Adaptive phenotypic plasticity in an island songbird exposed to a novel predation risk

Avian nest site selection and levels of parental care require assessments of numerous fitness costs and benefits. Nest site selection in open cup-nesting species is considered a relatively conservative trait; most species and genera are confined to nesting within particular vegetation strata. The nesting stratum further determines risk to nest predation, the principal cause of reproductive failure. We document predator-induced plasticity in nest site placement and levels of parental care in orange-crowned warblers (Vermivora celata) on an island lacking avian nest predators. We show a shift from ground nesting, characteristic of mainland populations, to off-ground nesting that appears adaptive relative to higher predation levels of ground nests. By altering the perceived nest predation risk via experimental introduction of a model avian predator prior to nest building, we demonstrate that warblers shift nest sites to more concealed ground locations. Moreover, warblers differentially adjust nest visits to feed nestlings in the presence of the predator: reducing feeding more at less concealed off-ground nests than at more concealed ground ones. Both shifts in nest site placement and feeding rate adjustments suggest adaptive phenotypic plasticity in response to increased perceived predation risk, providing evidence that birds continuously assess variation in the fitness costs and benefits of their behavioral decisions.