Introduction

A bird embryo can generate only a limited amount of heat (Romijn & Lokhorst, 1960; Tazawa et al., 1988). The ability of the egg to dissipate heat is also limited (Spotila, Weinheimer & Paganelli, 1981; Turner, 1985; Sotherland, Spotila & Paganelli, 1987). Consequently, the temperature of an egg can be controlled only slightly by the embryo and deviate from ambient temperature by roughly -0.5 °C to +3 °C at most (Turner, 1985; Sotherland et al., 1987). Most bird embryos must develop within fairly narrow temperature limits (Barott, 1937; Drent, 1970, 1973, 1975; Deeming & Ferguson, Chapter 10), and environments that provide these temperatures are rare. It follows that an egg will rarely be at an ideal temperature for development unless fluxes of heat into and out of it are somehow supplemented and controlled. The management of heat flux into and out of the egg, effected by the brooding parent, is incubation: all other attributes of incubation (gas and water exchange and turning) are, at root, secondary to this one function.

In this chapter, I wish to address the fundamental problem of how incubation ‘works', i.e. how heat flow between an egg and incubating parent is managed. Although the thermal interaction of the embryo and the incubating parent has been extensively studied, it remains poorly understood. The physics and physiology of temperature of incubated bird eggs are examined, and the energy cost to the incubating parent of keeping the egg at that temperature, is determined.