Avian Reproduction over an Altitudinal Gradient. II. Physical Characteristics and Water Loss of Eggs

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Abstract
Although the effective diffusion coefficient for gases is inversely related to barometric pressure, rates of daily water loss during natural incubation and final water contents of pipped embryos are independent of altitude in eggs of redwinged blackbirds (Agelaius phoeniceus) breeding from sea level to 3,050 m. These results indicate that the increase in diffusion of water vapor from the egg has been offset in some manner in populations of this species breeding at high altitudes. Eggs of redwinged blackbirds and robins (Turdus migratorius) breeding from sea level to 3,050 and 3,450 m, respectively, were analyzed for eggshell conductance, egg mass, water content, shell thickness, and incubation temperature. Of these, eggshell conductance showed the only meaningful relation to barometric pressure and may serve to control rates of gaseous diffusion at high altitudes.