Increased eggshell porosity in replacement clutches of the Black-headed Gull Larus ridibundus

Abstract

Water vapour permeability, pore density and thickness of the eggshells from initial and replacement clutches of the Black-headed Gull (Larus ridibundus) were measured and compared. The permeability of the eggshells of replacement clutches was, on average, higher than that of either the corresponding initial clutches or the control sample of late initial clutches initiated during the same period as replacement clutches . Thus, the difference cannot be explained merely on the basis of any seasonal trend. The difference was caused by the higher number of pores and not by the thinner shells of the replacement eggs. Eggshell porosity increased during the first days of incubation . Changes in eggshell porosity can probably serve as an early signal of changing environmental properties affecting eggshell quality.