Determining the reproductive behaviour of individual brown-headed cowbirds using microsatellite DNA markers.

The reproductive behaviour of brood-parasitic birds has been of long-standing interest to evolutionary biologists but key features such as the mating system, patterns of host use and degree of overlap of female egg-laying ranges are largely unknown for particular species. Here we use microsatellite DNA markers to obtain such information for one of the best-studied brood parasites, the brown-headed cowbird, Molothrus ater. Parentage analyses of young and adults from a marked population at Delta, Manitoba for 1 year revealed the following key features of the reproductive behaviour of this species. (1) Monogamy is the predominant genetic mating system in this population, although matings by a single male with more than one female occur infrequently. (2) There is substantial variance in reproductive success among individuals, with only 23 out of 34 (68%) of all resident females and 21 out of 54 (39%) of all resident males in the study population identified as producing offspring. (3) Nonbreeding adults are seen less frequently and have shorter periods of residency on the study site suggesting they may form a subpopulation of 'floaters'. (4) Roughly half the females on site laid their eggs in the nests of a single host; most strikingly, the most fecund female laid all 13 of her eggs in red-winged blackbird, Agelaius phoeniceus, nests. This raises the possibility that cowbird populations may consist of combinations of females some of whom are host generalists and others host specialists. (5) Female egg-laying areas rarely overlap. (6) Realized annual fecundity of females may be lower than previously suggested. These results provide important information that can be used to clarify the mechanism and the selection pressures that have led to the evolution of different features of the reproductive behaviour of these birds. Copyright 1999 The Association for the Study of Animal Behaviour.