Nesting in association with species that aggressively attack predators may reduce nest predation in some birds. For example, nesting ducks sometimes benefit from aggressive defence by nesting Canada Geese (Branta canadensis). Although Common Eiders (Somateria mollissima) commonly nest with gulls (Larus spp.), the costs and benefits for eiders of this association remain uncertain. Over two years, 32 instances of kleptoparasitism of Herring Gulls (Larus argentatus) by Canada Geese were recorded in arctic Canada. The Canada Geese nested amongst Common Eiders and interrupted Herring Gulls while taking eider eggs. The geese displaced Herring Gulls from approximately ten percent of all eider eggs taken from nests, and ate egg contents themselves. Kleptoparasitism may provide Canada Geese with an important exogenous food resource, but the small overall number of eggs stolen from gulls (less than five percent of all eider eggs laid) limits the potential consequences of additive compensatory predation pressure by gulls within the eider colony.
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Vol. 25 • No. 2