Fitness costs and benefits of cowbird egg ejection by gray catbirds

Gray catbirds (Dumetella carolinensis) eject over 95% of brown-headedcowbird (Molothrus ater) eggs placed into their nests. Ejectionbehavior could be maintained by selection from either: (1) cowbirdparasitism, if the costs of accepting a cowbird egg outweighthe costs of ejecting it, or (2) conspecific parasitism, ifsuch parasitism occurs naturally and results in ejection. Thisstudy tested the above hypotheses by measuring the cost ofacceptance of cowbird parasitism (n= 38 experimentally introducedcowbird chicks) and of cowbird egg ejection (n = 94 experiments),as well as the frequency of natural conspecific parasitismamong 229 catbird nests observed and the frequency of conspecificegg ejection (n = 27 experiments). The conspecific parasitismhypothesis was not supported because catbirds accepted all foreignconspecific eggs placed into their nests, and no natural conspecificbrood parasitism was detected at any nests. The cowbird parasitismhypothesis was strongly supported because the cost of acceptinga cowbird chick (0.79 catbird fledglings) is much greater thanthe cost of ejecting a cowbird egg (0.0022 catbird fledglingsper ejection).