Incubation Water Loss in King Penguin Egg. II. Does the Brood Patch Interfere with Eggshell Conductance?

Abstract

A new model of water vapor exchange in avian egg is proposed, in which water is lost by egg only through the part of the eggshell that is uncovered by the brood patch. Experimental data from king penguin incubation are used for analysis, as in this species the single egg is in close and continuous contact with the brood patch on 25% of its shell area. The inclusion of water lost from the brood patch is necessary for the model to fit with experimental data. This model, called 'fractional model" as nest and egg are divided into two fun ctionally diferent parts, is compared with a classical model in which egg and nest are considered as homogeneous. In each model, a water loss from the brood patch is either considered or not. The fractional model, with the inclusion of brood patch water loss, appears preferable because it more accurately reflects the observed incubation conditions of the king penguin and is in better agreement with experimental data throughout incubation. The major consequences of such a model could be extended to other bird species: an actual shell water vapor conductance under the brooder that is reduced; a heterogeneous nest that provides a temperature for the developing embryo different from that for egg water loss; and a possible influx of water through the upper part of the egg.