Nest predation and cowbird parasitism in Song Sparrows

Obligate brood parasitic birds may improve their reproductive success if they cause the failure of nests that they had not parasitized, because many host species renest soon after failed nesting attempts and replacement nests may be available for future parasitism. Presently there is conflicting evidence on a key correlate of this parasite-predator hypothesis, namely whether parasitized nests survive at higher rates than non-parasitized nests. Using data collected by volunteers for the Cornell Nest Record Program and by examining nest survival in a northeastern population of Song Sparrows (Melospiza melodia) where parasitism by Brown-headed Cowbirds (Molothrus ater) was common, I found that parasitized sparrow nests in the nestcard sample were more likely to survive at the early stages of the nesting cycle than non-parasitized nests. In addition, while overall reproductive success in the focal study population did not differ significantly between parasitized and non-parasitized hosts, non-parasitized nests were significantly more likely to fail due to predation prior to hatching than non-parasitized nests. Whether the correlation between the presence of parasitic eggs and decreased predation occurs due to cowbirds causing the failure of non-parasitized nests or, alternatively, preference for safe host nests and/or the protection of parasitized nests by parasitic females, will require direct observations and experimental manipulations.