Coal tits increase evening body mass in response to tawny owl calls

Current theory predicts small birds should have a reduced body mass when daytime predation risk is high. However, the influence that nighttime predators might have on changes in body mass or daytime foraging behaviour has not been addressed. We therefore studied the effect of changes in perceived nocturnal predation risk on the body mass of captive coal tits. In a soundproof room, eight coal tits were housed in individual cages and an experiment was performed in which the birds were subjected to two treatments. First, morning and evening body mass was monitored following nights that were quiet. Second, these parameters were measured following nights when the call of a tawny owl had been played once per hour. Evening body mass was 3% greater on days following owl-disturbed nights, but morning body masses did not differ between treatments. To ensure this result was a response to the owl calls per se, and not a general response to increased disturbance, a second experiment was necessary. Here the coal tits were exposed hourly to the calls of a nightjar, a non-predatory nocturnal bird, but no increases in body mass were observed compared to quiet nights. We suggest the coal tits increased body mass in response to owl calls to offset increased nighttime energy expenditure in attentive behaviour. Received: 26 July 1999 / Received in revised form: 30 October 1999 / Accepted: 29 November 1999