David B. Lank

The snow geese of La Pérouse Bay : natural selection in the wild

Introduction The Lesser Snow Geese of La Perouse Bay Population structure and gene flow Fitness components model of Snow Goose life cycle Pairing, mating, and parental care Annual variation in fitness components Age effects Heritability of quantitative traits Plumage colour Clutch size The timing of reproduction Egg size Body size Synthesis and conclusions References Author index Subject index

BODY SIZE VARIATION IN LESSER SNOW GEESE: ENVIRONMENTAL PLASTICITY IN GOSLING GROWTH RATES'

We examined the influence of timing of reproduction and brood size on growth rates of goslings of nidifugous Lesser Snow Geese (Anser caerulescens caerulescens) using data collected at La Perouse Bay, Manitoba from 1978 to the present. Gosling growth rates declined significantly during the season, and the declines were independent of the parental genotype. Goslings from larger broods grew significantly faster, but there was no significant annual variation in the seasonal growth rate patterns with either hatch date or brood size. The hatch date and brood size effects could not be accounted for by systematic differences in either egg size or body size of the female parent. Goslings that grew more slowly due to the effects of hatch date or brood size were significantly smaller as adults. Because gosling growth rates influence final adult size, they may have a significant effect on various life history traits in this species.

Signaling Individual Identity versus Quality: A Model and Case Studies with Ruffs, Queleas, and House Finches

We develop an evolutionary model that predicts that characters selected to signal individual identity will have properties differing from those expected for indicator signals of quality. Traits signaling identity should be highly variable, often display polymodal distributions, not be condition dependent (i.e., be cheap to produce and/or maintain), not be associated with fitness differences, exhibit independent assortment of component characters, and often occur as fixed phenotypes with a high degree of genetic determination. We illustrate the existence of traits with precisely these attributes in the ornamental, conspicuously variable, and sexually dimorphic breeding plumages of ruff sandpipers Philomachus pugnax and red‐billed queleas Quelea quelea. Although ruffs lek and queleas are monogamous, both species breed in high‐density aggregations with high rates of social interactions (e.g., aggression and territory defense). Under these socioecological conditions, individual recognition based on visual cues may be unusually important. In contrast to these species, we also review plumage characteristics in house finches Carpodacus mexicanus, a nonterritorial, dispersed‐breeding species in which plumage ornamentation is thought to signal quality. In keeping with expectations for quality signals, house finch plumage is relatively less variable, unimodally distributed, condition dependent, correlated with fitness measures, has positively correlated component characters, and is a plastic, environmentally determined trait. We briefly discuss signals of identity in other animals.

Long-Term Decline in Body Size in a Snow Goose Population: Evidence of Environmental Degradation?

(1) The effect of environmental factors on annual body size variation was investigated in a breeding population of lesser snow geese (Anser caerulescens caerulescens L.) La Perouse Bay, Manitoba, using data from 1969 to the present. The population size has increased from approximately 2000 to 10 000 pairs during this period (Cooch et al. 1989). (2) Annual mean gosling body mass, tarsus and culmen length (measured at fledging) declined significantly by approximately 16% (240 g), 4% (3.4 mm), and 2% (0.93 mm) respectively in cohorts hatching between 1976 and 1988. The decline in gosling size has led to a significant decline in size of locally bred adults. Annual mean adult female body mass, tarsus and culmen lengths declined by approximately 15% (270 g), 6% (5.4 mm), and 4% (2.3 mm), respectively, in cohorts hatching between 1969 and 1986. (3) Gosling growth and subsequent adult size was also reduced in years with cold, wet weather after hatch. (4) The decline in gosling size was not dependent on the pattern of annual variation in egg or hatch mass, nor systematic changes in annual mean hatch date or post-hatch weather. Goslings reared by individual adult females showed a decline in size over time, suggesting that the general decline reflects a non-genetic change in gosling growth rates during the fledging period. The most probable non-genetic factor is a long-term reduction in food available to the geese at La Perouse Bay. This reduction in food availability may reflect over-exploitation of primary food plants by the geese.

Western sandpipers have altered migration tactics as peregrine falcon populations have recovered

The presence of top predators can affect prey behaviour, morphology and life history, and thereby can produce indirect population consequences greater and further reaching than direct depredation would have alone. Raptor species in the Americas are recovering since restrictions on the use of dichlorodiphenyltrichloroethane (DDT) and the implementation of conservation measures, in effect constituting a hemisphere–wide predator–reintroduction experiment, and profound effects on populations of their prey are to be expected. Here, we document changes in the behaviour of western sandpipers (Calidris mauri) at migratory stopover sites over two decades. Since 1985, migratory body mass and stopover durations of western sandpipers have fallen steadily at some stopovers in the Strait of Georgia, British Columbia. Comparisons between years, sites and seasons strongly implicate increasing danger from the recovery of peregrine falcons (Falco peregrinus) as a causal factor. A decade–long ongoing steep decline in sandpiper numbers censused on our study site is explained entirely by the shortening stopover duration, rather than fewer individuals using the site. Such behavioural changes are probably general among migratory shorebird species, and may be contributing to the widespread census declines reported in North America.

BLOOD ISOTOPIC (δ13C AND δ15N) TURNOVER AND DIET-TISSUE FRACTIONATION FACTORS IN CAPTIVE DUNLIN (CALIDRIS ALPINA PACIFICA)

Abstract Avian studies are often interpreted using dual (e.g. 13C, 15N) isotope models, assuming turnover of both isotopes occur at similar rates, but only a few studies have quantified turnover rates for more than one of those isotopes simultaneously. To test the generality of previous turnover and fractionation estimates and assumption of synchronous C and N patterns of turnover rates, we captured Dunlin (Calidris alpina pacifica) wintering in the Fraser River Delta, British Columbia, and derived isotopic turnover rates and diet-tissue fractionation factors by experimentally manipulating diet. Birds (n = 15) were initially fed a terrestrially derived diet (mean δ13C: −24.7‰, mean δ15N: 3.5‰) for 54 days. A treatment group (n = 11) was then switched to a marine-derived diet (mean δ13C: −18.3‰, mean δ15N: 13.7‰); a control group (n = 4) was maintained on the terrestrial diet for a further 59 days. An exponential model described patterns of isotopic turnover for 13C and 15N, and turnover rates and half-lives of the two isotopes were correlated, confirming the assumption of synchronous patterns of turnover for those isotopes. The half-lives for 13C and 15N in Dunlin whole blood were 11.2 ± 0.8 days and 10.0 ± 0.6 days, respectively, and are among the lowest values obtained to date for wild birds. Variation in turnover rate among individuals was not related to indices of body condition.

Sexual selection, arrival times, philopatry and site fidelity in the polyandrous spotted sandpiper

Summary1.Spotted sandpipers have a resource defense polyandrous mating system in which females compete for control of breeding territories and mates, and males provide most of the parental care.2.Data from a nine year study of a local breeding population showed that females preceded males to the breeding ground in spring. Within each sex, experienced local breeders arrived before new breeders. The mean arrival time of new females coincided with egg-laying of first clutches by experienced females, and at this time new females experienced their greatest success at entering the breeding population. Arrival of new males coincided with the onset of incubation by experienced males.3.Breeding males and females returned from year to year at similar rates. Birds returned at lower rates following breeding failure. This effect was strongest among birds after their first local breeding year, i.e. inexperienced birds, and weakest among experienced females.4.Significantly more locally hatched females than males returned and bred on the study site in their first year. Similarly, there were more locally hatched female than male chicks which returned but did not breed.5.These results are consistent with the hypothesis that the intensity of sexual selection drives the arrival timing of individuals on breeding grounds, and affects patterns of natal philopatry. The importance of territorial control in obtaining mates causes females to precede males, which is a complete reversal of the normal avian pattern. In both sexes, inexperienced birds arrive when intrasexual competition is lessened by the commitment of older birds to reproductive or parental activities. Greater female natal philopatry may reflect an increased importance of site familiarity for females as they attempt to become breeders in this resource defense polyandrous system.

WESTERN SANDPIPERS (CALIDRIS MAURI) DURING THE NONBREEDING SEASON: SPATIAL SEGREGATION ON A HEMISPHERIC SCALE

Abstract The nonbreeding distribution of Western Sandpipers (Calidris mauri) was documented using 19 data sets from 13 sites along the Pacific and Atlantic coasts of the Americas. Western Sandpipers showed latitudinal segregation with regard to sex and age. Females wintered farther south than males. A “U” shaped pattern was found with respect to age, with juveniles occurring at higher proportions at both the northern and southern ends of the range. Distribution of sexes might be affected by differences in bill length and a latitudinal trend in depth distribution of prey. For age class distribution, two different life-history tactics of juveniles might exist that are related to the higher cost of feather wear for juveniles compared to adults. Most juveniles complete three long-distance migrations on one set of flight feathers whereas adults complete two. Juveniles may winter either far north, thereby reducing feather wear induced by ultraviolet light, migration, or both, or far south and spend the summer on the nonbreeding area.

Long-term decline in fecundity in a snow goose population: evidence for density dependence?

(1) A population of lesser snow geese (Chen caerulescens caerulescens L.) has been studied at the breeding colony at La Perouse Bay, Manitoba, from 1968 to the present. (2) Annual mean clutch size, when adjusted for annual variation in mean laying date and intraspecific nest parasitism, has declined significantly by 0.72 eggs, or 16% of the initial annual mean, over the period of the study. Annual rates of egg loss due to predation, egg hatchability, and fledging success have remained unchanged. (3) The rate of the long-term decline in mean clutch size was independent of female age and breeding experience, and was not related to systematic changes in the age-structure of the breeding population. Individual females had clutch size declines parallel to the decline of the population mean. (4) Mean clutch size is negatively correlated with the size of both the breeding colony and the total flyway population, both of which have increased significantly. The decline in clutch size may reflect increased intraspecific competition for food during spring migration.

Intraspecific nest parasitism and extra-pair copulation in lesser snow geese

Intraspecific nest parasitism and extra-pair fertilization contributed significantly to individual reproductive success in a colony of plumage polymorphic lesser snow geese,Chen caerulescens caerulescens, studied for 14 years in northern Manitoba, Canada. Parasitic females preferred to lay in or adjacent to occupied, defended nests, rather than undefended nests. Nesting females usually rolled eggs that had been laid nearby into their nests. Nest-attendant males typically attacked the intruding females mate, who usually remained at a short distance, thereby drawing the resident male away from the nest. Plumage genetics were used to estimate rates of both nest parasitism and extra-pair fertilization. Nests of homozygous dominant blue morph pairs produced 4·03% homozygous recessive white goslings (N = 4938), which must have been unrelated to both parents, providing information on the rate of nest parasitism. Nests of homozygous recessive white morph pairs produced 2·12% blue goslings (N = 45 777), which must have been unrelated to either one or both parents, providing information on the rate of nest parasitism plus extra-pair fertilization. Using colony-wide estimates of the colour ratio of goslings produced (28·5% blue) and of the blue allele frequency in males (21%), it was calculated that nest parasitism accounted for 5·6% of hatching goslings, and that 2·4% of goslings were fathered by extra-pair fertilization.