9 Maintenance of egg temperature

Contact incubation of eggs is one of the most conspicuous features of avian biology. Its purpose is the maintenance of a warm and steady egg temperature, and during incubation, the parent bird undergoes remarkable changes in its behaviour and physiology, all seemingly directed to meeting this need. For example, the pectoral skin of incubating birds, commonly the female, but in some instances the male as well, develops into a fleshy and well-vascularised brood patch, naked of feathers (White and Kinney 1974; Grant 1982; Tøien et al. 1986; Chapter 8). While incubating, the brood patch is pressed against one surface of the egg, warming it, a process known as contact incubation. The transfer of heat into the egg is regulated through adjustments to both blood flow through the brood patch and heat production by the parent, mediated through temperature sensors in the skin of the brood patch (Collias 1964; White and Kinney 1974; Midtgard et al. 1985; Chapter 8). Likewise, many birds construct nests constructed to insulate the eggs against losses of heat from their exposed surfaces. In some instances, the nest completely encloses the egg in its protected environment, although many nests are cup-shaped, open at the top to accommodate the parent while it sits on the eggs (Collias 1964; Grant 1982; Skowron and Kern 1984). Many birds are steady incubators, sitting on the eggs without interruption from the completion of the clutch to hatching. However, many are intermittent incubators, leaving the nest periodically during the day to feed or defend the territory around the nest. These absences seem timed to limit the extent to which the eggs cool during the absence: in colder conditions, the absences are shorter, while warmer conditions are correlated with longer absences (Turner 1994a; Chapters 6 and 15). Incubation involves a transfer of heat between the parent and embryo, which in principle can be understood as a physical process mediated by the physiology of both parent and embryo. The intention of this chapter is to provide a brief overview of the physical and physiological principles underpinning the maintenance of egg temperature. These principles have potentially far-reaching implications for the ecology and life histories of birds. Thus, it is essential that they are properly understood from the outset. Many attempts to make this link rely on erroneous assumptions about the ways heat flows through eggs, usually from a misplaced desire for simplicity. The transfers of heat between parent and egg are marvellously subtle, however, and it is argued that these subtleties are important features in how parent and offspring manage the flows of heat between them. Understanding these subtleties is impossible if they are simplified away from the beginning.