Influence of Group Size and Habitat Type on Reproductive Success in Common Murres (Uria aalge)

On Tatoosh Island, off the Olympic Peninsula in Washington, Common Murres (Uria aalge) nest either in small subcolonies located in crevices in vertical, rocky cliff walls, or in large subcolonies located on the island top, usually at cliff's edge. Nesting murres are subject to predatory pressure from Glaucous-winged Gulls (Larus glaucescens) and Northwestern Crows (Corvus caurinus), which are resident egg predators, as well as from Peregrine Falcons (Falco peregrinus) and Bald Eagles (Haliaeetus leucocephalus), which are transient predators on adults. Although subcolony size dilutes the effects of egg predators, the presence of raptors in the system negates this simple effect of dilution. Relative to conspecifics in crevice subcolonies, murres in cliff-top subcolonies are more likely to be attacked by raptors, as well as to temporarily evacuate the subcolony in response to the presence of raptors. During subcolony evacuations, egg predators gain unhindered access to abandoned eggs. As a result, murres nesting in cliff-top subcolonies experience both delays in phenology and lowered reproductive success relative to murres nesting in crevice subcolonies. Despite continued raptor-facilitated egg predation, the majority of the murre population nests in cliff-top subcolonies, which are less than 20 years old and are growing rapidly. This suggests that new colonists are constrained by a lack of accurate information into making incorrect decisions about which subcolonies to join.