Breeding and Natal Dispersal of the Goldeneye, Bucephala clangula

Breeding and Natal Dispersal of the Goldeneye, Bucephala clangulaAuthor(s): Hilary Dow and Sven FredgaSource: Journal of Animal Ecology, Vol. 52, No. 3 (Oct., 1983), pp. 681-695Published by: British Ecological SocietyStable URL: http://www.jstor.org/stable/4447 .

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Journal of Animal Ecology (1983), 52,681-695

BREEDING AND NATAL DISPERSAL OF THE GOLDENEYE, BUCEPHALA CLANGULA

BY HILARY DOW* AND SVEN FREDGAt

*Department of Zoology, University of Newcastle-upon-Tyne, NE] 7R U and tSvenska Jagarefirbundet, tstermalmsgatan 39, S100-41 Stockholm, Sweden

SUMMARY

(1) Breeding and natal dispersal were examined for female goldeneye breeding in nest boxes in Sweden. Females changing nest box between years dispersed a median distance of 0 75 km to the new nest box (or, to the tenth nearest nest box). Individuals (N = 17) which returned to the natal area to breed, nested in the vicinity (median = 0 73 km) of the box in which they had hatched.

(2) Neither the proportion of females changing nest box, nor the distances moved between nest boxes, varied among years.

(3) Forty-two percent of females reoccupied the same nest box in successive years. Females which failed to breed successfully were less likely to return to the same nest box than females which bred successfully.

(4) Females which moved to another nest box were less likely to hatch their broods successfully than females which returned to the same nest box. When they moved they produced smaller clutches and broods, and bred later in the season than when they returned to the same nest box.

(5) Among females which moved, those which had failed to breed successfully moved further to another nest box than those which had bred successfully. In the case of females which lost clutches to predators, moving away from the vicinity of the previous nest box may have been advantageous, since boxes in which clutches were preyed-upon tended to be preyed-upon in the following year and also tended to occur close together.

(6) The greater the distance from the previous nest box at which females bred, the less likely they were to start egglaying early in the season.

(7) Females whose previous nest box was occupied by another female moved further to new nest boxes than females whose previous nest box was left empty.

INTRODUCTION

Adults in many migratory populations of birds exhibit a high degree of fidelity to their previous nest sites and their surviving offspring frequently return to breed in the natal area (e.g. review by Baker 1978). In many cases, e.g. common terns Sterna hirundo (L.) individuals which return to their previous nest sites raise more offspring than those which move (Austin 1945, 1949). Previous experience is known, however, to influence nest site fidelity since individuals that have poor breeding success in one year are less likely to reoccupy the same site in the next year than successful pairs, e.g. bobolinks Dolichonyx oryzivorous (L.) (Martin 1974), great tits Parus major (L.) (Harvey, Greenwood & Perrins 1979), and sparrowhawks Accipiter nisus (L.) (Newton & Marquiss 1982).

* Present address: Culterty Field Station, Newburgh, Ellon, Aberdeenshire, AB4 OAA.

0021-8790/83/1000-0681$02.00 ? 1983 British Ecological Society

681



Breeding dispersal of goldeneye

In birds it is usually the male, or the pair, which return to the previous nest site (Greenwood 1980). It has proved difficult in many avian studies (e.g. Thomas 1980; Ollason & Dunnet 1978) to discriminate between the reproductive effects of fidelity to the same nest site, and those of faithfulness to the same mate. Greenwood (1980) suggests that waterfowl (Anatidae) differ from most other birds in that homing to the natal site and the previous breeding site is female rather than male-biased, e.g. long-tailed duck Clangula hyemalis (L.) (Alison 1975, 1977). Because drakes (males) neither select, nor defend, the nest site (Kear 1970) it is possible, in ducks, to examine the causes and consequences of female fidelity to the same nest site separately from those of fidelity to the mate. There have been, as yet, few investigations of this kind in ducks (exceptions include Mihelsons et al. 1970; Doty & Lee 1974) so the factors which cause ducks to change nest site and the extent and reproductive consequences of such movements are poorly understood.

The purpose of this paper is to examine breeding and natal dispersal in one species of hole-nesting duck, the goldeneye Bucephala clangula (L.). The data are derived from a 22-year study of goldeneyes breeding in nest boxes in a wildlife management area in Sweden. General accounts of the breeding biology of the goldeneye are given by Carter (1958), Eriksson (1979) and Dow (1982), but a summary of some features pertinent to the analyses in this paper are presented here. Goldeneyes usually return as pairs to the breeding grounds in April. Females select a nest site and then lay the clutch over 1-2 weeks: some females are known to lay eggs parasitically in the nests of other females. The drakes neither investigate nor defend the nest site, although they may wait in its vicinity whilst the female is egglaying. The female incubates the clutch for around 30 days, during which time the male deserts and leaves the breeding grounds. It is believed that very few females re-lay if the breeding attempt fails. Broods leave the nest hole 24-48 h post-hatching and the (precocial) young may be reared up to 3 km from the nest hole.

Although female goldeneye are known to return to the same nest hole in successive years (e.g. Johnson 1967; Rajala & Ormio 1970; Nilsson 1971) empirical demonstrations of the extent of their fidelity to them are lacking. The objectives of the analyses presented here are, firstly, to provide data on the degree of fidelity displayed by females to the previous nest hole. Secondly, an investigation is made of the features of a female's reproductive performance which correlate with either the probability of changing nest hole, or the distance females move to a new nest hole. Thirdly, the reproductive consequences of both fidelity to the same nest hole and of movements between nest holes are examined.

In addition to information concerning breeding dispersal, data are also presented on the extent of fidelity displayed by females towards their natal area.

METHODS

Study area The data are derived from a long-term study of female goldeneye duck breeding in

Varmland, Central Sweden (Lat. 59?34'N; Long. 14?8'E). Records cover the period from 1959 to 1980. Goldeneyes bred each year in nest boxes permanently located around water-bodies in a study area of 50.5 km2 (Fig. 1). The area is described in more detail by Fredga & Dow (1983). The numbers of nest holes (94.4% nest boxes; 5.6% natural tree holes; N = 107) available each year has fluctuated but increased gradually from thirty-one (1959) to a maximum of ninety-nine (1978) (Table 1). Since the area was extensively planted with commercial timber the density of natural tree holes was low. Ten such cavities were located which would have been suitable for goldeneyes, but only six of these were

682



H. Dow AND S. FREDGA 683

f\ River a Tree hole

* Nest box

OQ Lake Floating bog

o 1 km

FIG 1. Dispersion of nest boxes (;; n = 101) and natural tree holes (A; n = 6) within the study area.

TABLE 1. Annual changes in the number of available nest sites, the numbers of females breeding and moving to new nest sites, and the average dispersal distances

they moved

Number of Total Females Median nest sites number of Number of moving nest dispersal (boxes & breeding marked box between distance

Year tree holes) attempts females years (%) (km) 1959 31 22 9 1960 66 25 11 66.7 1-86 1961 65 32 26 78.0 0.47 1962 64 31 25 64.7 0.93 1963 62 25 17 41.7 1.27 1964 58 24 19 7.6 0.87 1965 73 26 15 45.4 0.96 1966 70 25 17 40-0 0.47 1967 80 35 15 85.7 0.84 1968 81 38 11 100.0 0.85 1969 90 31 20 70.0 1.21 1970 86 39 25 66.7 0.53 1971 91 33 16 53.8 0.80 1972 90 32 12 62.5 1-32 1973 91 32 13 40.0 0.49 1974 92 31 19 63.6 0.82 1975 90 35 22 45.4 0.24 1976 90 44 26 81.3 0.70 1977 97 35 20 41.7 0.46 1978 99 35 23 53.8 1.15 1979 97 36 27 76.4 0.63 1980 94 28 22 43.7 0.55




occupied by females during the study. There may have been more natural tree holes which were never found. The number of nest boxes located at lakes beyond the boundaries of the

study area was low, hence it differed from surrounding areas by nature of its high density of nest sites.

Collection of breeding data

A preliminary survey of the breeding population was carried out in 1959 and this was followed by more detailed studies from 1960 to 1980. Visits were made to each nest box in late April or May and again at the end of every breeding season. Occupied nest boxes

(goldeneye eggs or down present) were subdivided into the following categories. (i) Successful: a clutch was incubated and one or more young hatched. (ii) Unsuccessful: no young hatched; (a) a clutch was present but the female had

deserted it; (b) there was goldeneye down present in the box but the eggs had been preyed- upon. It is believed the major predator of goldeneye eggs was the pine marten, Martes martes (L.), since no remains of eggs were left in the nest box.

All nest boxes were completely cleaned out and relined with dry decayed wood chips at the end of each breeding season. Accurate records were kept, from 1960 to 1980, of the numbers of goldeneye eggs and young in each nest box. Hatching dates were recorded from 1968 onwards and the date on which the first egg of the clutch was laid (egglaying date) was determined from these (by backdating). In 1979 and 1980 those nests in which additional eggs were laid parasitically by other goldeneye females were identified (see Dow 1982 for details).

Since (i) neither the means nor the variances of clutch and brood sizes differed significantly among years, (ii) there were no significant differences among years in the distribution of egglaying dates and (iii) the size of the breeding population has remained relatively constant over the study period, the data for all years have been pooled in the

analyses. The dates of egglaying did vary among years so these have been incorporated into the analyses in this paper as the number of days before (negative) or after (positive) the mean date (zero) in each year. Analyses of clutch size, brood size and egglaying date are restricted to those instances in which females incubated their clutches successfully and hatched at least one young.

Incubating females were caught in all years by trapping them at the nest box. It is believed that all females which bred successfully were ringed and identified (Table 1); the exceptions were females breeding in natural cavities. Individuals which deserted their clutches, or whose clutches were preyed-upon, were not normally identified, and adult breeding males could not be caught. Young were individually marked, using light-alloy wing-tags affixed through the patagial membrane, during the 48 h period after hatching from 1968 onwards.

In most cases the real age of females could not be determined and so the females' breeding age was used. This represents the number of years since the first recorded breeding attempt of that individual. Several females had intermittent breeding histories, i.e. they were recorded as 'missing' from the breeding population during at least one year of their breeding lives. Females breeding for the first time had smaller clutches and broods and bred later in the season than more experienced females (Dow 1982); therefore, in several cases, analyses are performed separately on these two groups of females.

Movements between successive nest sites are described as movements from the 'old' box to the 'new' box. In order to measure the extent of breeding dispersal (the distance from the old nest box to the new nest box), the coordinates of each nest box were measured from a

684



H. DOW AND S. FREDGA

1:10 000 aerial survey map of the area. The distances between all nest boxes were measured and each box was ranked, on a nearest-neighbour basis, in relation to every other box. Boxes were classified into five spatial groups (cluster groups) according to the distance separating nearest-neighbours, using a hierarchical clustering technique (Ward's Method) recommended by Everitt (1974). The spatial distribution of nest boxes in the study area was highly clumped around the water-bodies with most nest boxes close together and some relatively isolated (Fig. 1). The distance (km) females moved between successive nest boxes may, therefore, be a misleading measure of breeding dispersal. Consequently two additional, complementary measures of breeding dispersal were made for females which moved nest box:

(i) the nearest-neighbour rank (see above) of the new box from the old box and the probability of females occupying one of the two nearest boxes to the old box;

(ii) the probability of females moving to a new box within the same cluster group of boxes as the old box.

Means are given throughout this paper + one standard error. Probability values associated with the Mann-Whitney U-Test and Wilcoxon Matched Pairs Signed Ranks Test are two-tailed and corrected for ties.

RESULTS

What is the extent offidelity to the previous nest box, or its locality? Among females returning to breed in the study area 42% (N = 240) returned to the

same nest box (Fig. 2). In the case of those individuals which moved nest box between years, 24% (N = 139) moved to the first or second nearest-neighbour nest box, and in 80% of instances to a nest box within the same cluster group of boxes as the old box. On average they moved 0.75 km (median distance) from the old box to the new box, or, alternatively, to the tenth (median rank) nearest box from the old box (Fig. 3(a) and (b) respectively). There was always a surplus of vacant nest boxes available for females since,

130-

120-

110-

100-

S 90-

E 80- a)

70-

K 60-

50-

z 40- 30- 20- 10- IO-

I 2 3 4 5 6

Number of successive years In the same nest box

FIG. 2. The numbers of females that returned to the same nest boxes in successive years.

685



686 Breeding dispersal ofgoldeneye

60- (a) 50-

40-

30-

20-

10-

1-0 2-0 3-0 4-0 50 6-0

Distance moved (km) o

- 70- (b) = 60- z

50-

40-

30-

20-

10-

10 2b 30 40 50 60 70 80 90 th

Distance moved (nearest-neighbour rank)

FIG. 3. Frequency distributions of the distances (a) and the nearest-neighbour ranks (b) that females moved from the old nest box to the new one.

on average, only 40.8% of the nest boxes were occupied annually (Table 1). The proportion of females moving nest box and the dispersal distances they moved did not vary significantly among years and were not related to the number of vacant nest boxes.

What are the reproductive consequences offidelity to the previous nest box?

Comparisons were made of the reproductive performance of females in years in which they remained in the same nest boxes with the performance of the same females in years in which they moved boxes. When females remained in the same boxes they tended to produce larger clutches, more young, and started egglaying earlier in the season than when they moved boxes (Table 2). The greater the number of consecutive years in which females bred in the same nest box the earlier the dates on which they started egglaying (rs = -031; P < 0.001; N= 53; females of breeding age one were excluded from this analysis). There

TABLE 2. Comparisons of the reproductive performance of females breeding in the same nest box as the year before with their performance when they moved nest box

between years Clutch size Brood size Egglaying date

Moved nest box 8.80 + 0.36 8.43 + 0.35 +3-51 + 1.42 Returned to the same nest box 9.89 + 0.35 9-05 + 0-43 -6-00 + 1.64 Difference in reproductive performance if

return to same boxt + 110 + 0.47 +0-62 + 0.46 -9-50 + 1-79 (more eggs) (more young) (days earlier) **N = 40 *N = 40 **N = 22

Egglaying dates are given as the number of days before (-ve) or after (+ve) the mean (zero). P values are from Wilcoxon Matched Pairs Signed Ranks Test: * represents P < 0-05 and **P < 0-01. t Values given are the changes observed within the same females.



H. Dow AND S. FREDGA

were, however, no progressive increases in clutch and brood sizes with increasing numbers of years in the same nest box (rs = 0.08 and 0.07 respectively; P > 0.1 in both cases; N = 214).

Females which moved nest box were less likely to hatch their broods successfully than those which returned to the same nest box (Table 3; x2 = 8.80, P < 0.01; N = 240). They were, however, not as likely to be parasitized by other goldeneye females (Table 4; Fisher Exact Test; P < 0.05). Parasitism is less frequent in clutches that are laid later in the season (Dow 1982) and it has not been possible to discriminate between the effect of changing nest box and the effect of breeding later in the season on the likelihood of being parasitized.

TABLE 3. The probability of females failing to hatch a brood in relation to whether or not they returned to the same next box in successive years

Moved to Stayed in new nest box same nest box

Hatched brood successfully 116 96 Failed to hatch brood 23 5 Probability of failing to hatch brood 0.16 0.05

TABLE 4. The number of females that were parasitized in relation to whether or not they moved nest box between years (data are from 1979 and 1980 only)

Stayed in Moved to same nest box new nest box

Parasitized 9 7 Not parasitized 3 14

Why dofemales move to new nest boxes?

A female's breeding experience in one year was strongly correlated with the probability of changing nest box in the following year. Females which failed to breed successfully were more likely to move to a new box the next season than females which bred successfully (Table 5). In addition, females which had been recorded as 'missing' from the breeding population (i.e. not recorded breeding in the preceding year, probably indicating a failure to breed) were less likely to return to the nest box they had used before than females which

TABLE 5. The probability of a female moving nest box in relation to the outcome of the breeding attempt in the previous year

Previous year This year

Probability of Move to a Stay in same moving to a

new nest box nest box new nest box N Unsuccessful breeding attempt 16 2 0-89 18

X2= 12-5 P < 0.001

Successful breeding attempt 78 95 0-45 173 Z2= 31-6 P < 0.001

After 'missing' interim year 45 4 0.92 49

687



688 Breeding dispersal ofgoldeneye

TABLE 6. The numbers of females which bred successfully and the numbers which failed to do so in relation to (i) whether or not they moved nest box and (ii) the

outcome of their breeding attempt the year before

Breeding attempt this year Successful Unsuccessful N

Females which moved nest box between years: After an unsuccessful breeding attempt 13 3 16 After a successful breeding attempt 68 10 78 After 'missing' interim year 35 10 45

139

Females which returned to the same nest box: After an unsuccessful breeding attempt 1 1 2 After a successful breeding attempt 91 4 95 After 'missing' interim year 4 0 4

101

bred successfully. It appears to have been advantageous for females which failed to breed successfully to move box since 81% (N = 16) of those which did so bred successfully in the new box (Table 6). It is not possible, however, to compare their breeding performance with that of other females which stayed in the same nest box after failing to breed because there were only two known individuals in which this was recorded (Table 6).

Preferences for nest boxes may be altered by some or all of a female's previous experience in prior years; 14.1% (N = 78) of those females which changed nest boxes in the year following a successful breeding attempt returned to nest boxes they had used successfully in earlier years. Increasing breeding experience did not, however, alter the tendency to change nest boxes; females were as likely to move nest box after the first breeding attempt as after a later breeding attempt (Z2 = 0.66; P > 0.5).

It is not clear why those females which hatched broods successfully in one year moved nest boxes in the next year. It does not appear that they had had a poorer reproductive performance than females which bred successfully and returned to the same nest site. Comparisons were made of the clutch sizes, brood sizes and egglaying dates of females in the year prior to 'a move' or 'a stay' for two groups of females: (i) those of breeding age 1 and (ii) those of breeding age > 1. In the latter group separate analyses were made of data from those females which had remained in the same nest box for at least two consecutive years, and those which had not done so. In all cases no significant differences were found between females which subsequently stayed and those which subsequently moved (Mann-Whitney U-Test; P > 0.29 in each group). Similarly a matched-pair comparison within individual females failed to reveal any significant differences between the clutch sizes, brood sizes, and egglaying dates recorded in the year prior to moving and those recorded in the year prior to returning to the same nest box (Wilcoxon Matched Pairs Test; P > 0.05 in each case). Nor was there any indication that the females which moved box were constantly changing nest boxes throughout their lives; they were as likely to have had two consecutive breeding attempts in their previous nest box as other females which re-occupied their previous nest box (X2 = 0.74; P > 0.3).

Howfar dofemales disperse when they move nest box?

The factors which correlated with a female's decision to change nest box (see above) also correlated with the distance it moved to the new box (Table 7). Females which had



TABLE 7. How far females moved to a new box in relation to the outcome of their previous breeding attempt in the old nest box

Dispersal distance to new box

Probability of Probability of the new box the new box

Breeding Mean nearest- Median nearest- being in the being the 1st attempt Mean distance Median distance neighbour rank neighbour rank same cluster or 2nd O

last from old box from old box of new box of new box group of boxes neighbour to year to new box (km) to new box (km) from old box from old box as the old box the old box N >

(i) 1-18 ? 0.11 0.75 14-98 ?+ 154 10-00 0.80 0.24 139 (ii) 1-71 + 0-37 1-45 26-62 + 5.98 22-00 0-62 0.00 16

(iii) 1.00 + 0-11 0.79 11-41 + 1.64 5.3 0-86 0-29 78 (iv) 0.98 + 0-18 0.59 9-27 + 1.44 3.5 0.93 0.34 45 (v) 1.15 + 0-18 0.86 15-68 + 2-90 10-5 0.75 0-22 33

(vi) 1.28 + 0-24 0.71 17-28 + 2.92 11.3 0.77 0.25 45 ?

(i) All females which moved box. (ii) Females which had failed to breed successfully.

(iii) Females which had bred successfully. (iv) Females which had bred successfully and whose old nest box was left empty. (v) Females which had bred successfully and whose old nest box was occupied by another female.

(vi) Females which were 'missing' in the previous year.

a.' 00 'IC




100- A

80 /-

r 6- //

40- // O - /0-

E 20- /// - o- <

I I I I I ] 0 10 20 30 40 50 60 70 80 90th

Rank of nearest-neighbour nest box

FIG. 4. Cumulative frequency plotted against dispersal distance (measured in terms of the nearest-neighbour rank of the new box from the old box) for females moving nest box between years: (i) *?O when the previous breeding attempt was unsuccessful; (ii) A A when the previous breeding attempt was successful and the old nest box empty; (iii) 0 0 when the

previous breeding attempt was successful and the old nest box occupied by another female.

unsuccessful breeding attempts in one year (deserted the clutch or lost the clutch to a predator) moved further in the next year than those which bred successfully and moved (Fig. 4; Mann-Whitney U-Test; distance (km): z = 2.1; P < 0.04; N = 94; nearest-neighbour box rank: z = 2.0; P < 0.03; N = 94). Following a year in which they were recorded as 'missing' from the breeding population, individuals moved a distance (from their previous nest box to their new nest box) intermediate between that of successful females and that of unsuccessful females (Table 7), and did not differ, in this respect, from either of these two groups of females (Mann-Whitney U-Test; P > 0.1 in each case).

Females may have dispersed after losing a clutch to a predator because those occupying a box in which a clutch had been preyed-upon in the preceding year appeared more likely to lose their clutch than those occupying a box in which a clutch had been hatched successfully the year before (Fisher Exact Test; P < 0.05; Table 8). Boxes in which clutches had been preyed-upon tended to have nearest-neighbour boxes (first to fifteenth) with a higher rate of predation than boxes in which clutches were never preyed-upon (Wilcoxon Matched-Pairs Test; T = 14; P < 0.01; Fig. 5). Furthermore, the rate of predation showed a progressive decline with increasing distance (nearest-neighbour rank) from boxes in which clutches had been preyed-upon (rs = -0.83; P < 0-01; N = 15).

There is a suggestion that females avoided the vicinity of their old nest boxes if they were occupied by other females. Those whose previous nest box was occupied moved further to their new boxes than those whose previous box was left empty (Table 7, Fig. 4; Mann-Whitney U-Test, nearest-neighbour ranks: z = 2.41; P < 0.008; N = 78).

TABLE 8. The number of females which bred successfully and the number which had their clutches preyed-upon in relation to the occupancy of the nest box in the

preceding year Occupancy in preceding year

Occupancy in this year Successful Preyed-upon Successful 60 6 Preyed-upon 13 6

690




0:3

04I

1 3 5 7 9 11 13 15 th

Rank of nearest-neighbour nest box

FIo. 5. The rate at which clutches were lost to predators (mean number per 22 years) in the 1st, 2nd, 3rd . . . 15th nearest-neighbour nest boxes from: (i) boxes in which clutches were never preyed-upon (0); and (ii) boxes in which at least one clutch was preyed-upon during the study

period (0).

What are the reproductive consequences of the distances females disperse when they move nest box?

Females which remained in the same cluster group of boxes as the old box had earlier egglaying dates than females which moved to a new nest box beyond the cluster group of the old box (Table 9; Mann-Whitney U-Test; U = 160.5; P < 0.05; N = 64). Data presented in Table 10 suggest that the greater the distance females moved to a new box (from the old one), the less likely they were to lay early in the breeding season. A test for detecting a linear trend in proportions (recommended by Snedecor & Cochran 1967) indicated that in the case of females which moved nest box there was a decreasing tendency to breed early with increasing distance from the old nest box (z = 2-02; P <

TABLE 9. The date of egglaying-number of days before (-ve) or after (+ve) the mean date (zero)-in relation to whether females moved to a new nest box within the same cluster group of boxes as their previous nest box or to a different cluster

group of nest boxes

Egglaying date

New nest box Mean + S.E. Median

Same cluster group -0-63 ? 1.09 -0-8 Different cluster group +7-62 + 2.67 +7.2

TABLE 10. The numbers of females which bred on/earlier than, or later than the mean egglaying date in relation to how far they moved from the old nest box to the

new one

Distance moved to new nest box

Egglaying date Same nest box <0.5 km 0.5-1-0 km > 10 km

Earlier than/on the mean date 39 15 7 7 Later than the mean date 17 11 7 17

691




0.03). There were, however, no changes in the sizes of clutches and broods with increasing distance moved from the old box to the new box (rs = 0.10; P > 0 1; N = 111; in both cases).

What is the degree offidelity to the natal area? There is evidence that female preference for a breeding locality was influenced by

previous experience of the natal area. To date (1980) seventeen adult females, which were wingtagged as young have returned to breed in the natal locality. The first nest boxes these females occupied as breeding adults were, on average, 1.05 + 0.24 km; median = 0.73 km (or alternatively the x = 14th; median = 9th nearest nest box) from the box in which they were hatched. Almost all (94%) occupied a nest box within the same cluster group as the natal box.

One wingtagged male was recovered as a 2 year old (drowned in a fishing net) in May 1982, in Vasterbotten, 575 km north of the study area. None of the wingtagged females were reported during the breeding season from any other localities in Sweden. However, on nine different occasions during the study, females together with their broods (N = 9 females; total of 84 young), were transported to other areas in Sweden, over 50 km from the study area. Four of these females subsequently returned to breed in the study area (two returned to the same nest box) although none of their offspring did so. Some of the young were also subsequently recorded (N = 8; all shot); five in the vicinity of the area in which they were reared (three in their first autumn and two in their third autumn); and three from the overwintering grounds.

DISCUSSION

It is clear that goldeneye females produce more young and breed earlier in the season if they return to the same nest site in consecutive years rather than move to a new nest site. Females with higher reproductive success are believed to be those which hatch large broods early in the year since Dow (1982) has shown the survival of young goldeneye to be independent of the brood size in which they hatched but lower among individuals hatched later in the year. The factors that translate goldeneye familiarity with a nest hole, or an area, into increased reproductive success are unknown. Hinde (1956) and Lack (1966) suggest the advantages of familiarity are likely to be: a reduction in time spent searching for new nest sites; more profitable exploitation of local food resources; and greater awareness of resident predators. It is important to note that the male goldeneye plays no role in either selecting, or defending, the nest site or its vicinity, so the increase in female reproductive success associated with returning to the same area is not contingent upon fidelity to the same mate.

In addition to the reduction in reproductive success incurred when females move into new areas to breed, there are presumably other costs involved in changing nest site. Locating alternative holes may be difficult when the density of cavities is variable and often low. Prince (1968) in a study area in New Brunswick found a density of natural tree holes suitable for breeding goldeneye of around 2.4-7.7 cavities ha-'. In commercial timber stands, with few natural tree holes, the availability of nest boxes is likely to be both patchy and unpredictable.

Under certain circumstances there may be reproductive advantages of moving away from the previous breeding site. In the case of the goldeneye it has been found that individuals dispersed following a failure to breed, or if their clutch was preyed-upon. This trait has also been reported in mallards Anas platyrhynchos (L.) (Doty & Lee 1974) and

692




great tits (Harvey, Greenwood & Perrins 1979). The major predator of goldeneye eggs in Sweden is thought to be the pine marten (Eriksson 1975, 1980) which is a relatively long-lived mustelid that retains its home range from year to year (Hawley & Newby 1957; Soutiere 1979). Dispersal following a failure to breed would, therefore, appear to be an adaptive response by goldeneyes, serving to reduce the subsequent risk of breeding failure.

It was surprising that so many females should move nest site following a year in which they hatched young successfully, apparently with similar success to females which returned to the same nest site. There may have been variance in fledging success (after leaving the nest box) which was related to the location of the box and caused females to move the next year; but we have no data to examine this. However, it is worth pointing out that nest sites are not necessarily adjacent either to areas in which females feed in the spring (Carter 1958; Dow 1982), or areas in which they raise their young (Siren 1952; Eriksson 1978), so it is improbable that the availability of food in the immediate vicinity of the nest site influenced the female's decision to change nest box. Females may have moved as a result of being handled, for ringing purposes, in the previous breeding season, but this seems unlikely since females moved nest box in some years and stayed in the same nest box in other years.

Direct observations of competition for particular nest sites between females-which may have caused them to change nest box-were not recorded in the present study, but are reported by Grenquist (1963). There is, however, some circumstantial evidence for avoidance of certain occupied nest sites, since females whose previous nest box was occupied by another female moved further to their new box than females whose previous nest box was left empty. Unlike other hole-nesting birds, such as pied flycatchers Ficedula hypoleuca (Pallas) (Von Haartman 1971), there is no suggestion of territorial defence around the goldeneye nest hole (Carter 1958), nor is there any indication of spacing between nests (Fredga & Dow 1983). It may be that females move from their old nest boxes in order to avoid being parasitized by other female goldeneyes. Andersson & Eriksson (1982) demonstrated experimentally that, in a given year, extensive intra-specific nest parasitism (eggs added to nest by experimenter) caused goldeneye females to reduce their clutch size, or to desert the nest. If the costs of moving are less than the costs of parasitism then we might expect females to change nest nest boxes and move away under such circumstances.

The fidelity of female goldeneye to the natal area agrees with findings in other species of ducks, e.g. wood ducks Aix sponsa (L.) (Bellrose, Johnstone & Meyers 1964), and mallards (Doty & Lee 1974). No assessments were made in the present study of the reproductive consequences of females returning to the natal area to breed, although presumably this trait enhances their reproductive success (but see also Greenwood, Harvey & Perrins 1979a). The lack of information concerning male post-juvenile or breeding dispersal prevents the measurement of either the frequency or consequences of inbreeding. This is a factor which both Packer (1979) and Greenwood, Harvey & Perrins (1979b) suggest might influence the extent of dispersal away from an individual's natal area. Yet there is little evidence that male ducks exhibit fidelity to either the natal area or the previous breeding locality (e.g. Alison 1975, 1977), although they may, occasionally, return to an area with their old mate (e.g. Dwyer, Derrickson & Gilmer 1973).

ACKNOWLEDGMENTS

We are grateful for help from Svenska Jagareforbundet and for assistance in the field from Varmlands lans Jaktvardsf6rbund, especially Nils-Olof Olsson, Sven Carmland and Dan

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694 Breeding dispersal of goldeneye

Sandstrom. For advice and comments on drafts for this paper we thank Stewart Evans, Mats Eriksson and Ian Patterson. Hilary Dow was financed by a S.R.C. Studentship.

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391-459. Academic Press, New York & London.

(Received 15 April 1982)



Documents

Breeding and Natal Dispersal of the Goldeneye, Bucephala clangula