Auk 125:149-157.
Abstract.—Birds are known to modify their foraging behavior in relation to food availability.
Once understood, these relationships can be used to draw inferences about relative food
availability and habitat quality. We measured foraging behavior of Surf Scoters(Melanitta perspicillata)
and White-winged Scoters (M. fusca) feeding on clams during winter in the Strait of Georgia,
British Columbia, to evaluate the relative quality of the foraging landscape for wintering scoters.
Because clam biomass does not increase appreciably during winter through growth or recruitment,
scoters are faced with a depleting and potentially exhaustible food supply. Along with this temporal
variation, clam densities vary widely by site. We considered the influence of variation in clam density
on scoter foraging behavior, along with other factors known to affect foraging in other sea duck species,
such as season,sex, age, and environmental attributes. Clam-capture success (clams captured per dive)
and foraging effort (minutes underwater per hour) of Surf Scoters were not related to variation in clam
density. Clam-capture success of White-winged Scoters was unrelated to clam density; however, their
foraging effort was negatively related to clam density, though varying by only 4min across the range
of observed clam densities. For both species, foraging behavior was generally more strongly related
to other factors, especially seasonal and age effects.These results suggest that (1) observed variation
in clam density was relatively minor fromthe perspective of foraging scoters and (2) our study site
constituted high-quality winter habitat in which scoters were not constrained by food availability.
Received 28 June 2006, accepted 30 March 2007.Keywords: clam density, food availability,
Melanitta fusca, M. perspicillata, scoter foraging behavior, sea duck, Surf Scoter,White-winged Scoter.