Adaptations of polygynous breeding in the bobolink, Dolichonyx oryzivorus

Abstract

In some species of polygynous songbirds, males assist in feeding nestlings of their first (primary) female but ignore offspring of subsequent (secondary) mates. A number of adaptations important in minimizing the disadvantages of a secondary status in female Bobolinks (Dolichonyx oryzivorus) have been discovered.In Bobolinks, clutches of secondary females are significantly smaller than those of primary females, thereby adjusting brood size to correspond better with food delivery capabilities of secondary hens. Commencement of incubation with deposition of the penultimate egg allows additional brood size adjustment, for the resulting hatching asynchronism promotes efficient culling of the late-hatching chick in times of food shortage, without endangering the entire brood. Secondary females differ from primary pairs in food exploitation techniques by capitalizing on insects found close to the nest and by showing less discrimination in food selection. Such behavior permits compensation in the rate of food delivery trips to secondary nestlings so as to equal the tempo maintained by both members of the primary pair, yet results in quantitative disparities in food brought to primary and secondary nestlings. Flexibility in male feeding responses, resulting in partial male assistance at secondary nests in situations where secondary brood size is exceptionally large, is also important in helping to maximize the reproductive performance of birds engaging in this mating strategy. © 1974 by the American Society of Zoologists.