Adaptation of the Avian Egg to High Altitude

Abstract

Theoretical predictions and experiments on eggs of domesticated birds indicate that the diffusion coefficient of gases is inversely proportional to barometric pressure. Therefore potentially lethal losses of CO2 and water vapor from eggs laid at high altitude might result if the increased tendency of gases to diffuse at reduced barometric pressure were not counteracted in some manner. Limited data from two wild populations indicate that water loss from eggs is independent of altitude over a 3000 m elevational gradient. Four different possibilities are discussed by which compensation for increased diffusion of water vapor might be achieved at high elevations 1) a reduction in eggshell conductance (Gh2o) 2) an increase in the initial water content of the eggs 3) an increase in shell thickness, and 4) alteration of water vapor pressure in the nest microenvironment or incubation temperature by variation in parental behavior. Mean Gh2o of eggs of two precocial and four altricial species breeding above 2800 m is significantly reduced below values of related birds breeding at lower elevations, but no change in initial water content or shell thickness has been observed in such eggs. Observations of parental behavior in species breeding over wide elevational gradients have not yet been made. Identification of the mechanisms by which eggshell structure is modified to achieve a reduced Gh2o the environmental cues used by females to determine the elevation of the breeding location and the rapidity with which shell structuie can be modified awaits further research.