The role of nest predation shaping nest site selection and clutch size in songbirds: a comparative study between Santa Catalina and Santa Cruz Islands

PELUC, S. I.

Bodega Bay, CA

Simposio; Mathias Symposium; 2006

Nest predation is the major source of nestling mortality in passerines and should therefore be a prime determinant of traits such as nest site selection and clutch size. I compared those traits between populations of the Orange-crowned Warbler (OCWA) breeding on Santa Cruz and Santa Catalina Islands. Contrary to mainland breeding OCWAs, which are exclusive ground nesters, OCWA breeding on these islands show an unusual plasticity in nest site selection, nesting over a wide range of vegetation strata. Moreover, the islands populations are exposed to different nest predator communities: Santa Cruz is the only Channel Island where the Island Scrub-Jay, a major nest predator, breeds; while Jays are absent from Catalina and avian nest predators are very uncommon. This scenario yields two naturally dissimilar nest predation risks for OCWAs which should result in differences in their nest placement and clutch size. Our preliminary results show that as expected, OCWAs on Santa Cruz Island exposed to the visually oriented avian predators, preferentially nest on the ground and tend to have smaller clutches. On Catalina Island, warblers exposed to lower predation risk and where jays are absent, preferentially nest above the ground and tend to have larger clutches. In addition to providing valuable information on how nest predation affects avian productivity through its effects on nest site selection and clutch size, the present study generated useful information on the breeding biology of a poorly known endemic subspecies of the Orange-crowned Warbler.