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 singing2–4, 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 paired6,7. 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.