Determining the reproductive behaviour of individual brown-headed cowbirdsusing microsatellite DNA markers

The reproductive behaviour of brood-parasitic birds has been of long-standi ng interest to evolutionary biologists but key features such as the mating system, patterns of host use and degree of overlap of female egg-laying ran ges are largely unknown for particular species. Here we use microsatellite DNA markers to obtain such information for one of the best-studied brood pa rasites, the brown-headed cowbird, Molathrus ater. Parentage analyses of yo ung and adults from a marked population at Delta, Manitoba for I year revea led the following key features of the reproductive behaviour of this specie s. (1) Monogamy is the predominant genetic mating system in this population , although matings by a single male with more than one female occur infrequ ently. (2) There is substantial variance in reproductive success among indi viduals, 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 produ cing offspring. (3) Nonbreeding adults are seen less frequently and have sh orter periods of residency on the study site suggesting they may form a sub population of 'floaters'. (4) Roughly half the females on site laid their e ggs 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 combi nations of females some of whom are host generalists and others host specia lists. (5) Female egg-laying areas rarely overlap. (6) Realized annual fecu ndity of females may be lower than previously suggested. These results prov ide important information that can be used to clarify the mechanism and the selection pressures that have led to the evolution of different features o f the reproductive behaviour of these birds. (C) 1999 The Association for t he Study of Animal Behaviour.