THE FREQUENCY OF CONSPECIFIC BROOD PARASITISM AND THE PATTERN OF LAYING DETERMINANCY IN YELLOW-HEADED BLACKBIRDS

In altricial birds, conspecific brood parasitism (CBP) is disproportionately common in species that nest in colonies. We investigated the frequency of CBP in a colonial icterid, the Yellow-headed Blackbird (Xanthocephalus xanthocephalus). Using two criteria for detecting parasitism, we found no cases of CBP in a sample of 69 nests monitored for 522 nest-days. To make sense of this finding, we consider why specific forms of CBP may be lacking in this species. Egg removal experiments demonstrated that Yellow-headed Blackbirds are determinate layers, so a nesting female that also laid eggs parasitically would either suffer a reduced clutch size in her own nest or suffer a delay in initiating her own clutch. Neither territories, nor nest-sites, are limiting for female Yellow-headed Blackbirds, so parasitism by floater females is not expected. Although the destruction of clutches during laying was common during the study, we failed to see parasitism associated with this nest loss. Current information suggests that nest loss may not play an important role in promoting conspecific parasitism in most species.