The dawn chorus is linked with female fertility in the Willow Tit

We studied the dawn chorus in the Willow Tit Parus montanus, a monogamous and territorial passerine. We expected that if the song is part of sperm competition, a male should invest most in singing during the period when his female is fertile. A total of 57 pairs of Willow Tits was observed during the spring of 1992. To express singing effort, two variables characterising the dawn chorus were measured for each morning: proportion of time devoted to singing and the onset of singing. Time singing peaked about ten days before commencement of laying, and decreased afterwards. The earliest singers were observed during the laying-period. Time singing differed between male age classes; old males sang more than yearlings. Our results indicate that the dawn chorus in Willow Tits functions as interand/or intrasexual communication used in sperm competition. Since adult (high quality) males tended to invest more in singing than yearlings, it is also possible that the dawn chorus is honest advertisement of male quality.