Water-vapor pressure in nests of the San Miguel Island Song Sparrow

The water-vapor pressure (P_N) in nests of the San Miguel Island race of Song Sparrows (Melospiza melodia micronyx) averaged 16 torr, but varied considerable between nests and within individual nests during successive days of incubation. Large daily fluctuations occurred throughout the incubation period and did not parallel concurrent changes in ambien vapor pressure (P₁). Daily rates of water loss from nest eggs (M_H2O) averaged 28 mg day^-1, but also varied considerable within and between nests and did not correlate with changes in P₁. M_H2O increased 6-33% after the third day of incubation. P_N was significantly higher and M_H2O significantly lower in nests located in sheltered gullies than in nests from a windswept slope. These data suggest that Song Sparrows do not regulate P_N to achieve hatching success.