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

Abstract

Eggs of Pied Flycatchers (Ficedula hypoleuca) lose water at a slow, constant rate ($\dot{M}_{H_{2}O}$) 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 ($G_{H_{2}O}$) of the eggshell increases (linearly) fourfold between the time the egg is laid and the end of incubation. Linear increases in $\dot{M}_{H_{2}O}$ and $G_{H_{2}O}$ have previously been associated with the eggs of large, precocial birds rather than those of altricial songbirds. At least 88% of the increase in $\dot{M}_{H_{2}O}$ during the incubation period can be accounted for by changes in the egg's $G_{H_{2}O}$. 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 $\dot{M}_{H_{2}O}$ and $G_{H_{2}O}$ are not due to losses of cuticle from th...