Heredity 76 (1996)100—107 Received29 June 1995
Genetic and environmental effects on the timing of wing moult in the barnacle goose
KJELL LARSSON Department of Zoology, Uppsala University, Vilavagen 9, S-752 36 Uppsa/a, Sweden
Geneticand environmental effects on the timing of wing moult were analysed in a breeding barnacle goose (Branta leucopsis) population recently established in the Baltic area. Start of wing moult of adults was found to be correlated with number of fledged young produced and start of wing moult of their breeding partners. Date of birth and age were not significantly correlated with start of wing moult although the length of the interval between hatching date of broods and start of wing moult was correlated with age. Repeatability estimates were significantly different from zero showing individual consistency of start of wing moult between years. Offspring—parent regressions and full-sib analyses yielded significant heritability esti- mates for start of wing moult. No indications of maternal effects were found. An especially high degree of resemblance between one-year-old full-sibs indicated the presence of a common environment effect on start of wing moult.
Keywords:Brantaleucopsis, common environment, heritability, maternal effects, reproductive success, wing moult.
Introduction species. Even though he was able to show that the differences between the studied bird populations Therate and direction at which quantitative traits had a genetic basis, this does not necessarily indicate can evolve in natural populations as a response to that the phenotypic variation within populations is new selection pressures are dependent on the herita- heritable. However, by using captive blackcaps bility and genetic correlations between traits (Lande, (Sylvia atricapilla) from one partially migrating 1979; Falconer, 1981). In birds, many morphological population in an artificial selection experiment, traits have been found to be both highly heritable Berthold (1988) showed that the degree of migratory and genetically correlated with each other (Grant, activity responded to selection. The realized herita- 1986; Boag & van Noordwijk, 1987; Larsson & bility estimates obtained were high, suggesting that Forslund, 1992; Merilä & Gustafsson, 1993). For the intrapopulation variation observed under labora- traits more closely connected with lifetime repro- tory conditions to a large extent reflected genetic ductive success, such as clutch size, number of young variation (Berthold, 1988). Because heritability esti- produced, or life span, the few available heritability mates may be different under different environ- estimates are usually nonsignificant or low (Gustafs- mental conditions (Gebhardt-Henrich & van son, 1986; Mousseau & Roff, 1987; Boag & van Noordwijk, 1991; Hoffmann & Parsons, 1991; Noordwijk, 1987; but see van Noordwijk et al., 1980; Larsson, 1993), a further step towards a better Schluter & Gustafsson, 1993). general understanding of the inheritance of timing In recent years, studies have also started to focus of moult and other important activities in the annual on the inheritance of physiological and behavioural cycle would be to analyse if such traits are also traits which might influence the timing of important heritable under natural conditions. events in the annual cycle (van Noordwijk et a!., In natural populations of birds, a significant 1981; Berthold, 1989; Lambrecht & Dias, 1993). For degree of resemblance between related individuals example, by studying captive birds from geograph- may result not only from the effects of shared genes ically separated populations, as well as their hybrids, but also, for example, from the effects of a shared Berthold (1989) found evidence for genetic effects environment during growth or from maternal effects on the control of moult, migratory restlessness and (Falconer, 1981; Larsson & Forslund, 1992; Schluter other migratory activities in several passerine bird & Gustafsson, 1993; Gustafsson & Merilä, 1994).
100 1996 The Genetical Society of Great Britain. TIMING OF WING MOULT IN THE BARNACLE GOOSE 101
Thus, to be able to conclude that an observed Flocks of adult and juvenile barnacle geese were resemblance between relatives results from shared captured on moulting localities by a rounding-up genes, it is necessary to analyse the importance of technique in the middle of July each year. Captured possible environmental causes of resemblance. birds were individually marked with engraved Furthermore, knowledge about the effects of age, coloured plastic leg rings which allowed individual reproductive activities and other environmental and identification at a distance. When determining the social circumstances on the studied traits is also of age of captured birds, I distinguished between great importance in order to understand the causes adults, defined as birds one year old or older, and for resemblance between relatives measured at juveniles. Sex was determined by cloacal inspection different stages of life. (Owen, 1980). The aim of this paper is to examine genetic and The ninth primary feather, counted descendently environmental effects on start of wing moult in a (Ginn & Melville, 1983), of captured moulting adult population of barnacle geese (Branta leucopsis) birds was measured to the nearest 1 mm by inserting breeding in the Baltic area, Sweden. It has been a plastic ruler between the ninth and tenth primaries found previously that the mean date for start of wing as deeply as possible and measuring the distance to moult in this 23-year-old temperate-breeding popu- the tip of the ninth primary. By comparing the lation differs greatly from the mean date for start of lengths of the ninth primaries of 102 birds captured wing moult in Arctic-breeding barnacle goose popu- twice during a season it was estimated that the mean lations (unpublished). It has also been found that growth rate of this primary was 6.8 mm/day the mean start of wing moult in the study population (SD =1.58,n =52)in females and 7.3 mm/day varies between years and correlates with the timing (SD =1.33,n =50)in males (unpublished). Start of of breeding (unpublished). wing moult was estimated by calculating the date at Here, I extend the previous analyses, first, by which the length of this primary was zero, using its examining how the start of wing moult in males and length at capture and its sex-specific average growth females correlates with environmental and social rate. For birds which had dropped the old primary, factors such as date of birth, age, breeding success, but with the new primary not visible, the date of and start of wing moult in their breeding partners. start of wing moult was defined as the day of Secondly, I analyse the within individual variation by capture. Of the captured birds, 2.7 per cent had not calculating repeatability of start of wing moult for lost their old primaries on the day of capture. For the same individuals in different years. Thirdly, I these late-moulting birds, start of moult was defined estimate heritability of start of wing moult using as the day after the day of capture. The length of offspring—parent regressions and full-sib analyses. the new ninth primary of birds at capture varied Fourthly, I analyse and discuss some possible path- considerably (mean =62.5mm, SD =38.40,range ways by which the obtained heritability estimates 0—275 mm, n =4722,all years pooled). Therefore, might have been inflated by a common environ- possible confounding effects of individual variation mental effect. in primary growth rate on the estimates of start of wing moult will most likely be small. Relative start of wing moult was calculated by subtracting year- Materialsand methods specific mean values of start of wing moult from the Barnaclegeese were studied on breeding and moult- calculated individual values. The repeatability of ing sites along the coasts of the island of Gotland in relative start of wing moult was calculated for indivi- the Baltic between 1984 and 1994. This Baltic-breed- duals measured in at least two different years ing barnacle goose population, which most probably (Lessells & Boag, 1987). originated from the Arctic Russian population of Measures of reproductive performance, i.e. hatch- barnacle geese, has increased rapidly in numbers ing date of broods and number of fledged young, during the last 20 years (Larsson et al., 1988; were obtained for marked adults using observations Larsson & Forslund, 1994). The largest colony in the made on the breeding grounds (Larsson & Forslund, Baltic area is situated on three small islands named 1991, 1994). Hatching dates of broods of marked Laus holmar (57°17'N; 18°45'E) and comprised pairs were determined either by direct observations about 1730 breeding pairs in 1994. Details of the of pairs leaving their nest or by estimating the age of number of breeding pairs and breeding success in newly hatched young observed on nearby grazing the main colonies have been presented previously grounds, and then backdating (for details see (Larsson et a!., 1988; Larsson & Forslund, 1994). Larsson & Forslund, 1991; Larsson et a!., 1995).
The Genetical Society of Great Britain, Heredity, 76, 100—107. 102 K. LARSSON
Relative hatching dates of broods were calculated by several years, but only one randomly chosen brood subtracting year-specific mean values of hatching was included in the analyses. Thus, the same parents dates of broods from the observed values. If a never appear more than once in offspring—parent colour-marked juvenile was observed together with regressions, and full-sib resemblance has been marked adults after catching (see below) I assumed calculated from one brood only. that the date of birth of that juvenile was the same To analyse whether maternal effects or common as the hatching date of the brood of those marked environment effects were influencing the degree of adults, which in turn had been determined earlier in resemblance between related individuals I compared the same season. Relative date of birth of an indivi- heritability estimates obtained from different dual was defined as the difference, in days, between subsamples of birds. Higher heritability estimates its date of birth and the yearly mean hatching date from regressions of offspring on mothers than from of broods. regressions of offspring on fathers would indicate The number of fledged young was defined as the the presence of a maternal effect. Heritability esti- number of young observed on the date closest to mates obtained from full-sib analysis are expected to July 20 (within days) which is about one week be higher than estimates obtained from offspring— before juveniles are able to fly. Complete breeding parent regressions if there is dominance variance or failure was sometimes recorded at an earlier date. if the rearing environment, which full-sibs share, Barnacle geese usually start breeding at two to four affects start of wing moult of offspring (Falconer, years of age (Owen, 1980; Larsson, 1992). One-year- 1981; Larsson & Forslund, 1992). I also analysed the old birds do not breed. resemblance between related birds for which start of Family relationships were determined from obser- wing moult was measured at different ages. Separate vations of marked individuals on the breeding analyses of one-year-old offspring, which never grounds and on the grazing grounds during the two breed, and of offspring of four years old or more, weeks following catching. Grazing families were which almost always make breeding attempts, may clearly distinguishable from other family groups. reveal possible confounding effects of timing of DNA-fingerprinting and direct observations have breeding on the heritability estimates obtained. shown that intraspecific nest parasitism and adop- tion of young after hatching occur in this population resulting in about 17 per cent of the fledged young Results not being the true offspring of either their social mothers or fathers (Forsiund & Larsson, 1995; Start of wing moult in relation to date of birth and Larsson et a!., 1995). No case of extra-pair fertiliza- age tion has been detected (Larsson et a!., 1995). Relativestart of wing moult did not correlate with Heritability estimates (h2) for relative start of wing relative date of birth in either males or females moult were obtained by regressing the mean values (r =0.06,n =83,P>0.5; r =0.02,n =102,P>0.8, for adult offspring on female and male parents sepa- respectively). In an additional analysis, when only rately (h2 =twicethe slope) and on mean values of measurements of relative start of wing moult of both parents, i.e. midparent values (h2 =slope) one-year-old birds were used, the same result was (Falconer, 1981). Differences between slopes of found (r =0.07,n =55,P>0.6, males; r =0.15, regressions were tested for significance by a homo- n =55,P>0.2, females). geneity of slopes model (SAS Institute Inc., 1985). Relative start of wing moult was measured in Heritability estimates were also obtained from analy- birds of between one and ten years of age. The ses of adult full-sibs from broods of two or more sibs variation in mean relative start of wing moult among (Falconer, 1981). The presence of young not related age classes was small compared to the variation to either their social mothers or fathers in broods between individuals within age classes (Fig. la). (see above) is expected to deflate the heritability There was no significant relationship between rela- estimates. However, because extra-pair fertilizations tive start of wing moult and age for either males or seem to be absent or very rare the slopes of the females (Fig. la). For birds of two years old or more offspring—father and offspring—mother regressions that bred and hatched young, there was a significant will be deflated by a similar magnitude. Several of positive relationship between age and length of the the individuals included in the heritability analyses interval between hatching date of broods and start were measured in different years. In those cases only of wing moult (Fig. ib). This positive relationship in one randomly chosen measurement per individual both males and females can be explained by older was used. Some of the studied parents had broods in birds generally hatching their broods earlier than
The Genetical Society of Great Britain, Heredity, 76, 100—107. TIMING OF WING MOULT IN THE BARNACLE GOOSE 103
b-0.09,P>0.5 • =males U) (a) > 10 b=-014P>O.2 o =females 4- 5- 0 E 0•TT OI' C) 0 —C-' ji C TUW -5 - 1' 0 11111 4- liii 1 I I- 4- - (I) -10 a, 4- a) 15 168 102 85 73 55 44 36 30 22 9 a) 162 105 105 91 78 75 41 39 18 6
I I I I I I I I
U) 1' 0.60, P <0.001 0 (b) b 1.40P<0.001 0 45
40
CCC)035
a).30 Fig. 1 Relative start of wing moult .C (a), and length of interval between a,C0 hatching date of broods and start of 25 wing moult (b) in relation to age of —— individual barnacle geese. Bars repre- —c20 6 23 24 21 18 17 12 14 3 sent1 standard deviation. Sample 14 33 36 41 35 16 13 6 3 sizes are given for males (upper line) a, I 1 I I 4- and females (lower line) along the C 1 3 4 5 6 7 8 9 10 x-axis. Regression coefficients (b) are 2 based on mean values for males and females, respectively. Age (years)
younger birds (r =—0.31,n =424,P <0.001, males; significantly shorter for females with large broods r =—0.36,n =759,P <0.001, females; pooled data than for females with small or no broods (Fig. 2b). from 1985 to 1994) (Forslund & Larsson, 1992). In contrast, the length of this interval was signifi- cantly longer for males with large broods than for males with small or no broods (Fig. 2b). In males, Startof wing moult in relation to brood size the lack of correlation between relative start of wing Therewas a significant relationship between brood moult and number of fledged young, despite the size at fledging and relative start of wing moult in significant positive correlation between the length of females (Fig. 2a). On average, females with larger the interval between brood hatching date and start broods started their wing moult earlier than females of wing moult, and the number of fledged young, with smaller or no broods. No such relationship was can be explained by the hatching date of broods found in males. The length of the interval between being correlated with brood size at fledging hatching date of broods and start of wing moult was (r =—0.24,n =2531,P <0.001, pooled data from
The Genetical Society of Great Britain, Heredity, 76, 100—107. 104 K. LARSSON
= 0.29, P>0.20 • =males U) (a) b =-o.95, P'<0.05 =females > 10 o
5 0 E ci 0 I C
0 -5 I I-
U) -10
-15 w 532 140 126 65 40 12 582 151 129 68 41 11
U, -c0 2 (b) '4->...0c a, 40 I
Fig. 2 Relative start of wing moult II::. I (a), and length of interval between hatching date of broods and start of t25 wing moult (b), in relation to number a,tfl of fledged young produced by bar- 1 20 387 70 73 37 23 nacle geese. Bars represent 433 69 78 37 24 7 standarddeviation. Sample sizes are
______I I I I I givenfor males (upper line) and females(lower line) along the x-axis. r 0 1 2 3 Regression coefficients (b) are based on mean values for males and Number of fledged young females,respectively.
1985 to 1994) (Forslund & Larsson, 1992). Broods (P<0.001; test of homogeneity of the two correla- that are large at fledging have generally hatched tion coefficients). earlier than smaller broods. Repeatabilityof start of wing moult Synchronizationof start of wing moult within pairs Repeatabilityof relative start of wing moult was Therelative start of wing moult of partners within calculated for birds which were captured and meas- breeding pairs was positively correlated. The corre- ured in at least two different years. The first analy- lation between partners in pairs which failed to sis, in which I included all birds which were produce fledged young (r0.59,n =169,P-<0.001) measured twice, gave a repeatability estimate of 0.49 was significantly higher than the correlation between (F12481249 =2.93,P <0.001). In two additional analy- partners in breeding pairs which produced at least ses, I selected measurements that had been made one fledged young (r =0.20,n =283,P<0.001) when the bird was one year old and not breeding
The Genetical Society of Great Britain, Heredity, 76, 100—107. TIMING OF WING MOULT IN THE BARNACLE GOOSE 105 and when the same bird was at least four years old. slightly positive even though the length of the inter- The repeatability estimate obtained when the first val between hatching date of broods and start of measurement was at one year of age and the second wing moult was strongly negatively correlated with at four or more years of age was slightly lower hatching dates of broods (unpublished). In this (repeatability =0.30,F65,66 =1.84,P<0.01). The study, I found a significant relationship between repeatability estimate obtained when both measure- number of fledged young and start of wing moult. I ments were taken after four years of age was slightly also found significant correlations between start of higher (repeatability =0.58,F490491 =3.71,P
Table 1 Heritability estimates (h2) for relative start of wing moult in barnacle geese
Offspring measuredwhen Offspring measured when All offspring one year old four or more years old n h2±SE P n h2±SE P n h2±SE P
Midparent—offspring 141 0.23±0.08 <0.01 96 0.18±0.08 <0.05 57 0.33±0.14 <0.05 Father—offspring 166 0.29±0.11 <0.01 107 0.22±0.11 0.05 71 0.32±0.20 0.10 Mother—offspring 179 0.23±0.11 <0.05 114 0.23±0.11 0.05 80 0.38±0.18 <0.05 Full-sib 71 0.12±0.20 0.26 35 0.86±0.26 <0.01 21 0.23±0.44 0.30 n =numberof families
The Genetical Society of Great Britain, Heredity, 76, 100—107. 106 K. LARSSON breeding barnacle geese, Owen & Ogilvie (1979) than in randomly chosen individuals, may have found, as I did in this study, that partners which affected the timing of the first wing moult in failed to produce fledged young moulted more one-year-old birds. synchronously than partners which successfully In conclusion, this study showed that the large produced fledged young. individual variation in timing of start of wing moult Even though there is evidence which shows that can be explained to a considerable extent in terms of traits such as timing of moult can be significantly external environmental variation and social circum- affected by various environmental factors and social stances. However, my comparisons of related adult circumstances this does not necessarily mean that individuals also showed that a significant part of the genetic factors are unimportant or that such traits phenotypic variation most probably can be ascribed would respond slowly to phenotypic selection. In this to additive genetic variation. Thus, if the timing of study, the repeatability estimates for relative start of wing moult influences the lifetime reproductive wing moult were significantly different from zero success of individuals one can expect evolutionary and showed that the same individuals tended to start changes in the date of start of wing moult in this their wing moult at similar dates in different years. population. The mean start of the wing moult of The repeatability estimates were especially high Baltic-breeding barnacle geese occurs about three after the birds had reached the age of four. Thus, weeks earlier than that of Arctic-breeding barnacle part of the total phenotypic variation observed in geese, from which the Baltic birds most probably the population can be explained in terms of consist- originated about 23 years ago (unpublished). Thus, ent differences between individuals. The heritability it is possible that at least part of the observed differ- estimates for relative start of wing moult were also ence in mean date of start of wing moult actually generally significantly different from zero. This indi- represents a recent microevolutionary change in the cates that part of the phenotypic variation within the Baltic population. However, to be able to confirm population reflects additive genetic variation. The that these population differences indeed reflect heritability estimates obtained from offspring—father genetic differences, other types of analysis, for regressions were not significantly different from example transplant experiments (cf. Rhymer, 1992), those obtained from offspring—mother regressions. are needed. Hence, maternal or paternal effects on this trait seemed to be absent or weak. The heritability estimate obtained from analyses Acknowledgements of measurements of one-year-old full-sibs was Ithank Par Forslund, Juha Merilä, Ben Sheldon and considerably higher than estimates obtained from Staffan Ulfstrand for discussions and comments on analyses of older full-sibs and from offspring—parent the manuscript. Financial support was provided by regressions. Therefore, it is likely that some compo- the Swedish Natural Science Research Council, Olle nent of the environment shared by full-sibs affected och Signhild Engkvist Stiftelser, and the Swedish the timing of wing moult when the birds were one Environmental Protection Agency. year old, but that this effect disappeared or became overshadowed by other factors later in life. In some bird species, moulting has been found to begin References exactly one year after birth and subsequently at BERTHOLD,r'. 1988. Evolutionary aspects of migratory about 12 month intervals (Gwinner, 1986). It is behavior in European warbiers. .1. Evol. Biol., 1, therefore possible that the annual moulting rhythm 195—209. might be affected by seasonally varying external BERTHOLD, p1989.Genetics of migration. In: Gwinner, E. factors to which an individual is exposed during (ed.) Bird Migration, pp. 269—280. Springer-Verlag, ontogeny. However, there were no significant corre- Berlin/Heidelberg. lations between relative date of birth and relative BLACK, J. M. AND OWEN, M. 1989. Parent—offspring rela- tionships in wintering barnacle geese. Anim. Behav., 37, start of wing moult in either one-year-old or older 187—198. birds. Thus, the high degree of resemblance between BOAG, P. T. AND VAN NOORDWLTK, A. .1987.Quantitative one-year-old full-sibs cannot be explained by effects Genetics. 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