The Avian Egg: Incubation Time and Water Loss

The large difference in incubation time among bird eggs, ranging from a minimum of 11 days to nearly 90, has aroused man's interest since antiquity. In her critical review of the history of our knowledge of incubation periods, Margaret M. Nice (1954) wrote, "The people who have been concerned with incubation periods fall into three groups: the guessers, the copyists, the investigators." Guessers came first and these have been busily quoted since Aristotle for more than 20 centuries. It was not until Evans (1891) and Heinroth (1922) made their own observations that reliable data began to accumulate. In spite of the inaccuracies since Aristotle's time, and the many exceptions which are now well recognized, there is an obvious general correlation between egg weight and incubation period. These have been presented by Heinroth (1922), Needham (1931), and Worth (1940) in graphic form and were reinvestigated in this presentation on the basis of newer information in the literature. It is of interest to note in retrospect that all these correlations are essentially similar, that the standard error of estimates is large, and that there are many exceptions. This merely illustrates that the many factors which determine the incubation period are not understood. For these reasons we analyzed other correlates of egg size such as the gas conductance of the egg shell and particularly the water loss properties of eggs, problems which Heinroth had already mentioned some 50 years ago in his classical treatise on incubation time. On the basis of water vapor conductivity measurements of the egg shell previously presented (Ar et al. 1974), the daily weight losses of eggs during natural incubation reported by Drent (1970), and the reported incubation periods, one is now able to derive new relationships which apply to eggs in general. These indicate that incubation time for a given egg weight is inversely porportional to the water vapor conductance of the egg shell. Furthermore, during natural incubation all eggs, regardless of size, lose approximately 18% of their initial weight and the mean water vapor pressure difference between the egg and the microclimate of the nest is 35 torr.