Silke Nebel

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.

Differential predator escape performance contributes to a latitudinal sex ratio cline in a migratory shorebird

Sexual segregation outside the mating season is common in vertebrates, and has been attributed to sexual differences in predator escape performance in ungulates and fish, but not in birds. Here, we tested the hypothesis that sex- and latitude-specific predator escape performance underlies the differential nonbreeding distribution of western sandpipers (Calidris mauri), a migratory shorebird. Females overwinter further south along the American Pacific coast, creating a latitudinal cline in sex ratio. Escape performance is reduced with increasing body mass, and birds generally carry less fat further south. Western sandpipers with poor escape performance were therefore predicted to prefer southern sites to reduce the risk of mortality posed by predators. Data from four nonbreeding latitudes showed that wing loading, used as an index of escape performance, was overall higher for females, and that it decreased with latitude in both sexes. Within latitudes, wing loading was lower at smaller, and presumably more dangerous, sites. Flight response to a predatory attack was longer in the south. Mortality risk offers a novel and candidate explanation for differential distribution patterns in western sandpipers and possibly other avian migrants.

Migration ecology and morphometrics of two Bar-tailed Godwit populations in Australia

Abstract Bar-tailed Godwits (Limosa lapponica) are long-distance migratory shorebirds. About a third of the global population migrate to Australia during the non-breeding season where they occur mainly in the north-west and east. Using measurements of bill, wing, tarsus and total-head (head and bill), we show that birds from the two main Australian non-breeding regions belong to separate populations. Individuals in north-western Australia are subspecies L. l. menzbieri, which breeds in north-eastern Russia, whereas those in south-eastern Australia are subspecies L. l. baueri, which breeds in northern and western Alaska. Resightings and recoveries of marked birds showed that there is little exchange between the two populations on the non-breeding grounds. They also take different routes during northward migration: L. l. menzbieri was more frequent on staging areas in the western Yellow Sea and largely absent from Japan, whereas L. l. baueri was more numerous in the eastern Yellow Sea and was often recorded in Japan. L. l. baueri left on northward migration 1–2 weeks earlier than L. l. menzbieri and gained more body mass before departure, presumably in preparation for a longer migratory leg. Our data suggest that on northward migration, L. l. menzbieri and L. l. baueri make direct flights of 5400–6200 km and 8200–8500 km, respectively, to reach staging areas in East Asia. The route of Limosa l. menzbieri on its southern passage is similar to that of northward migration, with stop-overs in East Asia. Limosa l. baueri, however, make an ∼10400-km nonstop flight across the Pacific Ocean from Alaska to eastern Australia, one of the longest non-stop migratory flights known amongst birds.

Foraging behaviour of western sandpipers changes with sediment temperature: implications for their hemispheric distribution

Migratory shorebirds need to replenish their energy reserves by foraging at stop-over sites en route. Adjusting their foraging behaviour to accommodate variation in local prey availability would therefore be advantageous. We test whether western sandpipers (Calidris mauri), a sexually dimorphic shorebird, adjust their foraging behaviour in response to local changes in prey availability, as inferred by changes in diurnal time and sediment temperature. Both males and females showed quantitative changes to foraging mode in relation to each of these variables. Probing, for example, which is used to exploit infaunal prey, was significantly more common at higher temperatures. The results presented here are consistent with the notion that western sandpipers can adjust their foraging behaviour in response to variation in prey availability. Further, we speculate that temperature-induced changes to prey location may contribute to the striking sexual segregation observed for this species during the non-breeding season.

Differential migration of shorebirds in the East Asian–Australasian Flyway

Abstract Differential migration involving intraspecific segregation of ages or sexes on the non-breeding grounds is common among migratory birds. Most of the existing data have, however, been collected in Europe and the Americas and very little is known about such migration patterns in the East Asian—Australasian Flyway, which links eastern Siberia with Australia and New Zealand. Spatial segregation of males and females during migration and at non-breeding grounds has clear implications for conservation management, as the loss of habitat predominantly used by members of one sex will disproportionately reduce effective population size. Here, I review the published data on differential migration in shorebirds in the East Asian—Australasian Flyway and discuss these data in the context of alternative hypotheses for differential migration and their relevance to existing conservation programs.

Molecular sexing of prey remains permits a test of sex-biased predation in a wintering population of western sandpipers

Population sex ratios in monogamous birds are often male biased. One factor that can affect population sex ratios is sex–biased predation. However, most estimates of sex–biased predation in birds have focused on species with obvious sexual colour dimorphism or body size dimorphism. Data on sexually monomorphic birds are generally lacking. In the present study, we adopt a PCR–based sexing procedure to help test for sex–biased predation in a wintering population of western sandpipers (Calidris mauri), a shorebird that shows only subtle sexual size dimorphism. Specifically, by comparing the a priori determined sex ratio of live birds wintering at a site in western Mexico to the molecular estimate obtained from depredated birds at this same site, we were able to perform a population–specific test for sex bias in predator–induced mortality. The proportion of females estimated from living (ca. 25%) versus dead (ca. 24%) individuals was in fact not significantly different, indicating that the strong male bias in this population is not due to differential predation. However, molecular sexing of prey remains is a hitherto unexploited test of sex–biased predation in birds, and is potentially applicable to any species for which prey remains can be gathered. We discuss our results in the context of alternate ecological hypotheses for population sex biases.