Do Female Red‐Winged Blackbirds Limit Their Own Breeding Densities?

In territorial, polygynous species such as the Red—winged Blackbird (Agelaius phoeniceus), either of two processes might limit densities of breeding females: (1) females resident on a territory might repel new settlers (aggressive density limitation), or (2) prospective settlers might in their own interests avoid already—settled territories (passive density limitation). Two removal experiments were performed to test for either type of limitation in a population of Red—winged Blackbirds in northwestern Pennsylvania. First, randomly selected primary females were removed before nesting began. The number of newly settling females was no greater on removal territories than on controls, while remaining harems were significantly smaller on removal territories. Nesting success was no greater on removal territories, despite the reduction in breeding density. Second, one to three females were removed from randomly chosen territories later in the breeding season. Again, the number of new settlers was no greater on removal than on control territories, while remaining harems were significantly smaller on removal territories. Nesting success was no different on removal and control territories, but nest predation was significantly higher on removal territories. Two further experiments tested specifically for aggressive limitation. First, levels of aggressiveness shown towards taxidermic mounts by primary females were related to settlement patterns on their territories. Aggression score was found not to correlate with either harem size or with the length of the delay between the nest starts of the primary and secondary females. Second, primary females were treated with testosterone to increase their aggressiveness. Treated females gave significantly higher levels of aggressive displays than controls. However, harem sizes were no smaller on territories of treated females, nor did secondary females nest later on such territories than on control territories. Thus, there is no evidence that female Red—winged Blackbirds in this population are limiting their own breeding densities either passively or aggressively. The absence of passive limitation and of any negative density dependence in reproductive success weighs against the acceptance of the polygyny threshold model for this species.