Second North American Sea Duck Conference - Patuxent Wildlife ...

SECOND NORTH AMERICAN SEA DUCK CONFERENCE
FACTORS DETERMINING HERRING GULL
FORAGING SUCCESS WITHIN AN ARCTIC COMMON EIDER DUCK COLONY
Karel Allard¹, H. Grant Gilchirst², and Antony Diamond¹
¹Atlantic Cooperative Wildlife Ecology Research Network, P.O. Box 45111, University of New
Brunswick, Fredericton, NB E3B 6E1 CANADA; h77ct@unb.ca
²Canadian Wildlife Service, National Wildlife Research Centre, 1125 Colonel By Drive, Raven
Road, Carleton University, Ottawa, ON K1A 0H3 CANADA
Reproductive success of many colonial nesting birds is partly determined by loss of eggs and
young to avian predators that nest within colonies. We investigated the ecology of herring gulls
(Larus argentatus) foraging within a common eider (Somateria mollissima) colony in the Canadian
Arctic. We hypothesized that predation rates of eider eggs and ducklings by gulls vary in relation
to environmental conditions, prey alternatives, and risk of injury. 1) We studied gull predation of
eider eggs and quantified how herring gull numbers, search rate, and attack success varied with
environmental (weather, tide, available light, etc.) and behavioral (eider reproductive phenology, nest
density, etc.) factors. Herring gulls did not force incubating hens off their nests, and took eggs only from
unattended clutches. Consequently, gulls foraged more intensively and were most successful during
eider egg-laying when hen nest attendance was intermittent. 2) We studied gull predation of ducklings
during passage of eider broods from nest to sea. We assessed the effect of environmental variables
on search and duckling capture rates by gulls, paying particular attention to annual variation. Capture
rates were low and variable across years, but lowest when lemming abundance was high, despite
consistent encounter rates between gulls and eider broods. Frequency of eider defensive response
to foraging gulls was higher in low lemming years, and highest in the year following the lemming
abundance peak. Otherwise, gulls searched most intensively and were most successful during windy
conditions, when they could hover over broods. 3) We manipulated food availability within foraging
territories actively defended by pairs of resident gulls. Territories varied in both size and in numbers
of eider nests they contained. Both territory attendance and defense was largely determined by prey
abundance within territories. Territorial residents were responsible for more than half of all egg
depredations within the colony, and ultimately protected nesting eiders from other gulls by actively
excluding them from their feeding territories. Our findings stress the importance of assessing costs,
returns and risks for predators before making broad generalizations about their influence on colonial
nesting prey. Our results also suggest that risk of injury may influence the foraging behavior of
herring gulls as they attempt to maximize the trade-off between energy gain and foraging costs.
62 ANNAPOLIS, MARYLAND, USA NOV. 7-11, 2005