Does testosterone determine dominance in the house sparrow Passer domesticus? An experimental test

Badges of status function in many birds within a social context to establish dominance hierarchies and reduce antagonistic encounters. In order to maintain the honesty of the signalling system, such badges must be costly to produce or to maintain. The chest bib of the house sparrow functions as a badge of status and changes in size are known to be controlled by testosterone levels. We sought to test the relative importance of testosterone as opposed to bib size in determining dominance within a group of male house sparrows. We did this by manipulating testosterone levels independently during both breeding and post-breeding seasons in experimental birds and examining the effect of testosterone titre, as well as corticosterone titre relative to bib size on dominance levels. Dominance hierarchies within the groups were tested during both the breeding and post-breeding phases. We compared the results of these tests with dominance among intact (unmanipulated) birds. Results suggested that the breeding season dominance levels were largely determined by testosterone levels as well as bib size, whereas the post-breeding dominance levels were determined by postbreeding testosterone titre and previous breeding season dominance level. Within unmanipulated birds, basal corticosterone levels were significantly, negatively correlated with dominance level, but only during the breeding season. The influence of breeding season dominance on post-breeding dominance suggests social history is important in determining dominance interactions as well as current testosterone levels and bib size.