Authors
  • Eckert, Christopher G.
  • Weatherhead, Patrick J.
Universities

Summary

The ideal dominance distribution model predicts that competition between individuals of a species for territories will result in socially dominant individuals acquiring territories in higher quality habitat than their subordinates. Although the dispersion and relative reproductive success of male red-winged blackbirds (Agelaius phoeniceus) across habitats in eastern Ontario appears to conform to the ideal dominance distribution model, data from a study of three captive groups comprised of males from both high (marsh) and low (upland field) quality habitats failed to support the prediction that males from marsh habitat are dominant to those from upland habitat. Contrary to the prediction males from uplands were generally dominant to males from marshes. We found a significant positive correlation between dominance and both increased epaulet size and increased body size. Controlling for these positive effects, upland males remained generally dominant to marsh males. Measurements of independent samples of males from both habitats indicated that the overall distribution of males does not conform to ideal dominance. We suggest that the strong between-year territory fidelity shown by male red-winged blackbirds and chance events when they initially acquire territories may contribute to this lack of conformity.

Methodology

During the summer of 1985 we captured 3 groups of territorial males using mist nets and decoy traps. All males were defending territories in either highly productive marshes or marginal upland fields within a 10 km radius of the Queen's University Biological Station, located 40 km north of Kingston in eastern Ontario. Each group consisted of a similar number of males from both habitats (marsh/upland : group I = 5/3, group II = 5/ 5, group III = 4/6). Aside from habitat, and habitat-related differences in morphology, we wanted to minimize the effect of any other potentially confounding factors. To minimize the effects arising from differences in residency (Searcy 1979a; Yasukawa and Bick 1983) all males in each group were captured and introduced into the aviary on the same day (I: 6 June, II: 27 June, III: 9 July). To minimize any effects of prior acquaintance of some of the males within a group, males from the same habitat were captured from different sites so that no two males resided in the same marsh or field. However, both marsh and upland sites were within the same general locale, so that all males would have become aware of the different habitats available during territory establishment in the early spring. We should note that in our study area marsh and upland habitat occur in a small scale mosaic such that both habitats are always in close proximity.

Location