Do Female Red‐winged Blackbirds Engage in a Mixed Mating Strategy?

Abstract

AbstractParentage analyses of broods of nestling red‐winged blackbirds (Agelaius phoeniceus) revealed that extra‐pair fertilizations (EPFs) accounted for 24% of the offspring. 8% of attempted copulations and 13% of male courtship displays during observations of focal females were by extra‐pair males. In addition, mates and non‐mates often chased and occasionally made physical contact with females; 34% of those chases in which contact was made were extra‐pair chases. Females behaved variably during both within‐pair and extra‐pair events; females crouched less and resisted more frequently during extra‐pair courtship than during within‐pair courtship. All extra‐pair events, whether natural or induced by male removal, were either resisted or accepted by the female. In 318 focal female‐hours of observation during the fertilizable period, no female was ever seen in another male's territory soliciting a copulation. In addition, removal of females' mates resulted in frequent extra‐pair courtship and copulation; all of these occurred on the removed male's territory. Some females left their mates' territories on occasion — these forays were nearly always off the study area, no female was ever seen copulating with an extra‐pair male while on these forays, and neither the frequency nor the duration of female forays correlated with the frequency of extra‐pair fertilizations within broods. There were no associations between extra‐pair fertilizations and female age, settlement order, nest order, or clutch size. The number of fledglings produced from a nest was significantly positively associated with the number of sires of the brood. Fewer offspring apparently starved in broods that were multiply sired, yet males did not provide courtship feedings during either within‐pair or extra‐pair copulations, nor was any paternal care provided to young sired through extra‐pair matings. The frequency of infertile eggs was low (< 1%); in those instances of infertile eggs the territory owner sired some young in the same nest or another nest on his territory. Fewer broods were a mixture of within‐pair and extra‐pair paternity than expected by chance. Clear evidence implicating a mixed strategy on the part of females could not be gathered. Because females behaved variably and because not all costs and benefits to females of extra‐pair copulations could be measured, it remains possible that female behavior patterns are either (1) part of a mixed strategy, or (2) part of a strategy minimizing the costs of copulation.