Heating nest-boxes reveals an energetic constraint on incubation behaviourin great tits, Parus major

According to the 'energetic-bottleneck' hypothesis, incubation in birds is constrained by the availability of energy. Hence, uniparental incubators ar e predicted to respond to a change in energy supply by adjusting, positivel y or negatively, the time spent warming the clutch. Energetic constraints o n incubation in the great tit (Parus major) were demonstrated by heating ne sts, so that the night-time thermostatic component of daily energy expendit ure in females was reduced by comparison with a control group. Birds in hea ted nests increased the time allocated to incubation during the day by 55 m inutes, consistent with the predictions of the energetic-bottleneck hypothe sis. Daily energy expenditure of all birds was inversely related to night-t ime ambient temperature, and did not differ between warmed and control bird s on mild nights. When temperatures were low, however, escalation of daily casts was less for birds in heated nests. It is suggested that the balance of the energy budget may effect a proximate control on the constancy of inc ubation, with likely implications for reproductive success.