Effects of testosterone on growth, plumage pigmentation, and mortality in Black-headed Gull chicks

In the Black-headed Gull Larus ridibundus, sibling chicks defend small territories against conspecifics with testosterone-dependent aggressive behaviour. The energetic requirements for the performance of this behaviour may trade off against the energetic requirements for growth. There are indications that testosterone suppresses growth in birds and, therefore, regulate this trade-off. In this study, the effect of testosterone on growth and plumage pigmentation of Black-headed Gull chicks was analysed. Young chicks in small groups were treated for ten days with testosterone or sham treated. Testosterone-treated birds showed decreased growth rate (daily increase in body mass, head-bill length and tarsus-length) and a marked decrease in juvenile pigmentation of the plumage (tail-bar, back, and secondary coverts). Field measurements revealed a negative correlation between nest density, which correlates positively with aggressive behaviour of adults, and plumage coloration. Furthermore, these measurements showed an increase in mortality of chicks that had low levels of pigmentation early in life. The data suggest that chicks face a testosterone-regulated trade-off between growth and territory defence.