Hormones and behaviour: a comparison of studies on seasonal changes in song production and testosterone plasma levels in the Willow Tit Parus montanus

Abstract

Two previously published studies giving observations on the annual cycle of song production and seasonal variations in testosterone plasma levels in the Willow Tit Parus montanus were compared in an attempt to elucidate the still poorly understood connection between hormones and behaviour. In Willow Tit males, the annual cycle of song production runs parallel to seasonal variations in the plasma levels of testosterone, showing peaks in both spring and late summer. In females, increased testosterone levels could be established only in late summer, but, as in the males, the increased levels in late summer are due exclusively to juvenile birds. In view of the idea that song production in birds is closely associated with testosterone, these findings confirm the conclusions that most of the singing in spring is done by adult Willow Tit males, that the chief contributors to the song period in late summer are the juveniles of either sex, and that the singing of immature Willow Tits occurs mainly in connection with aggressive encounters . The endocrinological data agree with reports of increased song activity in young Willow Tit males and females in late summer.