Demography and breeding behaviour of brown-headed cowbirds: An examination of host use, individual mating patterns and reproductive success using microsatellite DNA markers

Abstract

<p>I analyzed six years of data collected from a marked population of a brood parasitic bird, the Brown-headed Cowbird (Molothrus ater ) at Delta Marsh, Manitoba. I used mark-recapture techniques and observation to document annual survival probabilities and population sex structure. I used genetic markers to describe host use patterns, individual mating patterns and reproductive success. In general, males had higher survival than females and the sex ratio of the population was 1.9 males per female. Analysis of the community-level patterns of host use indicated that host choice was nonrandom. The probability of parasitism on different host species may have been influenced by the relative availability of the nests of each host species, the degree of host nesting synchrony and the quality of the host species. Analysis of individual host use behaviour showed that most females used more than one host species, but there was evidence of nonrandom host use by some individuals in this population. The mating system was variable across years, but in general, more females than males were likely to have a single mate within a breeding season but neither sex was strictly genetically monogamous. The reproductive success of both males and females increased as individuals gained additional mates. In general, the host use and mating behaviours of individual cowbirds varied across years. This flexibility may allow individuals to adapt to annual and geographical variation in the environment.</p>