On the Relation between Egg-weight and Body-weight in Birds.

Abstract

SUMMARY. 1 Heinroth's important data on the body-weight and egg-weight of 432 species of birds have been re-examined, and the figures for ten selected groups, and for the total of all groups, presented in simplified graphic form. The body-weights range from about 2 g. to 90 kg., the egg-weights from about 0.3 g. to over 1 kg. The relative egg-weight decreases from 12.2 per cent. in the first class to 1.8 per cent. in that of highest body-weight. 2 As already shown by Eeinroth, relative egg-weight decreases as absolute body-weight decreases. 3 The relation between egg-weight (y) and body-weight (x) within any group of birds can apparently be expressed by an equation of the form y = bxk. However, while k approximates to 1.0 for small values of x, it gradually decreases with increasing body-weight, and the limiting value for large values of x appears to be about 2/3. 4 This may indicate that two antagonistic processes are at work in deciding relative egg-size, one (possibly biological advantage in the struggle for existence) favouring growth of egg-weight in linear proportion to body-weight; the other (possibly the physiological difficulty of increasing the bulk of the already enormous egg-cell) restricting growth of egg-weight to a linear proportion to surface (of body or egg). 5 As is well known, different groups of birds show different relative egg-weights. It is shown that the difference between groups in this respect remains approximately constant through the whole range of body-size. Special conditions of life, such as nidicolous and naked condition of young (e.g. Oscines), and, still more notably, reproductive parasitism on smaller species (some cuckoos) are correlated with special values for the relative egg-weight. 6 The curves for each group are on the whole similar in shape whether the group comprises only small or only large forms. Thus between the same absolute body-weights, egg-weight increases much less rapidly in the larger members of a group of low mean size than in the smaller members of a group of high mean size (e.g. Oscines versus Columbae; Oscines, Columbae and Limicolaeversus Anseres and Otidids). 7 There are indications in some groups of the superposition of two (or more ?) curves of the above mentioned type, one for the smaller, the other for the larger forms within the group (Anseres, Colymbiformes, etc.). 8 The importance of accurate weight-measurements for various problems of general biology is emphasized.