Testosterone and helping behavior in the azure-winged magpie(Cyanopica cyanus): natural covariation and an experimental test

In this study, we investigated patterns of natural covariation between testosterone and reproductive status in a cooperatively breeding bird species, the azurewinged magpie (Cyanopica cyanus). To assess the relationship between testosterone and breeding behavior,we also manipulated testosterone (T) levels early in the season (before breeding started) using testosterone-filled or empty implants.

Carlos de la Cruz et al, Behav Ecol Sociobiol (2003) 55:103-111