REVIEW Coevolution, communication, and host-chick mimicry in parasitic finches: who mimics whom? Mark E. Hauber & Rebecca M. Kilner Received: 2 June 2006 / Revised: 13 October 2006 / Accepted: 13 October 2006 / Published online: 21 November 2006 # Springer-Verlag 2006 Abstract Why do brood parasitic Vidua nestlings mimic the intricate gape patterns of their hosts’ young so precisely? The classic explanation is that mimicry is the outcome of a coevolutionary arms race, driven by host rejection of odd-looking offspring. Selection favors para- sitic nestlings that converge on the host young’ s mouth markings, and simultaneously benefits hosts whose mouth markings diverge from those of the parasite. The outcome is highly elaborate mouth markings in host young that are accurately mimicked by parasite nestlings. Our review of recent work provides mixed support for this traditional view and, instead suggest that complex mouth markings function to stimulate adequate provisioning, rather than to signal species identity. Thus, similarly elaborate gape morphologies in hosts and parasites could have evolved through nestling competition for parental care. According to this view, and in contrast with existing hypotheses, it is host young that mimic parasitic offspring, in order to compete effectively for food. Keywords Coevolution . Indigobirds . Parent–offspring conflict . Sibling rivalry Introduction Estrildid finches in Africa are exploited by 19 species of Vidua finch, obligate brood parasites, which typically victimize a single host species (Payne 1997). The elaborate and complex gape patterns displayed by host offspring are precisely matched by the parasite (Fig. 1), sometimes to the extent that the two species cannot easily be distinguished when they share the same nest (Sorenson et al. 2003). It calls to mind the accurate mimicry of host egg color and patterning shown by some host races of common cuckoos Cuculus canorus (Brooke and Davies 1988; Davies and Brooke 1989; Moksnes and Roskaft 1995), known to be the result of a coevolutionary arms race played out between parasite and host (Rothstein and Robinson 1998). Hosts defend themselves against exploitation by the cuckoo by removing odd-looking eggs from their nests, thus selecting female parasites whose eggs survive because they look like part of the host’ s own clutch (Davies 2000; Payne 2005b). The traditional view maintains that a similar signature forgery arms race is responsible for the evolution of the perfectly matching and elaborate gape morphologies in the parasitic Vidua finches and their hosts (Neunzig 1929; Nicolai 1964). In this review, we suggest an alternative to this classic (Lack 1968), textbook (Gill 2003) interpretation of mimicry evolution in Vidua finches. Recent observations in the laboratory and the field reveal that host estrildid finches Behav Ecol Sociobiol (2007) 61:497–503 DOI 10.1007/s00265-006-0291-0 Communicated by A. Cockburn M. E. Hauber (*) School of Biological Sciences, University of Auckland, PB 92019, Auckland, New Zealand e-mail: m.hauber@auckland.ac.nz R. M. Kilner Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK