Kunc, H. P., Foerster, K., Vermeirssen, E. L. M., & Kempenaers, B. (2006). Experimentally elevated plasma testosterone levels do not influence singing behaviour of male blue tits (Parus caeruleus) during the early breeding season. Ethology, 112(10), 984-992. doi:10.1111/j.1439-0310.2006.01254.x.