Breeding synchrony and nest predation in red-winged blackbirds

Three previous studies have disagreed on whether breeding synchrony increases nest success through reduced nest predation in Red‐winged Blackbirds (Agelaius phoeniceus) in eastern North America. We used data from an 11‐yr study of redwings in eastern Ontario, Canada, to duplicate the analyses of the one study (by D. F. Westneat) that did show that nesting synchrony reduced predation. We found no evidence that nest predation and nest success were affected by nesting synchrony. We did find significant effects of female age, clutch size, and clutch initiation date on nest success, but overall, our independent variables explained very little variation in nest success. We hypothesize that the difference between our results and Westneat's may be attributable to differences in the principal nest predators. Mammals appeared to be important nest predators in our study, as is typical for eastern populations of Red‐winged Blackbirds. Mammals appeared to be less important nest predators in Westneat's study, probably because that study area was fenced and the grass between ponds was mowed. Red‐winged Blackbirds mob avian predators, but have limited ability to defend nests from mammals. Group mobbing of avian predators could result in a positive relationship between nesting synchrony and nest success when birds are the most important nest predators. The greater importance of avian nest predators in western than in eastern populations of Red‐winged Blackbirds could account for some differences in social polygyny between those populations. However, our results indicate that nesting decisions by female redwings in eastern populations should not be affected by the nesting activity of other females. Thus, social polygyny in this population does not arise through an advantage of group living.