Edmonton . Boyd, H. & Ogilvie, M . A . 1972 . Breeding biology of feral Greylag geese in South West Scotland. Wildfowl, 23, 83-88. Brakbage, G . K . 1965 . Biology and behaviour of tubnesting Canada Geese. J. Wildl. Mgmt ., 29, 751-771 . Coulson, J . C . 1966 . The influence of the pair-bond on the breeding biology of the Kittiwake gull, Rissa tridactyla. J. Anim . Ecol., 35, 269-279. Coulson, J . C. 1972 . The significance of the pair-bond in the Kittiwake . Proc . XV Int. Ornithol. Congr., 424-433 . Davis, J . W. F. 1976 . Breeding successes and experience in the Arctic skua (Stercorarius parasiticus) . J. Anim. Ecol., 45, 531-537 .

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Dawkins, R. & Carlisle, T. R . 1976. Parental investment . mate desertion and a fallacy . Nature, Lond., 262, 131-133 . Drent, R. 1970 . Functional aspects of incubation in the Herring Gull . Behaviour Suppl., 17, 1-132. Elder, W. H. 1955 . The relation of age and sex to the weights of Pink-footed and Greylag geese . Wildf. Trust Ann . Rep., 6, 127-132 . Gardarsson, A . 1976 . Stofnstaerd og framleiosla heioagaesar (Anser brachyrhynchus) i pj6rsarverum 1971-1974 . Nattdrufraedistofnun Islands, Reykjavik. Gardarsson, A . & Sigurdsson, J . B . 1972 . Skyrsla um ranns6knir A heidagaes i pj6rsdrverum sumarid 1971 . Nattdrufraedistofnun Islands, Reykjavik . Geis, M . B. 1956 . Productivity of Canada geese in the Flathead Valley, Montana . J. Wild!. Mgmt ., 20, 409, 419 . Inglis, I. R . 1976 . Agonistic behaviour of breeding pinkfooted geese with reference to Ryder's hypothesis . Wildfowl, 27, 95-99. Johannsson, B., Kristinsson, H . & Palsson, J . 1974 . Skyrsla um grasafraediranns6knir i pj6rsdverum 1972 . Nattdrufraedistofnun Islands, Reykvajik . Kerbes, R . H ., Ogilvie, M . A . & Boyd, H . 1971 . Pinkfooted goose of Iceland and Greenland : a population review based on an aerial survey of Thj6rsArver in June 1970 . Wildfowl, 22, 5-17 . Kistchinski, A. A . 1971 . Biological notes on the Emperor goose in northeast Siberia . Wildfowl, 22, 29-34 . Lazarus, J . & Inglis, I . R . (in preparation) . The breeding behaviour of the pink-footed goose . Parental care and vigilant behaviour during the fledgling period. Maclnnes, C . D . 1962 . Nesting of small Canada geese near Eskimo Point, North West Territories . J. Wild!. Mgmt., 26, 247-256. Mickelson, P. G . 1975 . Breeding Biology of Cackling geese and associated species in the YukonKuskokwin Delta, Alaska . J. Wildl. Mgmt., 45, 1-35 . Munro, D . A. 1960 . Factors affecting reproduction of the Canada Goose, (Branta canadensis), 1958 . Proc . Int . Ornithol . Congr ., 12, 542-556. Newton, I . & Kerbes, R . H. 1975 . Breeding of Greylag geese (Anser anser) on the Outer Hebrides, Scotland . J. Anim . Ecol., 43, 771-785. Nie, N. H ., Hull, C . H ., Jenkins, J . G ., Steinbrenner, K . & Bent, D . H . 1975 . Statistical Package for the Social Sciences . 2nd edn . New York : McGrawHill . Ryder, J. P. 1971 . Distribution and breeding biology of the Lesser Snow Goose in Central Arctic Canada . Wildfowl., 22, 18-28 . Ryder, J . P. 1972 . Biology of nesting Ross's geese . Ardea, 60, 185-213 . Ryder, J . P. 1975 . The signifiance of territory size in colonial nesting geese : an hypothesis . Wildfowl, 26, 114-116 . Shephard, B . N ., Inglis, I . R . & Lazarus, J . (in preparation) . An ethogram of the pink-footed goose (Anser brachyrhynchus) . Sigurdsson, J . B . 1974. Ranns6kir a rarphattum og afkomu heidagaesar. Nattdrufraedistofnun Islands . Reykjavik.



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ANIMAL BEHAVIOUR, 25,

Skutch, A. F. 1962 . The constancy of incubation. Wilson Bull., 74, 115-152 . Weigland, J . P., Pollock, M . J. & Petrides, G . A. 1968. Some aspects of reproduction of captive Canada geese . J. Wildl . Mgmt., 37, 894-905 .

3

White, F. N. & Kinney, J . L. 1974. Avian incubation Science, N.Y, 186, 107-115 . (Received 1 August 1976 ; revised 8 November 1976 ; MS. number: 1566)