Nest predation and cowbird parasitism in song sparrows

Obligate brood parasitic birds may improve their reproductive success if th ey cause the failure of nests that they had not parasitized, because many h ost species renest soon after failed nesting attempts and replacement nests may be available for future parasitism. Presently there is conflicting evi dence on a key correlate of this parasite-predator hypothesis, namely wheth er parasitized nests survive at higher rates than non-parasitized nests. Us ing data collected by volunteers for the Cornell Nest Record Program and by examining nest survival in a northeastern population of Song Sparrows (Mel ospiza melodia) where parasitism by Brown-headed Cowbirds (Molothrus ater) was common, I found that parasitized sparrow nests in the nestcard sample w ere more likely to survive at the early stages of the nesting cycle than no n-parasitized nests. In addition, while overall reproductive success in the focal study population did not differ significantly between parasitized an d non-parasitized hosts, non-parasitized nests were significantly more like ly to fail due to predation prior to hatching than non-parasitized nests. W hether the correlation between the presence of parasitic eggs and decreased predation occurs due to cowbirds causing the failure of non-parasitized ne sts or, alternatively, preference for safe host nests and/or the protection of parasitized nests by parasitic females, will require direct observation s and experimental manipulations.