Clutches of Rufous Bush Chats Cercotrichas galactotes parasitised by Cuckoos Cuculus canorus contain larger eggs

Abstract

In southern Spain clutches of Rufous Bush Chats Cercotrichas galactotes that were parasitised by Cuckoos Cuculus canorus contained larger eggs than non-parasitised ones. Three alternative hypotheses are given that can explain this result. The first states that as a result of the larger size difference with the Cuckoo's egg, parasitism in smaller egg clutches is more easily detected by the host, which then ejects the Cuckoo's egg. Such incidence of parasitism would thus go easily unnoticed to the human observer. Yet, this hypothesis was rejected, since it is known that Rufous Bush Chats readily accept Cuckoo egg models larger than their own. The second hypothesis states that the observed higher average size of eggs in parasitised nests could be due to the removal of one of the smaller eggs of the host (it appeared that Cuckoos removed only one egg of the host), resulting in an increase of the average egg size of the remaining clutch. This hypothesis was tested by comparing average egg size of parasitised clutches with that of unparasitised clutches, but excluding the smallest egg in the clutch, and could not be rejected. As smaller eggs hatch earlier, eliminating smaller eggs would perhaps withdraw competition for food of the early-born Cuckoo chick with early born Rufous Bush Chat nestlings. The third hypothesis, which could not be rejected either, is that Cuckoos choose to parasitise host pairs of superior parental quality in the benefit of the Cuckoo chick. Superior quality is then related with large egg size.