Assessing the function of house sparrows' bib size using a flexible meta-analysis method

The black throat patch or bib of male house sparrows, Passer domesticus, is often referred to as a "badge of status" or a "badge" because previous studies have shown bib size to be correlated with the social status of males. Yet, little is known about how strong and robust this relationship is and how the strength of this relationship compares with that of other associations. We conducted a meta-analysis for 6 well-studied correlates of bib size: fighting ability, parental ability (egg incubation and food provisioning), age, body condition, cuckoldry, and reproductive success. We introduce a flexible meta-analysis method in this study that is better suited in the biological sciences than the methods usually employed in popular meta-analysis software because our method accounts for a common form of nonindependence of the data. The relationship between fighting ability and bib size was found to be strong and robust, and the relationship between age and bib size was moderate and robust. Also, body condition was weakly but significantly correlated with badge size. The other parameters showed nonsignificant small effects and/or large confidence intervals. Therefore, we conclude that bib size signals dominance and to a lesser extent age and possibly reflects body condition in house sparrows. There was weak evidence that bib size is currently under sexual selection because there was little association between reproductive success and bib size. This is surprising as the bib size probably affects the outcome of male--male competition. Empirical data on sparrow bib size could not be reconciled with sexual selection theory, although there is ample evidence that it is a condition-dependent trait. Copyright 2007, Oxford University Press.