Host use and fecundity of individual female brown-headed cowbirds

Descriptions of the laying behaviours of obligate brood parasites such as the brown-headed cowbird, Molothrus ater, typically fall into one of two categories. These categories are ‘shotgun behaviour’, where females are predicted to distribute their eggs apparently at random between available host nests, or ‘host selection behaviour’, where females lay their eggs discriminately, placing them into the most suitable host nests. To test these hypotheses, data on individual female fecundity and host use patterns are required, but until recently, such information has been largely unavailable. We used field observations and genetic parentage analysis to describe the fecundity and laying behaviours of individual female cowbirds over six breeding seasons at Delta Marsh, Manitoba, Canada. Although some females used more than one host species both within and between breeding seasons, other females parasitized specific host species regardless of their availability. This nonrandom host use suggests some degree of host selection in this population. We also found that realized female fecundity was low (X SD=2.3 0.6 eggs per female) compared with previously published estimates. Our results suggest that female laying behaviour lies on a gradient between the two extreme categories of pure ‘shotgun’ or ‘host selection’ laying behaviours, and females may optimize their reproductive effort by varying their behaviours as environmental conditions dictate. When high-quality hosts are not available for parasitism, cowbirds may switch to parasitizing lower-quality hosts. Flexibility in cowbird laying behaviour may be an adaptation that enables individuals to successfully exploit local and temporal variation in the availability of different hosts.