Summary
Female Red-winged Blackbirds (Agelaius phoeniceus) are often aggressive towards conspecific females during the breeding season. We hypothesize that the function of female-female aggression in this species is to guard the nonshareable portion of the male's parental investment.
The investment-guarding hypothesis predicts that a female should be more aggressive toward another female evincing interest in mating with the territory-owning male than toward a female simply perching within the male's territory. Results of mount presentations to females with active nests confirmed this prediction. Nesting females attacked a stuffed conspecific female mounted in a precopulatory, ‘soliciting’ posture significantly more often than a mount in a normal, perched posture.
The male's nonshereable parental care consists of provisioning his young, and most of this care is invested in the brood of his primary (first-to-nest) female. It is therefore predicted that primary females should be more aggressive than secondary (later-nesting) females. Female mount presentations also confirmed this prediction. Primary females attacked the soliciting mount significantly more often than secondary females.
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Yasukawa, K., Searcy, W.A. Aggression in female Red-Winged Blackbirds: A strategy to ensure male parental investment. Behav Ecol Sociobiol 11, 13–17 (1982). https://doi.org/10.1007/BF00297660
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DOI: https://doi.org/10.1007/BF00297660