The dawn chorus in the great tit Paras major is directly related to female fertility

Abstract

Previous explanations for the dawn chorus in birds include favourable acoustic conditions¹, low foraging profitability reducing the net cost of singing^2–4, high risk of territorial intrusion³ and unpredictability in overnight energy requirements leading to spare reserves at dawn⁴. All these hypotheses assume that males sing either to attract mates or to defend a territory and/or that singing and foraging compete for time⁵. In the case of the dawn chorus in the great tit Parus major none of these assumptions apply. A peak of song at dawn only appears in late spring when territorial boundaries are established and birds are paired^6,7. Conflict between singing and foraging cannot explain why males with more non-foraging time in longer days at northern latitudes roost earlier than birds at southern latitudes and still sing before dawn⁸. Female emergence from her nest hole terminates the dawn chorus in the great tit⁹. I show here that changes in the duration of male dawn song are closely related to changes in female fertility.