THE REPRODUCTIVE SUCCESS OF SECONDARY FEMALES RELATIVE TO THAT OF MONOGAMOUS AND PRIMARY FEMALES IN RED-WINGED BLACKBIRDS

We use two approaches to compare the reproductive success of secondary (S) females to the reproductive success of monogamous and primary (M/ P) females in two populations of the polygynous Red-winged Blackbird A gelaius phoeniceus. One approach is to calculate x, the mean reproduct ive success of all S females divided by the mean reproductive success of all M/P females. Estimates of x are corrected for seasonal declines in reproductive success of MIP females when necessary. Considering al l nests, the mean x is 0.98 for two years in a Washington population a nd is also 0.98 for four years in an Indiana population. Considering o nly successful nests, and pooling data over years, x is 0.98 for Washi ngton and 0.96 for Indiana. A second approach is to perform pair-wise comparisons of the reproductive success of each S female to the reprod uctive success of those MIP females that choose unmated males availabl e to, but not chosen by, the S female. In both redwing populations, in dividuals more often than not have higher success breeding as S female s than they would have breeding as M/P females with available unmated males. The implications of these results for polygyny models are discu ssed.