Grasp-ejection in two small ejecters of cowbird eggs: a test of bill-size constraints and the evolutionary equilibrium hypothesis

Many host species accept the costs of avian brood parasitism even though rejection appears to be the adaptive response. Acceptance is often attributed to an evolutionary lag in the appearance of rejection behaviour. Alternatively, the evolutionary equilibrium hypothesis explains acceptance as adaptive because the costs of rejection outweigh those of acceptance. Equilibrium assumes that bill size limits small hosts to more costly rejection methods, such as puncture-ejecting parasitic eggs. We tested whether warbling vireos, Vireo gilvus, and Baltimore orioles, Icterus galbula, the smallest puncture-ejecters of brown-headed cowbird, Molothrus ater, eggs, are capable of grasp-ejection by using solid, plaster cowbird eggs and video analysis. We also tested whether the depth of oriole nests prevents grasp-ejection. Vireos ejected significantly more plaster eggs than did orioles. Most (91%) plaster eggs were ejected by vireos with no cost and grasp-ejection was confirmed, indicating that bill size does not constrain grasp-ejection. The grasp index of warbling vireos was at least 33% smaller than that of previously known grasp-ejecters. Thus, many species should be capable of grasp-ejection, which indirectly supports evolutionary lag. Orioles ejected an intermediate proportion (45%) of plaster eggs and ejection method was not confirmed. Nest depth was not supported as an explanation for acceptance by orioles. Because a few large and small ejecters of cowbird eggs appear to use both grasp- and puncture-ejection, we suggest that puncture-ejection may have evolved first, representing an initial selection pressure for cowbird eggshell strength.