Bill Color, Reproduction and Condition Effects in Wild and Domesticated Zebra Finches

Abstract

Bill-color variability has the identical range and similar distributions in free- living Australian Zebra Finches (Taeniopygia guttata) and their wild-type domesticated de- scendants. Individual differences in bill color exist among adults of both sexes, both in nature and captivity. In laboratory birds, bill color changed over the course of the five-week breeding cycle, with lowest bill-color scores expressed at the end of the cycle. Longer-term patterns included a gradual decline of bill color over the course of multiple clutch attempts, followed by a rapid increase when resources for breeding were withdrawn. Among laboratory males, survivorship was clearly independent of bill color. Among females, bill color changed more rapidly in birds that subsequently died than in those that survived a two-year breeding experiment. High rates of reproduction were significantly associated with decline of male bill score, but not female bill score. The bill-color scores of laboratory males maintained on supplemented and basic seed diets for eight weeks did not diverge. Crowding of laboratory birds was associated with decreased bill color. For birds in nature, bill color tended to decline over the breeding season. Bill color of captive wild birds became more red over a six-week period when birds were fed ad libitum on the laboratory diet. Data for both laboratory and wild birds indicate that reproduction is associated with a decline of bill color in both sexes. Results of diet experiments were inconclusive, but helped to establish that the range of bill colors displayed by domesticated birds is similar to that encountered in nature. Our results, when considered in light of previous findings, suggest the possibility that bill color has different costs and benefits for the sexes and that genetic and/or physiological constraints prevent optimal phenotypic expression of bill color in Zebra Finches. Received 4 June 1990, accepted 21 July 1991.