An Altitudinal Cline in Conductance of Cliff Swallow (Petrochelidon pyrrhonota) Eggs to Water Vapor

The rate of diffusion of water molecules through air is inversely related to atmospheric pressure; diffusivity of water vapor, therefore, is greater at high altitudes than at low elevations. Assuming that conservation of water in avian eggs is essential for normal development and hatching of embryos, we predicted that eggs laid by birds nesting at high altitudes would have lower conductances to water vapor under standard conditions than eggs laid by conspecifics nesting at low elevations, thereby compensating for the increased potential for transpirational water loss from eggs at high altitudes. To test this hypothesis, we collected eggs of Cliff Swallows, (Petrochelidon pyrrhonota) along an altitudinal gradient of 1,627 m in north-central Colorado and adjacent Wyoming and determined conductances of the eggs to water vapor under controlled laboratory conditions. Water-vapor conductance of eggs decreases with increasing altitude of nesting sites, as predicted by our hypothesis. Additionally, eggs containing embryos with a well developed chorioallantoic membrane have higher conductances to water vapor than eggs lacking a well developed chorioallantois, indicating that conductance to water vapor increases during the course of development.