Relation of Avian Egg Weight to Body Weight

Abstract

JUST over 50 years ago Heinroth (1922) published the first extensive list of egg weights and adult body weights for 427 species of birds. Since then additional data have appeared and the most extensive list can now be found in Schonwetter's (1960-72) monumental description of bird eggs. With these as a major backlog, and data for the Fringillidae (Amadon 1943); the Sphenisciformes, Anseriformes, and Procellariiformes (Lack 1968); and Falconiformes (Mebs 1964), more than 800 egg weightbody weight correlates are now available. Our endeavor has been to describe the relationship between egg weight and body weight in mathematical terms that are amenable to further refinements when additional data became available. We have not been concerned with explaining the relationship as Lack (1968) has done in his elegant analysis, but have rather attempted to find out what common principles might emerge from this particular relationship. Our analytical approach is basically the same as suggested originally by Huxley (1923-24), namely to plot log (egg weight) against log (body weight) and to derive a regression equation that expresses egg weight, W, as a function of body weight, B, raised to a power: W = aBb. The additional data now available allow one to obtain individual regression equations for many orders and families. As Amadon (1943) had anticipated in his review of Huxley's analysis and as Lack (1968) has recently shown for many orders and families, each group of related birds has its characteristic proportionality constant, a. On the other hand, our analysis indicates that the power, b, is most likely the same for all groups, inamely 0.675. In addition, the relation of incubation time to body weight is derived (Rahn and Ar 1974); incubation time is shown to be proportional to body weight raised to the 0.166 power. Thus a 10-fold increase in body weight is in general associated with a 4.73-fold increase in egg weight and a 1.47-fold increase in incubation time.