Yellow warbler defenses are retained in the absence of brood parasitism but enhanced by experience with cowbirds

Yellow warblers, Setophaga petechia, respond to brood parasitic brown-headed cowbirds, Molothrus ater, by giving "seet" alarm calls and rushing to protect their nests. These responses are typically given in populations sympatric with cowbirds, but rarely in allopatric populations. Whether the lower expression in allopatry has a genetic basis or instead could be explained by behavioral plasticity has not been investigated. We found that warblers exposed to cowbirds in Montana (sympatric) responded significantly more often to cowbird models with seet calls (85.7%) and nest protection behavior (80.0%) than conspecifics breeding in long-term isolation from cowbirds in Alaska (allopatric, 27.3% and 16.7%, respectively). Warblers breeding in Idaho, which were locally isolated from cowbirds (naive) but presumed to be genetically similar to nearby parasitized warblers, were significantly less likely than sympatric warblers to respond to cowbird models with seet calls (30.3%) and nest protection (17.3%). The lower responsiveness of naive warblers implies behavioral plasticity in trait expression, indicating that the lower responsiveness of allopatric warblers relative to sympatric warblers could be explained entirely by plasticity. The lower expression of these defenses in the absence of cowbirds may contribute to their persistence by minimizing potentially costly recognition errors.