Reduced growth but not survival of chicks with altered gape patterns: implications for the evolution of nestling similarity in a parasitic finch

Pin-tailed whydahs, Vidua macroura, parasitize the parental care of common waxbills, Estrilda astrild, throughout southern Africa. Unlike most avian brood parasites, pin-tailed whydah nestlings resemble host young, precisely matching their elaborate gape patterns. Presumably, similarity arises through selection on parasites to avoid discrimination by host parents, but the mechanisms of this selection remain elusive. To investigate the capabilities of common waxbill parents to discriminate variation in nestling morphology, I manipulated the gape flanges of their chicks. Despite substantial overall nestling mortality, I found no significant differences in survival among young whose gape flanges were coloured black, were treated with a clear sham control, or were left unmanipulated. In addition, parents did not selectively eliminate chicks with conspicuous, natural gape pattern irregularities. I did find small but significant differences in growth between black and control + unmanipulated groups for two metrics (mass and head + bill length) but not for two others (tarsus and wing length). I also manipulated the costs to parents of provisioning young by clipping their flight feathers at some nests, a procedure that should have increased the probability of rejecting young with manipulated gapes. Counter to prediction, nestlings tended to survive better in nests where parents were handicapped. Common waxbill parents, like parents in many bird species, appear to respond to variation in chick phenotypes by adjusting provisioning rates towards young. This discriminatory behaviour may help to explain the detailed similarity of common waxbill nestlings and pin-tailed whydahs.