AVIAN EGG MORPHOMETRICS - ALLOMETRIC MODELS OF EGG VOLUME, CLUTCH VOLUME AND SHAPE

This paper describes three comprehensive new models of the allometric relationships between egg volume, clutch volume and shape, and body we ight. Mean egg dimensions, clutch sizes and adult body weights were ob tained for 326 species, mainly of four bird types: raptors (including owls), shorebirds, frogmouths (including nightjars), and storks (inclu ding the New World vultures). These are groups in which there is a wid e range of body sizes and of sexual dimorphism in body size (in direct ion and degree). Female body weight alone accounted for 92% of the var iation in egg volume. Sexual dimorphism in body size, phylogenetic rel ationship, and clutch size were significant contributors to the model of egg volume; their addition increased the explained variance to over 98%. The model was curvilinear (quadratic) in form, rather than linea r as assumed in previous models. Larger species laid smaller eggs than expected under a simple power function. For the fitted model, within bird types, generic groupings had parallel curvilinear slopes but diff ering intercepts. Between bird types, the slopes differed. Clutch volu me was scaled to body weight; all the bird types had a common slope, w hich was curvilinear. Body weight and dimorphism accounted for 89.5% o f the variation in clutch volume. For all bird types, eggs became prop ortionally longer in shape as body weight increased, according to a si mple power law. The relevance of these relationships to hypotheses on the evolution and adaptive significance of sexual dimorphism and to th e trade-off between egg size and clutch size is discussed briefly.