Costs and benefits of subadult plumage in mute swans: Testing hypotheses for the evolution of delayed plumage maturation

In some avian species, young birds capable of reproducing diminish their pr ospects of doing so by molting into a subadult plumage that accurately sign als their subadult status. Several hypotheses have been proposed to explain the evolution of delayed plumage maturation, but testing them usually has involved interspecific comparisons that are hard to interpret. Mute swans ( Cygnus olor) exhibit two phenotypes that differ in whether the birds have a gray subadult plumage (SAP phenotype) or molt immediately into an all whit e adult plumage (AP phenotype). The AP phenotype results from a recessive a llele on the X chromosome; both phenotypes occur in the same population and even in the same brood. We compared costs and benefits of both phenotypes in mute swans on the Chesapeake Bay in 1972-1980 and on Long Island Sound i n 1982-1989. Swans with the SAP phenotype had higher survival rates from ha tching to fledging than AP swans. In the fall, when AP cygnets began to mol t into their white plumage, their parents often attacked and drove them off while allowing SAP cygnets from the same brood to remain on their territor ies for several more months. SAP males had higher survival rates during the ir first 2 yr of life than AP males, but AP swans bred at a younger age tha n SAP swans. The only proposed hypothesis for the evolution of delayed plum age maturation that can explain its occurrence in mute swans is the status- signaling hypothesis. This hypothesis argues that males with subadult pluma ge honestly advertise their age and subordinate status while AP swans are c heaters and engaging in dishonest communication. SAP males acquire a longer period of parental care, suffer less aggression from older birds, and incr ease their survival but forgo the opportunity to breed at an early age. Thi s is a unique example of how a single gene resulted in either honest or dis honest communication, changed a bird's relationship with its parents and po tential mates, and altered the bird's chances to survive and to reproduce.