Black Oystercatchers: Body Shape Across Latitudes and Between the Sexes

Researchers working with a black oystercatcher. Image from the National Park Service's page.

Black oystercatchers (Haematopus bachmani) are large shorebirds of the intertidal zone, found from Alaska's Aleutian Islands to Baja California — a span of about 35 degrees of latitude. Individual birds vary widely in body form, such as bill size and wing shape. Researchers looked for patterns in those differences that could show whether a bird is a resident or a migrant, and where it comes from.

Why body shape might vary

Environment and behaviour are known to shape birds' bodies, which can produce differences between subspecies or between populations of a wide-ranging species. Two well-known patterns, confirmed across many groups of animals, link body form to latitude:

• Bergmann's rule — individuals at higher latitudes have larger bodies;
• Allen's rule — individuals at higher latitudes have smaller extremities.

Both are explained as ways to conserve heat in colder climates. Migration can also shape a bird: migratory birds tend to have longer, more pointed wings than residents.

Black oystercatchers differ in migration too. Many of those breeding in Alaska migrate — they are partial migrants — while nearly all breeding from British Columbia to the southern end of the range stay resident year-round.

What the study found

The researchers tested three explanations for geographic variation, using captures of roughly 250 birds at seven sites in two regions, Alaska and British Columbia.

• Consistent with Allen's rule, not Bergmann's. Birds in Alaska had shorter bills than those in British Columbia, and this held when body size was accounted for using wing length.
• No difference in wing shape, the trait expected to vary with migration, despite the regions' different migratory behaviour.
• Differences between the sexes and among sites suggest that several factors drive variation in the species' body form.

In other species, differences in body form and wing shape have been used to tell migrants from residents, or to identify where birds breed. Here, despite some differences between the regions, the analysis could only poorly assign birds to British Columbia or Alaska. The authors suggest that the wide variation within and between sites limits that, and that local conditions may matter more in shaping how the birds look.

Sources

• National Park Service, "Differences Between Resident and Migrant Black Oystercatchers." https://www.nps.gov/articles/000/differences-between-resident-and-migrant-bloys.htm
• The study: 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. Read the paper
• Words on this page from people and organisations outside the federal government are paraphrased; rewritten in hubnx's own words.