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The large difference in incubation time among vapor pressure difference between the egg and bird eggs, ranging from a minimum of 11 days the microclimate of the nest is 35 torr. to nearly 90, has aroused man' s interest since antiquity. In her critical review of the history RESULTS of our knowledge of incubation periods, Margaret M. Nice (1954) wrote, "The people who

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 SchSnwetter's (1960-72) monumental description of bird eggs. With these as a major backlog, and data for the Fringillidae (Areadon 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 : aB ø. The additional data now available allow one to obtain individual regression equat•ens 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, namely 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. ME•tIODS or ANALYSIS For each order, and in the case of Passeriformes for each family, the paired values of egg weight and body weight were converted to common logarithms, and the regression of log egg weight on log female weight was derived by linear least-squares analysis. In the ordinary method of least squares, it is assumed that the independent variable (here log B) is not subject either to biological variation or to errors of measurement. The residual scatter of the points around the regression line is therefore ascribed entirely to deviations of the dependent variable (here log W) from the 750 The Auk 92: 750-765. October 1975

In a series of recent works, attention has been paid to the functional properties of the avian eggshell: water vapor and respiratory gas conductances, water loss, metabolic rate and incubation time-all these major physiological characteristics of eggs may be closely and intimately related to egg mass, which, in turn, is allometrically related to eggshell structural properties such as thickness, porosity, mass, density and surface area (Wangensteen 1972, Ar et al. 1974, Rahn and Ar 1974, Paganelli et al. 1974, Rahn et al. 1974, Ar and Rahn 1978). These structural and functional relations of bird eggs reveal some variables of importance to the physiology of the embryo, including the gradient in water vapor pressure between egg and nest, the fractional water loss constant, the constancy of gas composition in the air cell, and total oxygen consumption per gram egg during incubation. The ability to hatch successfully is the outcome of a delicate equilibrium among several factors, some of which are inherited in the structure and function of the egg itself, while others are either imposed on the egg by the environment or controlled by the incubating parents.

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