An experimental investigation of egg rejection behavior in the grackles, Quiscalus
This study was an experimental investigation of the responses of grackles (Quiscalus) to foreign eggs in their nests. No evidence of conspecific brood parasitism was recorded at 797 Great-tailed Grackle (Q. mexicanus) nests, and I failed to induce this behavior by experimentally removing nests. Great-tailed Grackles are not indeterminate layers, an attribute often associated with conspecific brood parasites. Great-tailed Grackles rejected 8.1% of experimentally introduced conspecific eggs. Neither Bronzed Cowbird (Molothrus aeneus) nor Brown-headed Cowbird (M. ater) parasitism was recorded on Great-tailed Grackle nests. Cross-fostered Bronzed Cowbird nestlings, but not Brown-headed Cowbird nestlings, fledged from grackle nests, indicating that Great-tailed Grackles are unsuitable hosts for the Brown-headed Cowbird. Great-tailed Grackles rejected eggs via true egg recognition and populations sympatric and allopatric with Bronzed Cowbirds rejected 100% of model cowbird eggs. An allopatric population of the Boat-tailed Grackle (Q. major), a sibling species of the Great-tailed Grackle, rejected 100% of model eggs. Rejection in these grackles apparently evolved in response to Giant Cowbird (Scaphidura oryzivora) parasitism, and has been maintained by the Boat-tailed G ackle in the absence of parasitism for at least 10,000 years since it split from the Great-tailed Grackle. The Common Grackle (Q. quiscula), which lays the most variable eggs among the grackles, is the only grackle that has lost most of its rejection behavior. With extreme intraclutch egg variation, Common Grackles may be more likely to reject their own aberrant eggs, which would select against rejection behavior in the absence of parasitism. These results have significant implications for host-parasite cycles. Finally, a review of the correlates of egg rejection in Brown-headed Cowbird hosts revealed that historic contact, body size, taxonomic affiliation, and bill size are correlated with egg rejection. These results support evolutionary lag as an explanation for the acceptance of cowbird parasitism by most species, although hosts with eggs that resemble cowbird eggs may be in an evolutionary equilibrium with cowbirds.