Breeding synchrony and nest predation in Red-winged Blackbirds

Three previous studies have disagreed on whether breeding synchrony increas es nest success through reduced nest predation in Red-winged Blackbirds (Ag elaius phoeniceus) in eastern North America. We used data from an 11-yr stu dy of redwings in eastern Ontario, Canada, to duplicate the analyses of the one study (by D. E Westneat) that did show that nesting synchrony reduced predation. We found no evidence that nest predation and nest success were a ffected 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 a ttributable to differences in the principal nest predators. Mammals appeare d to be important nest predators in our study, as is typical for eastern po pulations of Red-winged Blackbirds. Mammals appeared to be less important n est predators in Westneat's study, probably because that study area was fen ced and the grass between ponds was mowed. Red-winged Blackbirds mob avian predators, but have limited ability to defend nests from mammals. Group mob bing of avian predators could result in a positive relationship between nes ting synchrony and nest success when birds are the most important nest pred ators. The greater importance of avian nest predators in western than in ea stern populations of Red-winged Blackbirds could account for some differenc es in social polygyny between those populations. However, our results indic ate that nesting decisions by female redwings in eastern populations should not be affected by the nesting activity of other females. Thus, social pol ygyny in this population does not arise through an advantage of group livin g.