Repeated Brood Parasitism by Brown-Headed Cowbirds (Molothrus ater) at Nesting Sites of Eastern Phoebes (Sayornis phoebe) across Non-consecutive Years

Abstract The temporal and spatial patterns of avian brood parasitism can critically influence host fitness, the coevolution of parasitic strategies and host resistance, and their reciprocal effects on population dynamics. This study examined spatial patterns of Brown-headed Cowbird (Molothrus ater) parasitism on Eastern Phoebe (Sayornis phoebe) hosts in 2009–2011 near Ithaca, NY, USA, and compared them with published data from 1999–2002 at the same nest sites, encompassing a period long enough for a turn-over of most of the breeding adult host and parasite populations. Relative to non-parasitized nests, host nests, that were more likely to be parasitized annually in 2009–2011, had also been parasitized more often during years 1999–2002. These findings are consistent with the hypothesis that Brown-headed Cowbirds, even across generations, show a consistent preference for particular Eastern Phoebe nest sites, although it remains unclear what the relative roles of the biological traits of the hosts occupying those sites versus the ecological and physical characteristics of the actual nest sites may be in yielding these patterns. Nonetheless, the results imply that knowing the site-specific historys of parasitism of reusable nests and breeding sites can be used to predict variation in the risk and future impact of cowbirds on this host species.