Summary
Observations of radio-tagged Brown-headed Cowbirds (Molothrus ater) indicated that individual males and females engage in extended associations through the breeding season. Persistent courtship by males directed at particular females and extended time spent together by certain males and females suggests that cowbirds form pair bonds. When individuals of the pair are together, females are more likely to initiate departures than males, and males are more likely to follow their mates after they depart. These observations suggest that pair bonds are being maintained by males. There were four cases of monogamy, as well as two bigamous and at least one trigamous relationship in the local population. Radio-tracking data showed that the range of a male encompassed most or all of that of his mate(s). The mating system of this population is compared with those of other populations in an attempt to assess factors influencing the diversity of cowbird mating systems. We suggest that cowbird mating systems may be influenced by both the sex ratio of the population as well as the habitat in which populations are found.
Methodology
This study was conducted on a 20-ha study site at the Queen's University Biological Station near Chaffey's Locks, Ontario, which is described in Teather and Robertson (1985). The study site was divided into 30 x 30 m quadrats and observations were plotted relative to this grid.
Trapping was conducted in 1981 and 1982 every day or every other day from the third week in April to the end of May and periodically thereafter until the end of the breeding season. Birds were attracted to areas baited with cracked corn and were captured in mist nets or Potter traps. Individuals were banded with Canadian Wildlife Service aluminum bands and a unique combination of colored leg bands. Since leg bands were not readily discernible if birds were walking in grass, wings were marked with one or two spots of acrylic paint corresponding to leg band sequence. This enabled individual identification of birds at distances up to 75 m using binoculars (7x) or up to 150 m using a spotting scope (20x).