Female Pied Flycatchers fail to respond to variations in nest humidity

We examined the possibility that a cavity-nesting passerine, the Pied Flycatcher (Ficedula hypoleuca), can modify the permeability of its eggs to water vapor facultatively in response to variations in nest humidity (P(N)). We found no significant relationship between the water-vapor conductance (G(E)) of flycatcher eggs and P(N). In addition, incubating female flycatchers failed to change their patterns of incubation when we modified P(N) experimentally. These data, coupled with large variations that occur in the G(E) of the eggs within individual clutches, suggest that Pied Flycatchers do not alter the structure of their eggs as they lay them and may not be aware of differences in P(N). It is more likely that G(E) is determined genetically and has been established by natural selection for eggs with high degrees of hatchability.