According to the 'energetic-bottleneck' hypothesis, incubation in birds is constrained by the availability of energy. Hence, uniparental incubators are predicted to respond to a change in energy supply by adjusting, positively or negatively, the time spent warming the clutch. Energetic constraints on incubation in the great tit (Parus major) were demonstrated by heating nests, so that the night-time thermostatic component of daily energy expenditure in females was reduced by comparison with a control group. Birds in heated nests increased the time allocated to incubation during the day by 55 minutes, consistent with the predictions of the energetic-bottleneck hypothesis. Daily energy expenditure of all birds was inversely related to night-time ambient temperature, and did not differ between warmed and control birds on mild nights. When temperatures were low, however, escalation of daily costs 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 incubation, with likely implications for reproductive success.
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