Age-dependent costs of cowbird parasitism in Yellow Warblers (Setophaga petechia)

Brood parasitism by Brown-headed Cowbirds (Molothrus ater (Boddaert, 1783)) often reduces the reproductive success of their hosts. We examined whether the ability of females to avoid or mitigate the costs of brood parasitism improved with age in a population of YellowWarblers (Setophaga petechia (L., 1766)) breeding near Revelstoke, British Columbia, between 2004 and 2011. Cowbirds parasitized 18% of Yellow Warbler nesting attempts and females rejected 24% of parasitized nests, principally by deserting the nest and initiating a new breeding attempt. We found no evidence that older females were better at avoiding parasitism ormore likely to reject parasitized nests than yearlings. On average, brood parasitism reduced clutch sizes by 0.8 eggs, had no effect on nest success, but reduced the number of young fledged from successful nests by 1.3 offspring. Despite age-related improvement in somemeasures of breeding performance, the costs of brood parasitism at each period of the breeding cycle did not vary with age. There was, however, some evidence, that brood parasitism reduced the annual productivity (total number of young fledged) of older females less than the annual productivity of yearlings suggesting that the cumulative costs of brood parasitism varied with age.