Many morphological and physiological parameters of avian eggs are known to be correlated with fresh egg weight, i.e, to exhibit allometric relationships. It has been predicted that part of the variability around some of these allometric relationships is the result of adaptations to incubation periods of different lengths. I performed multiple regressions of three of these parameters (the rate of water loss in the nest, conductance of the shell to gases and the rate of O2 consumption just prior to hatching) versus weight and incubation period to test these predictions. All these parameters were significantly correlated with weight and for eggs of the same weight inversely correlated with the length of the incubation period. Other investigators have reported allometric interrelations between weight, life span and other parameters. For example the average number of heartbeats is the same in mammals of all weights. The kind of interrelation reported in this study appears similar to this kind of allometric interrelation but is actually quite different because it concerns organisms of the same weight. I refer to this kind of interrelation between various parameters for eggs of the same weight as an isometric interrelation. The two kinds of interrelations are quite independent the existence of an allometric interrelation does not prove the existence of an isometric interrelation.