Fitness costs and benefits of cowbird egg ejection by gray catbirds

Abstract

Gray catbirds (Dumetella carolinensis) eject over 95% of brown-headed cowbird (Molothrus ater) eggs placed into their nests. Ejection behavior could be maintained by selection from either: (1) cowbird parasitism, if the costs of accepting a cowbird egg outweigh the costs of ejecting it, or (2) conspecific parasitism, if such parasitism occurs naturally and results in ejection. This study tested the above hypotheses by measuring the cost of acceptance of cowbird parasitism (n = 38 experimentally introduced cowbird chicks) and of cowbird egg ejection (n = 94 experiments), as well as the frequency of natural conspecific parasitism among 229 catbird nests observed and the frequency of conspecific egg ejection (n = 27 experiments). The conspecific parasitism hypothesis was not supported because catbirds accepted all foreign conspecific eggs placed into their nests, and no natural conspecific brood parasitism was detected at any nests. The cowbird parasitism hypothesis was strongly supported because the cost of accepting a cowbird chick (0.79 catbird fledglings) is much greater than the cost of ejecting a cowbird egg (0.0022 catbird fledglings per ejection).