Conspecific brood parasitism and egg rejection in Great-tailed Grackles

In this study, we tested whether conspecific brood parasitism (CBP) has sel ected for egg rejection behavior in the colonial Great-tailed Grackle Quisc alus mexicanus. No evidence of CBP was recorded at 797 Great-tailed Grackle nests, and we did not induce CBP by experimentally removing nests while gr ackles were laying. We determined experimentally that Great-tailed Grackles are determinate layers, an attribute opposite to that sometimes associated with CBP. Despite the absence of CBP, Great-tailed Grackles rejected 8% of experimentally introduced conspecific eggs, rarely rejecting or damaging t heir own eggs. Conspecific eggs added to nests during incubation tended to be rejected more frequently than eggs switched between nests, and eggs that differed the most from the host's eggs tended to be rejected sooner. There was no relationship between rejection and the stage of the nest cycle when experimental parasitism occurred; however, eggs were rejected faster when added during the prelaying and incubation stages than during laying. Eviden ce suggests, therefore, that egg rejection behavior in Great-tailed Grackle s has not evolved in response to CBP.