Water-Vapor Conductance and Shell Characteristics of Superb Lyrebird Eggs

Abstract

Superb lyrebirds exhibit a suite of breeding characteristics (a long incubation period, low incubation constancy, use of an enclosed nest, and winter nesting) which might cause an unusual pattern of gas exchange in the egg during incubation. This possibility was investigated by determining the egg's natural rate of water loss during incubation and its water vapour conductance, and by measuring shell parameters which influence these properties. The estimated fractional weight loss of the 61,5-g egg during incubation was 16%. This was similar to that in other species, as was the calculated water vapour pressure difference across the shell (27 Torr=3.6 kPa). The mean rate of water loss (194 mg day-') and the mean water vapour conductance (7.08 mg day-' Torr-' =53.23 mg day-' kPa-') of the egg were, however, relatively low. True shell thickness averaged 218 µm and pore density was 50-56 pores per square centimetre. Pores were unbranched and relatively evenly distributed. The fairly standard fractional weight loss over the 50-day incubation period resulted from the egg's low water vapour conductance which, in turn, stemmed mainly from a comparatively low pore density. It was concluded that the protracted incubation period, resulting from the low incubation constancy at low ambient temperatures, was probably the principal influence on the evolution of the small pore area and low water vapour conductance of the lyrebird egg.