TIME AND ENERGY IN THE INTERMITTENT INCUBATION OF BIRDS' EGGS

ABSTRACT

The intermittent incubation of an egg imposes costs on the parent in both time and energy. Heretofore, time and energy costs in intermittent incubation have tended to be considered separately, in part because no suitable theoretical framework has existed to describe how time and energy interact in the intermittent warming of an egg by a brood patch. This paper outlines a new approach to this problem, based upon the thermal properties of transient-state physical systems. Among the interesting results of this approach is a descriptive parameter, the thermal impedance, that provides a unitary description of how time and energy flow through an egg interact. Data describing the transient-state thermal properties of chicken eggs are presented, showing that an egg incubated by a brood patch behaves as a low-pass thermal filter, with a cutoff period of about 100 min. Birds have several ways of adjusting the time and energy costs of intermittent incubation. High efficiency of heat transfer from the brood patch to the egg requires a visitation cycle with a very long period, at least 2–3 time constants for at least 50% efficiency. The warmed mass of the egg can be limited by having short-period visitation cycles, but at the cost of efficient heat transfer into the egg.