Polygyny and extrapair fertilizations in eastern red-winged blackbirds (Agelaius phoeniceus)

Parentage of nestling red-winged blackbirds (Agelaius phoeniceus)from an eastern population was determinedusing DNA fingerprintingtechniques. Of 235 nestlings surveyed, 58 had fingerprints excludingthemale, but none excluded the female tending the nest. Data onpairing status during the female's fertilizable period was availablefor 232 offspring; 55 (25%of 1988 nestlings, 23% of 1989 nestlings)of those were sired through extrapair copulations. Of these55 offspring, 33 could be assigned to nearby territory holders;16 of the remaining nestlings may have been sired by nearbymales that were not captured. During both years, 44% of territorialmales had more than one female nesting simultaneously on theirterritory. The number of extrapair fertilizations gained bymales increased significandy with harem size in 1 year. Paternity(die proportion of nesdings on the territory sired by die territoryholder) showed a positive but nonsignificant increase widi haremsize in bodi years. There was no apparent cost in paternityfor males guarding two or more fertilizable females at the sametime. The broods of females that were fertilizable at die sametime anodier female was setding on die same territory tendedto have a greater proportion of extrapair fertilizations (0.37)than did die broods of odier females within harems (0.15). Establishedfertilizable females were chased significantly more by die territoryowner and by extrapair males when a new female was setding.There were no associations between a male's paternity or successat gaining extrapair fertilizations and his age or color-bandcombination. Overall, extrapair fertilizations had litde effecton die relationship between fledgling success and harem sizeand appeared to have a minimal impact on die overall intensityof sexual selection on males.