Opportunistic conspecific brood parasitism in a box-nesting population of Prothonotary Warblers (Protonotaria citrea)

Conspecific brood parasitism (CBP), although prevalent in some avian taxa, is easily overlooked when it occurs in low frequencies, and therefore the ecology of this behavior has only occasionally been described in passerines. We describe the occurrence of CBP in a population of Prothonotary Warblers (Protonotaria citrea) breeding in nest boxes, demonstrate associated fitness costs, and investigate parasite strategy. We genotyped individuals at 6 microsatellite loci and used Cervus software to determine log-likelihood of maternity (LOD scores) for offspring and social mothers. We set critical cutoff LOD scores at 95% confidence for exclusion of the social mother and assignment of a parasite mother from the breeding population. Of 805 nestlings (233 family groups during 2009–2013), we found that 12.7% had genotypes that were incompatible with their social mother. Females with unrelated nestlings (hosts) fledged fewer biological offspring within the host year than nonhost females despite fledging mo...