Co-evolutionary arms race between brood parasites and their hosts at the nestling stage

Traditionally it was thought that the arms race between brood parasites and their hosts was confined to the egg stage of the breeding cycle because many host species are able to reject mimetic parasitic eggs but they are unable to reject strongly different parasitic chicks. However, recently, new cases of chick rejection, discrimination, or mimicry, have been published confirming the possibility that an equivalent arms race to that found at the egg stage could ever be played out at the chick stage. Here, I review the evidence for the existence of a co-evolutionary arms race at the nestling stage. Recent findings include new deceiving strategies used by brood parasitic chicks, defensive strategies used by foster parents and adaptive strategies of host nestlings. This review shows that both chick discrimination and relationships between brood parasites and their hosts at the nestling stage are much more complicated than previously believed. At least in some brood parasite-host systems, an arms race at the nestling stage is working.