Eggshell conductances of avian eggs at different altitudes

Abstract

Previous studies have documented that the conductance to water vapor (${G_{{H_2}O}}$ ) of eggs collected at high altitudes from wild and domesticated birds is decreased below sea level values in approximate proportion to the decrease in barometric pressure (P_B) at the montane locations. This study tested whether this reduction could result from physiological adjustment of shell structures by the female in response to detection of a change of P_ß or a correlate thereof. The effect of a 28% reduction in P_B on ${G_{{H_2}O}}$ shell thickness, shell permeability to gases, and egg mass was tested by comparisons of eggs laid by Coturnix quail and Bengalese finches at sea level and after transport to 2900 m. The mean ${G_{{H_2}O}}$ of the eggs laid by the finches and 5 of 7 quail did not differ significantly from averages of eggs laid by the same birds at sea level. The ${G_{{H_2}O}}$ of the two remaining quail either increased above pre-transport levels or fluctuated in a manner that was not correlated with the direction of change of P_B. Average egg mass of one finch and several quail increased significantly following transport, but since this effect was also noted in control quail remaining at sea level for the entire period, the change is probably associated with aging. We conclude that the reduction in ${G_{{H_2}O}}$ observed in eggs laid by wild and domesticated birds at high altitudes most likely reflects long-term selection for females which lay genetically fixed shell structures appropriate for that habitat.