Predation rates, timing, and predator compositionfor Scoters (Melanitta spp.) in marine habitats

Anderson, Eric M.
Esler, Daniel
Boyd W. Sean
Evenson, Joseph R.
Nysewander, David R.
Ward, David H.
Dickson, Rian D.
Uher-Koch, Brian D.
VanStratt, Corey S.
Hupp, Jerry W.
Centre for Wildlife Ecology, Simon Fraser University
Canadian Wildlife Service, Environment Canada
Washington Dept of Fish and Wildlife
USGS - Alaska Science Center
Publication Date: 

Studies of declining populations of sea ducks have focused mainly on bottom–up processes with little emphasis
on the role of predation. We identified 11 potential predators of White-winged Scoters (Melanitta fusca (L., 1758)) and Surf Scoters (Melanitta perspicillata (L., 1758)) in North American marine habitats. However, of 596 Scoters marked with VHF transmitters along the Pacific coast, mortalities were recovered in association with just two identifiable categories of predators: in southeast Alaska recoveries occurred mainly near mustelid feeding areas, while those in southern British Columbia and Washington occurred mainly near feeding areas of Bald Eagles (Haliaeetus leucocephalus (L., 1766)). Determining whether marked Scoters had been depredated versus scavenged was often not possible, but mortalities occurred more frequently during winter than during wing molt (13.1% versus 0.7% of both species combined, excluding Scoters that died within a post release adjustment period). In two sites heavily used by Scoters, diurnal observations revealed no predation attempts and low rates of predator disturbances that altered Scoter behavior (≤0.22/h). These and other results suggest that predation by Bald Eagles occurs mainly at sites and times where densities of Scoters are low, while most predation by mustelids
probably occurs when Scoters are energetically compromised.

Citation Information: 
Can. J. Zool. 90: 42–50 (2012)