BADGE SIZE, PHENOTYPIC QUALITY, AND REPRODUCTIVE SUCCESS IN THE HOUSE SPARROW: A STUDY ON HONEST ADVERTISEMENT.

Honest signaling theory suggests that advertising traits must be costly to their bearer; thus, only individuals of high phenotypic quality can exhibit maximal expression of these traits. Males of the sexually dichromatic house sparrow, Passer domesticus, have a black throat patch that functions as a badge of status. I investigated whether badge size honestly shows phenotypic quality. Badge size increases with age and decreases with advancing fledging date in yearling males; thus, badge size was larger in older individuals even though age differences were small. Badge size also increased with physical condition independent of age. These results indicate that badge size functions as an honest signal, possibly because there are costs involved in its production. I also found that males with enlarged badges acquired more nest sites than either control males or males with reduced badges. However, males with enlarged badges possessing a nest site raised fewer fledglings per year than did males with reduced badges, suggesting that cheating has no selective benefit. Further studies that accurately measure the energy expenditure allocated to badge production and that quantify additional fitness components are needed to clarify how reliable badges are maintained.