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Bird eggs begin to lose weight as soon as they are laid but their volume and linear dimensions do not change during incubation. The volume of an egg can be estimated within 2% from the relationship: Volume = 0.51. LB2, where L is the length and B is the breadth (maximum diameter). The fresh weight of an egg can be estimated within 2% from the relationship: Weight = Kw' LB 2, where K w is a species-specific constant that can be determined empirically or calculated from published data. Received 25 April 1978, accepted 28 October 1978. IT is frequently useful to know the fresh weight of a bird's egg. One reason is that many aspects of the biology of bird eggs can be predicted from their weight and these predicted values can be used when empirical data are lacking. Alternatively, one way to detect adaptations to unusual situations is by comparing observed values with values predicted for an average egg. Some of the parameters that can be predicted from weight are metabolic rate (Rahn et al. 1974), incubation period (Rahn and Ar 1974), water vapor conductance (Ar et al. 1974), the daily rate of water loss (Drent 1970), surface area, density, and shell weight (Paganelli et al. 1974), and the relation of egg weight to adult body weight (Huxley 1923-24, Rahn et al. 1975). Additionally, accurate values of fresh egg weight are required for the calculation of fractional weight loss from the daily rate of water loss (Rahn and Ar 1974) and the estimation of incubation age (Westerkov 1950). However, fresh egg weight can only be determined at the time of laying because the egg immediately begins to lose weight by diffusion of water vapor. This daily loss is proportional to the 0.74 power of egg weight (Drent 1970) and totals about 16% of the initial weight by the end of incubation (Drent 1975). As a consequence, while a great deal of information is available on egg dimensions, there are few reliable reports of fresh egg weight. Fortunately, the linear dimensions of eggs do not change during incu- bation, and in the present paper I show that they can be used to predict egg volume and fresh egg weight. Several authors have shown that the volume of a bird egg can be estimated from its linear dimensions (Bergtold 1929, Worth 1940, Westerkov 1950, Stonehouse 1963), and Preston (1974) suggested a more complex approach. In the present paper, I evaluate the accuracy with which volume (V) can be predicted from linear dimen- sions (L = length, B = breadth or maximum diameter), using the equation: V = Kv' LB 2