Genetic Structure and Demographic Impacts of Oil Spills in Western and Clark's Grebes

Little is known about Western Grebe (Aechmophorus occidentalis) movement patterns, including breeding philopatry, winter site fidelity, migratory connectivity, and within-season movement. Such patterns are important to understanding grebe biology and determining how populations are limited, but are challenging to explore in this species. We conducted a preliminary study of grebe population structure and movement patterns using microsatellite data and banding recovery data. The six microsatellite loci used did not have the power to detect differentiation among two disparate breeding colonies (Eagle Lake, California; and Lake Wabuman, Alberta) and three wintering areas (coastal Washington, northern coastal California, and southern coastal California). Additionally, no significant differentiation was picked up between the Western Grebe and its closely related sister taxa, the Clark’s Grebe (A. clarkii). Future studies should include additional and more variable loci. Limited band recovery data suggest that, although migratory connectivity is not strong as birds from a given breeding locale migrating to multiple wintering regions, moderate levels of connectivity do occur. Birds from Manitoba – the only breeding population for which there is a sample size of birds banded and encountered – winter in highest frequency in the coastal and Puget Sound regions of Washington and British Columbia. These recovery data show Western Grebes to have plasticity in both their breeding and winter site fidelity, a trait predicted by the variability in breeding site condition and coastal winter food availability. Additionally, we observed relatively large-scale movements within winter seasons. These are among the only insights into Western Grebe movements, outlining the need for larger scale studies of movement and population structure, especially in light of current conservation issues faced by this species.