Water Loss, Conductance, and Structure of Eggs of Pied Flycatchers during Egg Laying and Incubation

Eggs of Pied Flycatchers (Ficedula hypoleuca) lose water at a slow, constant rate (Ṁ_H₂O) during egg laying but at a much higher, linear rate during incubation. Prepipping losses average 20% of the egg's mass when freshly laid. The watervapor conductance (G_H₂O) of the eggshell increases (linearly) fourfold between the time the egg is laid and the end of incubation. Linear increases in Ṁ_H₂O and G_H₂O have previously been associated with the eggs of large, precocial birds rather than those of altricial songbirds. At least 88% of the increase in Ṁ_H₂O during the incubation period can be accounted for by changes in the egg's G_H₂O. The high water loss that occurs during the second half of incubation appears to result from shell erosion, which reduces pore length and shell thickness, and perhaps increases pore size, in the equatorial and sharp regions of the egg. Increasing Ṁ_H₂O and G_H₂O are not due to losses of cuticle from the egg's surface or to increases in pore number, the number of open pores in the shell, or egg temperature during incubation. Flycatcher eggshells have four types of pores. They are usually open, frequently bent, and evenly distributed over the egg's surface.