Introduction

This chapter is an account of some thoughts and ideas that have been crystallised during the many years of working the field of avian egg physiology. It cannot and will not cover all aspects of the immense quantity of published work. My choice of references is much biased by their relevance to my way of viewing the issue of embryonic development in a cleidoic system – a problem nature has solved while we are still pondering how.

Water loss from avian eggs

Homeothermy and water loss from eggs

Birds have developed an active and intensive lifestyle, which requires homeothermy. Simultaneously, by maintaining a high incubation temperature, their embryos have evolved to develop fast in comparison with reptiles of the same size (Rahn & Ar, 1974; Blueweiss et al., 1978; Deeming & Ferguson, Chapter 10).

Thermoregulation in adult birds is associated with the presence of insulating plumage, which developed to replace reptilian scales. Regarding egg thermoregulation, most birds lay eggs in the still air which acts as an insulating medium in the nest, while many reptiles lay eggs in a soil substrate. This difference is not trivial: soil, and especially wet soil, both conducts heat well and has a high heat storage capacity (Table 14.1) (Hillel, 1978). Hence, eggs in direct contact with the soil exchange heat easily and cannot become much warmer than their surroundings, even if they were actively incubated.