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.