Aggression and dominance in winter flocks of shelduck Tadorna tadorna (L.)

Anim . Behav., 1977, 25, 44 7-459

AGGRESSION AND DOMINANCE IN WINTER FLOCKS OF SHELDUCK TADORNA TADORNA (L.) BY I . J. PATTERSON

Culterty Field Station, University of Aberdeen

Abstract. The frequency of aggression increased in late winter, without change in flock density . Dominance was shown in both captive and wild males and the wild population could be divided into highranking and low-ranking groups . Rank position within the former group was not related to subsequent performance, but high-ranking males were older, arrived earlier from migration, and hatched more broods than low-ranking ones . The latter were seen less frequently at a feeding station and may have been excluded from it by the high-ranking birds . The shelduck shows striking seasonal changes in social behaviour, gathering into huge flocks during the summer moult migration (Goethe 1961), forming smaller flocks on wintering grounds and on first return to breeding areas (Hori 1964), becoming territorial while nesting (Hori 1964 ; Young 1970 ; Williams 1973) and defending an area around the ducklings within a brood range (Hori 1964 ; Williams 1973) . Attack and threat are common in most seasons and Jenkins, Murray & Hall (1975) have suggested, mainly from counts of birds and measurements of rate of interaction, that aggressive behaviour in winter was important in limiting the size of the subsequent breeding population. Their hypotheses are : that areas of good feeding are limited, that there is a dominance hierarchy in the winter flock and that subordinate birds are excluded from the best feeding areas and eventually from the breeding population . The present study tests whether consistent dominance exists in a winter flock of wild colour-marked shelducks, whether such behaviour could exclude subordinate individuals from feeding areas, and examines its effects on survival and breeding . The study area was the Ythan estuary (Fig . 1) 20 km north of Aberdeen . The number of shelducks on the estuary fluctuated seasonally, with none present in autumn and peak numbers during the period April to June (Patterson, Young & Tompa 1974) . During each winter the number rose steadily as shelducks arrived from migration (Fig. 2) . Their dispersion has been described by Williams (1973) ; in 1970 and 1971 most of the birds flocked on the wide mudflat in the middle area of the estuary (Fig . 1(A)) with small numbers on a few other areas especially around a stream mouth near the sea (Fig . 1(B)) . In 1972 the flock concentrated higher on the estuary (Fig. 1(C)), and

this change continued in subsequent winters . The flocking phase ended during April as pairs dispersed to territories all over the estuary (Williams 1973) . Aggressive Interaction The simplest attack was a direct lunge with the head, resulting in a peck if the victim was close enough. Frequently the attacker erected the back feathers and lowered the head and neck in the head-down posture ; the bird might then remain stationary or walk, run or fly towards its opponent which would usually give way . Males, especially if their mates showed inciting behaviour, commonly preceded an attack by the head-pumping display (Boase 1935) . Interactions between two pairs were often prolonged and complex . All types of interaction could easily be recognized in a group of birds which were otherwise feeding or resting . The frequency of interaction was measured by selecting a group of 10 birds (usually five pairs) in the flock and counting the number of attacks between these birds in 5 min . The flock was watched from a car or hide at 100-150 m to avoid disturbance, and observations were discarded if the birds became alarmed during the 5-min period. Spacing between birds in the flock was measured at the start of each observation by estimating the distance to the nearest neighbour for each of the 10 birds in the group being watched and calculating the mean distance for the group . In addition I attempted to estimate the previous distance between birds which interacted, by continuously watching the positions of the males particularly and by noticing impending attacks in their early stages as birds converged, watched each other, or displayed slightly . 447



448

ANIMAL BEHAVIOUR, 25, 2

f

Fig . 1 . The Ythan estuary, Aberdeenshire, showing the areas used by the winter flock of shelducks (A, B and C) and the position of the observation hide (H) .



449

PATTERSON : AGGRESSION AND DOMINANCE IN SHELDUCK

Results In counts of attacks made over the whole winter (December to March), a large majority of the interactions were between males and they made 76. 7 per cent of the attacks on all categories (Table I) . Females tended to attack other females, and pairs to interact with other pairs . The small number of females which attacked males or pairs almost all had their mate displaying nearby at the same time . The frequency of interaction was very low until mid-February and then increased until the end of the flocking period in late March (Fig . 3), in parallel with the increase in total numbers on the estuary (Fig . 2) . The proportion of attacks preceded by running and flying also increased later in the winter (Table II) . There seemed to be a similar increase in the amount of biting and physical combat but this was difficult to measure . There was no change in mean distance between neighbours over the winter and birds which interacted did not differ in spacing from the mean spacing (Fig. 4). Both measures showed a very slight increase in spacing with season . Discussion These results suggest that the increase in frequency of attacks in March was not caused by increasing density within the flock as birds arrived from migration . Conversely they also

150 -

a

100 -

0

BIRDS

Ba__a

Bo

suggest that the attacks did not cause the birds to space further apart . The increase in attack frequency was unlikely to be caused by a decrease in food supply ; Buxton (1975) measured the density and biomass of the three commonest food items in the Ythan flock areas in 1971 and 1972 and showed that food abundance did not change over the winter . Low temperatures and especially freezing of the mud may make this food less available (Goss-Custard 1969), and there have been reports of large numbers of shelducks being found dead in emaciated condition in cold weather (Dobinson & Richards 1964 ; Harrison & Hudson 1964) . However, attack frequency on the Ythan was lowest in the coldest part of the winter and tended to increase (though not significantly) with increasing temperature . A much more likely explanation of the high frequency of attack in March, is that there was a progressive increase in aggressiveness from a low winter level to the much higher level which was shown on the territories from April onward (Williams 1973) . In late March, pairs began to leave the flock to stay permanently on territory, and several males made unusually large numbers of attacks in the last few days before leaving . An additional possibility is that most interaction might be between birds not familiar with each other and that the degree of mutual familiarity might decrease as the flock increases in size. Dominance Methods Preliminary observations were made on a group of hand-reared shelducks kept in a 10-m 2 enclosure with a small pool . All were marked with coloured plastic rings and interactions

~O 0

/ a te *-,o 1" 0

Table L Distribution of Aggressive Interactions Among the Sexes, Winter 1970. Data Winter 1971 were Very

50 0 0 e

off_ I DEC

Victim

JAN

Observation period t MAR FES 1970

Fig. 2. The total number of shelducks on the Ythan estuary in early 1970 . Data for 1971 were very similar . The vertical lines delimit the period when dominance observations were made .

Attacker

Male

Male Female Pair Total

Female

Pair

Total

548

55

24

627

34

102

6

142

4

0

45

49

586

157

74

818





4 50

ANIMAL BEHAVIOUR, 25, 2 120 day mean _L

11109INTERACTIONS PER 10 BIRDS

•

8-

•

0

1

7-

PER 5 MINS 6-

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•

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I

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10

15

20

25

30

5

10

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0

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S 15

20

w

I

I

I

I

i

I

25

5

10

15

20

25

1 30

MARCH

FEBRUARY 1972

Fig . 3 . Seasonal change in the frequency of aggressive interaction in the winter flock . The filled points represent single, 5-min counts of interactions and the vertical bars show one standard error above and below the mean (open points).

between them were recorded for 30 min after the daily renewal of their food. Observations on the wild flock were made from a hide (Fig . 1(H)) using grain as bait to attract the birds close enough to identify colourringed birds . The bait, which was supplied almost every day throughout the winter, also increased the density of the flock and the frequency of interaction so that data could be collected much more quickly than from the dispersed flock . Interactions seen between the same birds away from the bait were consistent with those seen at it . All interactions occurring in an observation period of several hours were recorded, including those between an unmarked and a marked bird as well as those between two marked birds . In the wild flock, only the males interacted sufficiently often to give enough data, and all the subsequent field data exclude females. Results (a) Dominance . Any shelduck initiating an attack was very likely to win the interaction .

In 137 interactions between the captive shelduck, only twice (1 . 5 per cent) did the victim retaliate in the remainder it fled immediately . Similarly, in the wild flock in 1972, retaliation occurred in only three (1 . 3 per cent) of 228 interactions between males . These results suggest that shelducks attacked only those subordinate to themselves . Any alternative explanations can be excluded by examining the consistency of results of interactions between pairs of wild marked males which had at least six interactions between them . In 46 (83 . 6 per cent) of 55 such pairs of males, one bird won all of the interactions. Since each pair had at least six encounters, all these results Table IL Locomotion during Aggressive Interactions, in Relation to Season (1972) Stand/walk Run/fly January-February

29

37

(56-1%)

March

23

140

(85 .9%)

X 2 = 22 . 15, P < 0 . 001 .



451

PATTERSON : AGGRESSION AND DOMINANCE IN SHELDUCK

differ significantly from a hypothesis of equality between the two birds (Binomial Test) . In the remaining nine pairs of males (Table III), the overall loser won only a single interaction in seven cases and the result differed significantly from equality in five cases. There was clearly a high degree of consistency in the results of interactions between any two marked birds and, since a large proportion of relationships were entirely one-way, dominance between male shelducks could be predicted from a single encounter with a fairly high probability . This made it possible to describe the dominance hierarchy in a wild population where it was difficult to record large numbers of encounters between every possible pair of individuals . (b) Hierarchy in captives . A group of four captive adult male shelduck, kept with an adult female and four juveniles, showed a straightline hierarchy (Table IV) . A group of nine, hand-reared juvenile males, kept with three juvenile females, similarly showed a straightline order . This group was observed on three occasions to check for changes in the hierarchy . On February 23 (Table V), all but one of the

males could be ranked . The second-ranking bird appeared to have a pair bond with one of the females and the two stayed together consistently . By March 9 the bird (E), previously fifth in rank, had paired with another female and had moved into top place . The other males remained in the same relative order except that the previously unranked bird (I) was now in the position vacated by E. By March 26, bird I was associating with the third female and had moved up into third position, so that the three paired males were in the top positions with the original top-ranking male (A) in fourth place . The hierarchy among these captive juveniles, although clear at any one time, was thus very changeable Most of the upward changes, however, were associated with the formation of pair bonds and probably resulted from a tendency for a paired male to attack other males coming close to his mate in the confined space of the enclosure . (c) Hierarchy in the wild flock. Two different methods were used to describe the hierarchy in the wild birds . The most satisfactory was similar to that used on the captives, i .e . inspection of the results of encounters between

80

76MEAN

T 1

T

5-

SPACING

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4-1

0

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0

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20

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JANUARY

I

30

I

I

I

5

10

I

15

20

25

FEBRUARY

5

10

1 .5

20

MARCH 1972

Fig . 4. Spacing between nearest neighbours, in relation to season . The filled symbols represent the mean spacing on each day in groups of 10 shelducks and the vertical bars represent one standard error above and below the mean . The open symbols represent the mean spacing between birds prior to an interaction between them .

3

30



452

ANIMAL BEHAVIOUR, 25, 2

each pair of marked birds but in practice a bird could only be placed in order if it had been seen interacting with a reasonable number of other individuals . The second method was based on the individual interactions by each bird, including those with unmarked opponents and those with marked birds which had not themselves enough data to be included in the rank . Each bird's rank position was estimated by the percentage of its encounters which it won . For both methods, the data on interactions were sorted, stored and analysed by computer, using the ICL4/70 of the Aberdeen University Table III. Proportion of Encounters Won by the Overall Winner in Pairs of Males. Each Line Represents One Pair of Marked Males Encounters won by Overall winner

Overall loser

P*

30

1

< 0 . 001

10

1

0 .006

9

1

< 0 .01

7

1

0 .04

5

1

5

1

5

1

12

4

5

5

Computing Centre . A preliminary ranking of birds, based on the percentage of encounters won, was modified by inspection of the observed dominance relationships between marked individuals until an order was achieved which gave the smallest number of inconsistencies . In both 1970 and 1971 it was possible to arrive at a rank order with a very small number of inconsistent results (Figs 5 and 6) . Nine males in the 1970 sample had enough data in 1971 to be ranked again (Fig. 7) . There was no correlation between their relative rank positions in the 2 years (Spearman r - 0 . 250, P > 0 . 1) . The higher-ranking birds in 1970 tended to remain high in 1971 but the middle birds in 1970 tended to lose rank relative to those low in 1970. These rank changes were not correlated with changes of mate or territorial status or with breeding success in the intervening period and are difficult to explain . These results show that the aggressive behaviour seen in the winter flock was associated with a clear dominance hierarchy between the males. An attempt was made to identify factors associated with the achievement of dominance . Factors Associated with Dominance The minimum age of each male was known, since most had been ringed as adult birds some years before this study began . A minority were ringed as juveniles or yearlings and were of known age . In neither year was there a significant correlation between rank and age, but many of the

< 0 . 01

0 . 04 Table V. Changes in Dominance Among Nine Handreared Juvenile Male Shelducks (Rank order on Different Days)

*P = probability of this result occurring by chance if the two birds are assumed to be equally likely to win (Binomial Test) . Table IV . Dominance Among Four Captive Male Shelducks November 1968. Each Number Represents the Encounters Won by the Bird in That Row over the Bird in that Column Loser : Total won

26/2/70

9/3/70

26/3/70

A

E(+ aY)

E(+ a9)

B (+ b Y)

A

B (+ b4)

C

B (+ bY)

1(+ cY)

D

C

A

A

B

C

E

D

F

-

9

6

8

23

F

I

C

B

11

2

13

G

F

D

C

-

4

4

H

G

G

-

0

H

H

14

40

Winner A

D Total

lost 0

9

17

I (not ranked)



PATTERSON : AGGRESSION AND DOMINANCE IN SHELDUCK

birds may have been older than their minimum age would suggest, so that better data on age are required . The weights of the birds, which should be related to both size and condition, were obtained by trapping as many as possible over the winter . In neither year was there any correlation between rank and weight. Similarly, there was no correlation in either year between rank and date of arrival on the estuary or between rank and breeding success in the previous season . Only one possible trend was discovered ; in 1971 the more dominant birds at the baiting station tended to be those which were closest to their eventual territory sites, although the correlation was not significant . There was no such trend in 1970 and more limited data in 1972 also showed no correlation . It was thus difficult to predict dominance in male shelduck from their physical characteristics or from their previous history, as has been possible in other species . If the increased aggression in March just before uptake of territories is considered as an early stage of territorial aggression, then a male close to his territory might tend to initiate more attacks which might confer dominance in the flock. The converse interpretation, that high rank allows a male to get a territory near the baiting station is less likely . Williams (1973) LOSER 1

2 3 4

1

1 n

2

7

7 4 1

8

9

1 / 1 3

1 5 7

10

II

12

13

14

15

16

1

17

1/1 3 1 21 4 1 1 4 6/ 1 2 1

8

found that 77 per cent of pairs returned to the same site they occupied the previous year, although there was no correlation between ranks in two years . Williams also found that pairs with territories on the central part of the estuary, near the baiting site (Fig . 1 (H)) were no more successful at hatching broods than pairs elsewhere so that there is no obvious advantage in getting a territory in this area . Dominance and Subsequent Performance There was no correlation in either 1970 or 1971 between rank and the date at which birds left the flock to take up territory. In 1970 all ranked males held territories ; in 1971, of the 28 ranked birds, the six which did not hold territories had a mean rank position of 14 . 8 (ranks 6, 8, 9, 19, 23 and 24) showing no correlation between rank and territorial status . In both years some of the territories were held on freshwater pools peripheral to the estuary . In 1970 the four males concerned had a mean rank of 4 . 8 (ranks 1, 4, 6 and 8) out of 18 and in 1971 five males had a mean rank of 16 . 2 (ranks 3, 12, 17, 22 and 27) out of 28 . There was thus a non-significant tendency for these peripheral territory holders to be high-ranking in one year, but not in the other . The breeding success of individual pairs was measured by counting the number of ducklings with them on the first day that the family was seen . (Frequent interchange of young between broods made it impossible to determine how many of each pair's young reached the flying

4 1 1 1 2

LOSER RANK 1 2 3 4 5 6 7 8

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8

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20 2

1 13 1 2 1

1

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414

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2

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3

3 2 1

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453

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229 29

3 2 4

2 1970

Fig. 5 . Dominance table for interactions seen in 1970 . The birds are numbered in rank order along the rows and columns. Each figure in the table shows the number of encounters won by the bird in that row over the bird in that column . Where a bird won only a majority of the encounters, the proportion won by each is entered as a proportion in the winner's row .

1971

Fig. 6 . Dominance table for interactions seen in 1971, arranged as in Fig. 5 .

ANIMAL BEHAVIOUR, 25,

454

RANK 1970

1971

1

005 531 23

00000

019 002

A08 530 383 013 563 440 0~_ ` , 010 ~~/ / 572 4 613 /f 3 80 019 000 321 330 007 624

000

/

f

018 057 Fig. 7 . Changes in rank order between years . The figures are the serial numbers of individual marked males arranged in rank order (from Figs 5 an 6) . Birds which were ranked in both years are joined by arrows .

PATTERSON : AGGRESSION AND DOMINANCE IN SHELDUCK

stage .) There was no significant difference between high- and low-ranking birds in either the proportion which hatched broods or in the mean size of the broods (Table VI). Lowerranking birds tended to have slightly larger broods. Marked birds which disappeared were assumed to have died, since none were found in searches of adjacent populations and none were recovered dead elsewhere in a breeding season . Survival was unrelated to rank . Of 18 males ranked in 1970, five failed to reappear in subsequent years ; these had a mean rank of 11 .0 (ranks 2, 7, 14, 15 and 17) . Similarly, two of the 28 males in 1971 failed to reappear, and had a mean rank of 12 . 5 (ranks 1 and 24) . Thus a male shelduck's position in the winter flock hierarchy was not significantly related to his subsequent performance in getting a territory, in breeding, or in survival. There are several possible explanations for this lack of correlation . Perhaps the simplest is that dominance position may not be functional but may arise as a side-effect of the gradual onset of territorial aggression in late winter. The males' tendency to attack will vary between individuals, due to several factors possibly including nearness to the eventual territory site, and this variation could produce a rank order which would have no relationship with subsequent events in the breeding season. A second possibility is that dominance might be functional in a more competitive situation, in hard weather, or at a higher population size . Both 1970 and 1971 were fairly mild from January to March and no observation was made during severe conditions when the birds would be crowded together on the small areas of remaining open mudflat . Population size in the breeding seasons of 1962-64 was about fifty per cent higher than in 1970-71 (Patterson et al . 1974) and competition for territory sites and Table VL Breeding Success in Relation to Dominance No . of males Half of rank order

Not Hatching hatching broods broods

Mean brood size ± sa

1970 upper 1970 lower

3 6

6 3

7.3 f 0.9 8 .2 f 1 . 1

1971 upper 1971 lower

5 4

7 7

5 . 0 f 1 .2 5.8 f 1.3

455

possibly other resources might well have been higher then . Both of these explanations, however, neglect the fact that in both years, especially in 1970, only some of the males in the population could be ranked . It is obviously important to consider the remainder . The Unranked Males The most obvious birds which could not be assigned ranks were the unmarked ones, which made up 44 . 3 per cent of the winter flock in 1970 and 24 . 1 per cent in 1971, after considerable ringing in the previous year. Since most of the marked birds were caught in winter in traps baited with food, they could have been a biased sample ; either the weaker members of the flock or conversely the more dominant ones, able to drive others from the food . Differences in dominance between marked and unmarked males can be measured by considering all the unmarked males together in their encounters with the marked males. If there is no bias in the sample a large group of unmarked males should win around 50 per cent of their encounters with marked ones . In 1970 the unmarked males won 57 .0 per cent of 272 encounters with marked birds, and in 1971 they won 47 . 3 per cent of 201 encounters . Neither of these percentages differs significantly from 50 per cent so that there is no indication that the marked sample was biased with respect to dominance . Williams (1973) has also shown that marked and unmarked birds in this population did not differ in breeding success in 1970-72 . A second important group which could not be ranked were those birds (about two-thirds of the territorial males) which were seen interacting with too few others to allow an estimate of their individual dominance positions . These unranked birds, like the unmarked ones, can be compared as a group with the ranked birds . The comparison was confined to males which subsequently became territory holders on the river in the same season, so that the two groups were equivalent sets of breeding adults . The unranked group would otherwise tend to include many young non-territorial males . Few interactions were observed between ranked males and unranked ones, but in both years the unranked birds were less successful, winning only 21 . 2 per cent of 19 encounters in 1970 and 12 . 5 per cent of 16 in 1971 (Table



456

ANIMAL BEHAVIOUR, 25, 2

VII) . In both years more ranked birds dominated unranked ones than the reverse, although the differences from equality were not statistically significant in either year or with the two combined. The ranked males hatched significantly more of their clutches than the unranked ones in both years . Broods were hatched by 61 . 5 per cent of 26 ranked males over the 2 years, but by only 15 . 6 per cent of 45 unranked males (Table VIII) . Survival in all of these territorial males was high in both years ; only a few individuals disappeared before the next season and there was no difference in the proportion disappearing between the ranked and unranked groups . Table VII. Encounters Between Ranked and Unranked Males. Each Line Represents One Pair of Marked Males Encounter won by : Ranked male Unranked male 1970

3 3 3 2 4 0 0 15 5

0 0 0 0 0 1 2 1 4 (21 . 1 %) 3

11 1 1 1 0 0 14 4

0 0 0 0 1 1 2 2(12-5%)

o Total encounters won Total dominant 1971

Total encounters won Total dominant

The differences in dominance and hatching success raise the question of what other differences existed between the ranked and unranked males. The groups differed strikingly in age, measured as before, with many of the ranked males but few of the unranked ones known to be at least 5 years old (Table IX). The difference was significant in 1970 and almost so in 1971 . On average, ranked males were first seen on the estuary 5 weeks earlier than unranked males in 1970 and 4 weeks earlier in 1971 (Table X) . The difference, which was significant in 1971 and approaching significance in 1970, may be related to the difference in age between the two groups. Williams (1973) found that older shelduck were seen significantly earlier in the winter, with a relationship still noticeable up to at least 8 years old. At first sight, late arrival might seem to explain the failure to rank some of the males, if they were not in the flock for long enough to be observed sufficiently . However, there was wide variation in the dates when birds were first seen, and in both years more than half of the unranked birds were seen before March . In both years this allowed five observation days spread over 4 weeks when these birds could have been observed at the baiting site . Table IX. Rank in Relation to Age Minimum age in years : >5

Not >5

1970 ranked males 1970 unranked males Fisher Exact Test : P < 0 .05

6 1

1 4

1971 ranked males 1971 unranked males Fisher Exact Test : P = 0.06

6 1

9 21

Table VIII. Breeding Success in Relation to Ranking ; Males Territorial on the Estuary No . of males : Hatching broods

Not hatching broods

1970 Ranked males Unranked males x 2 =5 . 12,P<0 . 05

7 5

2 15

1971 Ranked males Unranked males x 2 =8 .38, P<0 .01

9 2

8 23

Table X. Date of Arrival on the Estuary, in Relation to Ranking Date when first seen days after 1 December Mean f se 1970 ranked males 1970 unranked males d=1 . 84,0 . 1

0 . 05

52 .7 ± 6 . 1 88 .4 ± 9 . 1

1971 ranked males 1971 unranked males d = 2 .75, P < 0 .01

51 . 7 ± 6 . 8 79 .3 ± 6 . 6



457

PATTERSON : AGGRESSION AND DOMINANCE IN SHELDUCK

Occurrence at the bait was measured by the number of days on which a bird was seen there after the first sighting of that individual on the estuary. Among birds first seen before February, with at least 10 possible sightings at the bait, ranked males were seen on significantly more days than unranked birds with the same number of possible sightings (Table XI) . Away from the baiting site the unranked males in 1971 were seen almost as frequently as the ranked ones, and they were distributed similarly over the estuary. The eventual territory sites of the two groups were also distributed very similarly over the estuary . The total number of interactions recorded for each of the unranked males was significantly lower than for ranked males . Most unranked males seen at bait were seen there only once or twice and these had significantly fewer interactions than ranked birds which were seen on the same number of days (Table XII) . This difference is not surprising, since the ranked birds were selected as those which interacted with several other marked birds, and so would be expected to have had more interactions . The low number of interactions by the unranked birds could have been caused by their avoidance of encounters or merely by their being at the bait for only a short time on each occasion . These two possibilities cannot be distinguished since it was not possible to record the duration of individuals' visits to the bait .

ranking group interacted at bait sufficiently often to be assigned individual ranks but these differences in rank were not important in subsequent performance, at least in the existing relatively non-competitive situation . The subordinate birds occurred much less frequently at bait even though some of them were present on the estuary for much of the winter . Since most of them were seen at the bait at least once, it is unlikely that they were unaware of it ; of course they may not have been attracted by grain even though other individuals obviously were . All the birds present were scared off at the start of each observation period and the more subordinate may have been more reluctant to return. However, a more interesting possibility is that the subordinate males were excluded from the bait by the more dominant ones . The grain was scattered in a fairly restricted area of shore and the subordinates may have been unable to compete for feeding space. Such exclusion, if effective in naturally occurring competitive situations, could provide a mechanism for regulation of numbers . The results then support the hypotheses of Jenkins, Murray & Hall (1975), that dominant shelducks may exclude subordinates from feeding situations . Since Table XII. Number of Aggressive Interactions Recorded for Each Marked Male No. of interactions by individual males

Discussion The results show that the shelduck winter flock contained males of different status . A highTable XL The Number of Observation Days on which Individual Marked Birds were Seen at the Baiting Station . All had At Least 10 Possible Sightings No. of days seen at bait 0 1 2 3 4 5 6 7 R 9 10 11

Ranked males

Unranked males

1

2 5 1 1

1 1 3 1 3 2 2 1

1

Ranked males Birds seen once at bait

Birds seen twice at bait

Unranked males

56 52 34 22

12 9 6 6 4 3 2 2 1 0 0 0 Mann-Whitney U = 0, P = 0 .028 27 13 11

9 8 5 1

Mann-Whitney U = 0, P = 0 .028 Mann-Whitney U = 20, P < 0 .002 .



458

ANIMAL BEHAVIOUR, 25, 2

numbers continued to increase on the Ythan after March, such exclusion did not appear to limit the winter population, but the behaviour may have been effective in other situations . The difference in breeding success between the ranked birds and the other territorial males is striking . This may be directly related to the difference in dominance between the two groups ; the more dominant birds may have been able to compete successfully for a variety of resources such as the best nest sites . Preliminary observations in the nesting area suggest that in a small group of pairs nesting closely together, those with the most dominant males were most successful in hatching young . Other differences between the ranked and unranked males complicate the interpretation . The ranked males were older ; being more experienced and the presumably selected survivors of the year-classes, they may have been more efficient in searching for food, nest sites, brood areas, or other resources . Linked with the age difference, the earlier arrival of the ranked birds may have conferred some advantage . Williams (1973) has shown that early arrival was correlated with early occupation of territory, and that this in turn was correlated with early laying and more successful hatching . There is obviously a group of interlinked variables affecting breeding success and it is difficult to distinguish their separate effects on a relatively small amount of data . One possibility, which should not be neglected, is that the additional food supplied by the bait was sufficient to confer an advantage on the more dominant birds, able to annexe it for themselves . The amount of grain supplied was Table XIII . Weights of Ranked and Unranked Territorial Males in Winter (January to March) Year and category

Range of N Mean weights (g) (weight) weight (g)

SE

1970 Ranked 1150-1550 Unranked 960-1500 d = 2 . 565, P < 0 .02

13 17

1319 . 2 1201 .2

+ 352 ± 29 . 6

1971 Ranked 1120-1400 Unranked 980-1300 d = 2. 360, P < 0 . 02

14 10

1268 1205

± 20 . 4 f 27 . 9

There was no significant difference in weight between older and younger males, as defined in Table IX.

probably enough to give the regular attenders a large proportion of their daily food requirement for much of the winter and the grain probably needed much less effort to collect than the normal invertebrate food . Any feeding advantage should be reflected in the weights of the birds . Several individuals from both the ranked and unranked categories were trapped and weighed, some birds more than once. The mean weight of ranked males was significantly higher than that of unranked males in both 1970 and 1971 (Table XIII) and the difference cannot be explained by an age difference between the two groups . This again supports the idea that the dominant birds might exclude subordinate ones from the best feeding areas . Acknowledgments The Director and Staff of the Aberdeen University Computing Centre provided facilities and help with the development and use of computer programmes used in the analyses of most of these data . Programmes for sorting and storage of data were devised and partly written by Mr J . S . Knowles of the Centre, together with myself and Dr M . J . Williams of Culterty Field Station. Mr W . Murray helped greatly with trapping and marking of shelducks and Professor G . M . Dunnet, Dr D . Jenkins, Dr H. Milne and Dr A . Watson made helpful comments on drafts of this paper .

REFERENCES Boase, H. (1935) . On the display, nesting and habits of the Shelduck . Br . Birds, 28, 218-224 . Buxton, N . E. (1975). The feeding behaviour and food supply of the common Shelduck (Tadorna tadorna) on the Ythan estuary, Aberdeenshire . Unpubl . Ph .D . thesis, University of Aberdeen . Dobinson, H . M . & Richards, A . J. (1964) . The effects of the severe winter of 1962/3 on birds in Britain . Br . Birds, 57, 373-434. Goethe, F. (1961) . The moult gatherings and moult migration of Shelduck in north-west Germany . Br. Birds, 54, 145-161 . Goss-Custard, J . D . (1969) . The winter feeding ecology of the redshank Tringa totanus. Ibis, 111, 338356 . Harrison, J . & Hudson, M . (1964) . Some effects of severe weather on wildfowl in Kent in 1962-63 . Wildfowl Trust Ann. Rep ., 15, 26-32 . Hori, J . (1964) . The breeding biology of the Shelduck Tadorna tadorna. Ibis, 106, 333-360 .

PATTERSON : AGGRESSION AND DOMINANCE IN SHELDUCK Jenkins, D ., Murray, M . G . & Hall, P . (1975) . Structure and regulation of a shelduck (Tadorna tadorna (L .)) population . J. anim. Ecol., 44,201-231 . Patterson, I. J ., Young, C . M . & Tompa, F . S . (1974) . The shelduck population of the Ythan estuary, Aberdeenshire. Wildfowl, 25,161-173 . Williams, M . J. (1973). Dispersionary behaviour and

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breeding of Shelduck Tadorna tadorna L. on the River Ythan estuary. Unpubl . Ph.D. thesis, University of Aberdeen . Young, C. M. (1970). Territoriality in the common shelduck Tadorna tadorna. Ibis, 112, 330-335 . (Received 13 January 1976 ; revised 9 July 1976 ; MS. number : 1502)