Closer clutch inspection--quicker egg ejection: timing of host responses toward parasitic eggs

The prevalent, and so far most explored, host defense against brood parasitism is egg discrimination. Not only do the hosts differ markedly in their propensity to reject parasitic eggs but rejecters even vary in their egg rejection times. The focus of the present study was to investigate factors potentially responsible for high variation in timing of host egg rejection. As a model species, we chose the great reed warbler Acrocephalus arundinaceus, a cuckoo Cuculus canorus host, with female-restricted egg ejection behavior. We presented a cuckoo dummy near host nests and experimentally parasitized the clutches with a nonmimetic egg. Immediately afterward, we continuously video recorded host behavior to determine egg ejection times accurately. We fitted a regression tree model with the timing of egg ejection as a dependent variable and female-related characteristics (body condition, eggshell coloration, and behavior) as explanatory variables. Only female behavior toward the foreign egg proved to have a significant effect on the timing of egg ejection. Females devoting more time to clutch inspection ejected the egg significantly more quickly than females inspecting their experimentally parasitized clutches only briefly. We discuss our results in the context of known intra- and interspecific differences in host response times toward alien eggs and cognitive mechanisms involved in host egg discrimination processes. Copyright 2011, Oxford University Press.