Nestling mouth colour: ecological correlates of a begging signal

The mouths of begging nestlings vary widely in colour, ranging from yellow in robins, Erithacus rubecula, to red in reed buntings, Emberiza schoeniclus. Two functions have been suggested for bright nestling mouth colour: (1) it may improve the detectability of chicks, particularly in poorly lit nests and (2) within species, it may signal need. We tested these hypotheses in a comparative analysis, measuring the mouth colours of nestlings from 31 species under conditions of standardized light availability and food deprivation. Changes in mouth colour signalled need only among the seed-regurgitating finches. In these species there was a 'red flush' at the onset of begging, which became redder with increasing food deprivation. No other species showed these changes, including the closely related chaffinch, Fringilla coelebs, which feeds its young insects. We found no evidence that mouth colour was correlated with the light available in the nest. We did find, however, that nestlings in darker nests improved their conspicuousness through the relative colour and size of the flange that borders their brightly coloured mouths. Nestlings from darker nests had relatively wider flanges, which were whiter and less densely coloured in relation to their mouth colour, than those of chicks reared in better illuminated nests. Clutch size was not related to mouth or flange colour, or relative flange size. We suggest that nestling mouth colour has not been selected to make chicks detectable, but that this is the function of the surrounding flange. We also discuss reasons why signals of need through mouth colour are not more widespread. (C) 1998 The Association for the Study of Animal Behaviour.