Parasitism can be defined as a biological interaction in which one party benefits at the expense of another (Keeton and Gould 1986). However unlike predation, the parasite does not kill its host. In the case of avian brood parasitism, one bird lays its eggs in the nest of a member of its own or another species and then abandons its offspring to the care of its new foster parents. These foster parents or 'hosts' then raise the parasitic young at the expense of their own brood. Brood parasitic birds have reproductive advantages over those which provide parental care for their young because: 1) parental care provided by several fosterers allows a female to produce more successful offspring than she is capable of rearing herself in one season, and 2) parasite offspring are usually distributed among many host nests thereby increasing the probability that at least some offspring will escape predation (Payne 1977). Obligate interspecific brood parasitism, where females only parasitize the nests of other species, is a reproductive strategy adopted by approximately 1% of all bird species and is practiced by members of five families (Anatidae, Cuculidae, Indicatoridae, Icteridae, and Ploceidae). The degree to which interspecific brood parasites reduce host nesting success varies with the reproductive tactics of the parasite. For example, Common Cuckoo chicks (Cuculus canorus) eject eggs and young nest mates from the host nest with the help of an instinctive urge to push out of the nest anything that touches the sensitive shallow depression in the parasitic nestling's back (Lack 1968). Young African Greater Honey-guides (Indicator indicator) stab host nestlings to death with special mandibular hooks that drop off after two weeks of age (Friedmann 1955). In contrast, black-headed duck hatchlings seek only protection and warmth for 1-2 days post hatching and then leave the nest with no further cost to their host (Weller 1968). Finally, Brown-headed Cowbird nestlings intermediately affect host reproductive success by diverting parental resources such as food away from the host's young (Payne 1977). This loss of host fitness results in selective pressure for host defenses against parasitism such as egg ejection (Neudorf and Sealy 1992), clutch abandonment (Burgham and Picman 1989), or increased nest defense early in the nesting cycle (Burgham and Picman 1989, Briskie and Sealy 1989, Neudorfand Sealy 1992). To circumvent these host responses, adaptation in brood parasites has resulted in selective pressure for egg mimicry (Rothstein 1990), egg removal, or shorter incubation periods (Briskie and Sealy 1990, Payne 1977). The Brown-headed Cowbird is the most abundant and widely distributed obligate interspecific brood parasite in North America. Although the breeding behaviour of this bird has been widely studied, most findings are contradictory. The mating system of the Brown-headed cowbird has been described as ranging from monogamous (Laskey 1950, Dufty 1982a, 1982b, Yokel 1986), to promiscuous (Elliot 1980). Most studies also suggest that cowbirds parasitize multiple host species (Friedmann 1929, p 177-188, Jones 1941, McGeen & McGeen 1968, Elliot 1977, Fleischer 1985). However, a few suggest that some individuals may be host specialists (Walkinshaw 1949, McGeen & McGeen 1968). Few of these studies have used genetic techniques to determine the actual mating patterns and to investigate the breeding biology of males and females in a single marked population. The main objective of this study was to use molecular genetic DNA markers as well as behavioural observation to study the mating system and host specificity of a Brown-headed Cowbird population at Delta Marsh, Manitoba. More specifically, my objectives were to: 1) determine whether DNA microsatellite markers are useful for determining parentage in Brown-headed Cowbird populations 2) document the genetic mating system and the patterns of host use by individual females in a population of resident cowbirds. My findings provide the first evidence that microsatellites are useful for high resolution parentage analyses in brood parasitic bird species where there is no A priori information available on male or female parentage. In addition, they are the first to directly quantity the mating system of a Brown-headed Cowbird population and to suggest that individual females are best described as host generalists but may be showing some preference for host nests in one habitat over another. / Thesis / Master of Science (MSc)