Incubation Water Loss by Pied-Billed Grebe Eggs: Adaptation to a Hot, Wet Nest

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Abstract
Eggs of pied-billed grebes (Podilymbus podiceps) are incubated in warm, wet nests built of decaying vegetation 3-7 cm above the waterline. The daily mass loss ( ) of 52 grebe eggs in their nests was 157.6 mg · day-1 (±SD 13), resulting in a 16.4% loss of mass during the 23-day incubation period. Eggshell water vapor conductance (GH₂O) was 12.3 mg · day-1 · torr-1 (±SD 1.8), a value 2.7 times greater than predicted for other bird eggs of similar mass and incubation period. The high eggshell conductance is the result of a threefold increase in pore density. The bottom of the unincubated but covered nest cup remained at 31-33 C despite floating in water at 20 C and air temperatures ranging from 10.8 C to 23.6 C (June). Water vapor pressure around the eggs is elevated owing to the warm, wet nest, thus requiring an increase in eggshell conductance if the eggs are to lose sufficient water. Incubating adults leave the nests unattended (but covered) for prolonged periods of time. This behavior may be related to the maintenance of a suitable egg temperature by the warm nest material.