Body-Shape Adaptations in Black Oystercatchers: A Latitudinal and Sex-Based Analysis

Black oystercatchers have a wide range of morphological differences, such as size of bill and wing shape. Researchers looked at those differences to see if there were identifiable patterns that could tell us if they are residents or migrants and where they are from.

Black oystercatchers range from Baja, California to the Aleutian Islands in Alaska. Because of this broad geographical range, morphological differences between populations were expected. Based on captures of roughly 250 birds across 7 sites in British Columbia and Alaska, the authors found local and regional variation in the morphology of black oystercatchers, with birds from Alaska having shorter bills than those from BC. However, they found no evidence of differences in the wing shape, which is thought to vary with migration strategy. Black Oystercatchers in Alaska are partial migrants and BC Black Oystercatchers are considered residents. In other species, researchers have successfully used differences in morphology and wing shape to discriminate migrants from residents, or individuals from different breeding origins. Here, despite some difference in morphology between birds in BC and Alaska, their analysis had limited ability to assign birds to their origin in BC or Alaska. Limits in the ability to distinguish between birds from the two regions may be due to the considerable variation in morphology, both within and across sites, and indicate that local conditions may be more important in influencing morphological patterns.

Latitudinal gradients and sex differences in morphology of the Black Oystercatcher (Haematopus bachmani)

Abstract

Environment and behavior are widely understood to affect bird morphology, which can lead to differences among subspecies or populations within a wide-ranging species. Several patterns of latitudinal gradients in morphology have been described, though Allen's and Bergmann's rules are the most well-known and have been tested and confirmed across a diversity of taxa and species. These state that individuals at higher latitudes will have larger bodies (Bergmann's Rule) but smaller extremities (Allen's Rule) to conserve heat in colder climates. Migratory behavior also can influence avian morphology, particularly wing shape, where migratory birds tend to have longer, more pointed wings than residents. The Black Oystercatcher (Haematopus bachmani) is a large, partially migratory shorebird species restricted to intertidal habitats and distributed from Alaska to Baja California, spanning about 35° of latitude. A large proportion of Black Oystercatchers that breed in Alaska are migratory, where nearly all individuals breeding in British Columbia through the southern end of their range remain resident through the annual cycle. Their broad latitudinal range and diversity in migratory behavior may drive geographic variation in morphology. Here we evaluate three explanations for geographic variation in morphology of the Black Oystercatcher using data from seven sites across two regions: Alaska and British Columbia. We found evidence consistent with Allen's but not Bergmann's rule; birds in Alaska have shorter bills than those in British Columbia, and these findings held when controlling for body size using wing length. Despite regional differences in migratory behavior, we detected no difference in the wing shape of birds in Alaska and British Columbia. Differences between sexes and among sites suggest that multiple factors drive patterns of morphological variation in the Black Oystercatcher.

Roodenrijs, H., L. Ware, C. Rankin, M. Maftei, J. M. Hipfner, B. H. Robinson, D. Esler, H. Coletti, and D. J. Green. 2024. Latitudinal gradients and sex differences in morphology of the Black Oystercatcher (Haematopus bachmani). Ecology and Evolution 14(9): e70115.