21 - Why birds lay eggs

Summary

Introduction

It has long been a puzzle why ‘birds constitute the only vertebrate class that is both rich in species number and exclusively oviparous' (Blackburn & Evans, 1986). Viviparity has evolved in all other vertebrate classes and has evolved independently 93 times among the reptiles (Shine, 1985, Chapter 22), the ancestors of birds. The absence of avian viviparity has frequently been explained by citing morphological or physiological factors which presumably are incompatible with live birth. However, as Blackburn & Evans (1986) show, such arguments cannot be defended and thus argue that birds have achieved most of the advantages of viviparity ‘by such specialisations as endothermy, egg incubation, nest construction, uricotelism, shell pigmentation, parental care, altricial hatchlings, albumen provision, and calcareous eggshells'. Other interpretations are discussed by Anderson, Stoyan & Ricklefs (1987) and Dunbrack & Ramsay (1989).

Whatever the reasons for maintenance of oviparity, their success is obvious judging from their colonisation in all parts of the world and the large number of species that have evolved, more than double the number for mammals. One can also look at birds as successful competitors with mammals by adapting incubation behaviour, shell structure and shell porosity to various environmental conditions to deliver hatchlings which are behaviourly, functionally and in gross composition similar to mammalian neonates. Some common features are: 1) Avian embryos are maintained at temperature of about 36°C and mammalian embryos at 37°C.) During late stages of development, the O₂ and CO₂ tensions of the embryonic and extraembryonic blood vessels in avian embryos attain values similar to those in the mammalian fetus.