OBLIGATE BROOD PARASITES lay their eggs in the nests of other species, leaving incubation and care of the parasitic young to surrogate hosts. Because they lack parental obligations found in ~99% of other bird species (Payne 1977), re- search of obligate brood parasites has the poten- tial to contribute to our understanding of basic avian behavior, particularly regarding theories concerning mating and spacing patterns. The potential to increase fitness through parental care, despite the ensuing reduction of additional mating opportunities, is a critical component of avian mating system hypotheses (Orians 1969, Emlen and Oring 1977, Wittenberger and Tilson 1980). Monogamy is thus theoretically improb- able for brood parasites that are free from con- straints of parental care (Yokel 1986). Limiting ABSTRACT.—We characterized several equivocal aspects of the breeding biology of the brood parasitic Brown-headed Cowbird (Molothrus ater) at a study site in northeastern Illinois. A total of 175 offspring and a partial sample of parents were sampled and genotyped at six microsatellite loci. A combination of sibling-group and parentage assignment enabled us to characterize mating behavior and area used for reproduction by breeding adults, even for those not sampled. We assigned a mean of 4.7 (range 1–13) and 4.4 (range 1–16) offspring to 33 female and 32 male parents, respectively. Adults typically reproduced with a “primary part- ner” but up to three partners were common. Offspring females and males were spread over 9 and 12 ha, respectively. Half of the polygynous males produced offspring with an additional mate that parasitized nests near to or within areas overlapping those of their primary partner. That suggests that mate choice takes place at females’ egg-laying areas as opposed to more re- mote social areas. Multiple females frequently parasitized a single host nest and areas used for reproduction overlapped extensively for individuals of either sex. High frequencies of parasit- ism and superparasitism indicate a high cowbird density relative to that of hosts. Frequent promiscuity by cowbirds at our site contrasts with other studies reporting monogamy and may be due to higher densities of breeding cowbirds at our site. Recieved 15 March 2002, accepted 5 February 2003. RESUMEN.—En este estudio caracterizamos varios aspectos equívocos sobre la biología repro- ductiva del parásito de nidos Molothrus ater en el noreste de Illinois. Se muestrearon un total de 175 crías y una muestra parcial de los padres para determinar sus genotipos con base en seis loci de microsatélites. Usando grupos de hermanos y asignación de parentela, caracterizamos la conducta de apareamiento y el área utilizada para la reproducción por los adultos, incluso para aquellos no muestreados. Asignamos un promedio de 4.7 (rango 1–13) y 4.4 (rango 1–16) crías a 33 hembras y 32 machos, respectivamente. Los adultos generalmente se aparearon con una “pareja principal”, pero comúnmente tuvieron hasta tres parejas. La progenie de hembras y machos se extendió sobre un área de 9 y 12 ha, respectivamente. La mitad de los machos poligínicos produjeron progenie con una pareja adicional que parasitó nidos ubicados cerca o dentro de áreas que se sobreponían a las áreas de la pareja principal. Esto sugiere que la elec- ción de pareja se lleva a cabo en las áreas en donde las hembras ponen los huevos y no en áreas sociales más remotas. Frecuentemente, varias hembras parasitaron un mismo nido y las áreas utilizadas para la reproducción se sobrepusieron extensamente para individuos de ambos sexos. La alta frecuencia de parasitismo y súper-parasitismo indican una alta densidad de M. ater con relación a la de sus hospederos. La frecuente promiscuidad de M. ater en nuestro sitio de estudio contrasta con otros estudios que reportaron monogamia, lo que podría deberse a las altas densidades de individuos reproductivos en nuestro sitio de estudio. Department of Biological Sciences (MC 066), University of Illinois at Chicago, 845 W. Taylor Street, Chicago, Illinois 60607, USA BILL M. STRAUSBERGER 1 AND MARY V. ASHLEY BREEDING BIOLOGY OF BROOD PARASITIC BROWN-HEADED COWBIRDS (MOLOTHRUS ATER) CHARACTERIZED BY PARENT– OFFSPRING AND SIBLING-GROUP RECONSTRUCTION The Auk 120(2):433–445, 2003 1 Present address: Zoology Department, Bird Division, The Field Museum, 1400 S. Lake Shore Dr., Chicago, Illinois 60605-2496, USA. E-mail: bstrausberger@fmnh.org 433 Downloaded from https://academic.oup.com/auk/article-abstract/120/2/433/5562132 by guest on 11 June 2020