Plasticity in nest site selection in response to an avian nest predator: A manipulative experiment with Orange-crowned Warblers.

PELUC, S. I.; SILLETT, T. S.; ROTENBERRY J.

Quebec

Congreso; 122nd Meeting of the American Ornithologists' Union.; 2004

Nest predation is the major cause of reproductive failure for most open-cup nesting passerines. Selection should thus favor individuals who select safe nest sites and minimize their risk of nest predation. Ground nests usually face the lowest risk of predation, and species with flexible nest site choices should select the safest nesting stratum (e.g., ground instead of shrubs). I examined the importance of nest predation in shaping nest site selection in an open-cup nesting species, the Orange crowned Warbler (OCWA hereafter). In contrast to almost all other Vermivora breeding on mainland North America which are exclusively ground-nesters, OCWAs endemic to the California Channel Islands nest at variable heights (0 - 5 m), and breed in areas with few avian predators. I tested if the potential risk of nest predation influences nest site selection in OCWA breeding on Santa Catalina Island. I used vocalizations and mounts of the Western Scrub-Jay, a key nest predator on the mainland but absent from the Island, to experimentally increase the apparent presence of an avian nest predator in 12 OCWA territories (predator treatment). I exposed 12 OCWA pairs to vocalizations and models of a non-predator (House Finch), and 12 pairs to no decoy (control). I conducted the experiment on days when females were prospecting for nest-sites, but before they started nest building. OCWA exposed to the predator cues were expected to nest on the ground (safer stratum) relative to the non-predator or control birds, if nest site selection was influenced by predator presence. Nest heights differed significantly among treatments (Kruskal-Wallis X = 11.389, df = 2, P = 0.003). All females exposed to jays built nests 2 s on the ground. Nest heights of females exposed to finch presentations did not significantly differ from control females. The shift in nest site selection observed in OCWAs constitutes a potentially adaptive response to predator presence and provides convincing evidence that birds can recognize not only potential risks but also change their nesting behavior in a short time.