Nesting behaviour influences species-specific gas exchange across avian eggshells

Abstract

Carefully controlled gas exchange across the eggshell is essential for the development of the avian embryo. Water vapour conductance (G(H2O)) across the shell, typically measured as mass loss during incubation, has been demonstrated to optimally ensure the healthy development of the embryo while avoiding desiccation. Accordingly, eggs exposed to sub-optimal gas exchange have reduced hatching success. We tested the association between eggshell G(H2O) and putative life-history correlates of adult birds, ecological nest parameters and physical characteristics of the egg itself to investigate how variation in G(H2O) has evolved to maintain optimal water loss across a diverse set of nest environments. We measured gas exchange through eggshell fragments in 151 British breeding bird species and fitted phylogenetically controlled, general linear models to test the relationship between G(H2O) and potential predictor parameters of each species. Of our 17 life-history traits, only two were retained in the final model: wet-incubating parent and nest type. Eggs of species where the parent habitually returned to the nest with wet plumage had significantly higher G(H2O) than those of parents that returned to the nest with dry plumage. Eggs of species nesting in ground burrows, cliffs and arboreal cups had significantly higher G(H2O) than those of species nesting on the ground in open nests or cups, in tree cavities and in shallow arboreal nests. Phylogenetic signal (measured as Pagel's λ) was intermediate in magnitude, suggesting that differences observed in the G(H2O) are dependent upon a combination of shared ancestry and species-specific life history and ecological traits. Although these data are correlational by nature, they are consistent with the hypothesis that parents constrained to return to the nest with wet plumage will increase the humidity of the nest environment, and the eggs of these species have evolved a higher G(H2O) to overcome this constraint and still achieve optimal water loss during incubation. We also suggest that eggs laid in cup nests and burrows may require a higher G(H2O) to overcome the increased humidity as a result from the confined nest microclimate lacking air movements through the nest. Taken together, these comparative data imply that species-specific levels of gas exchange across avian eggshells are variable and evolve in response to ecological and physical variation resulting from parental and nesting behaviours.

Keywords: Avian eggshells; Life history; Museum specimens; Nest environment; Permeability.

Fig. 1.

Mean (±s.e.m.) surface-specific water vapour conductance (G_H2O) for 62 British breeding bird species, sourced from a single museum donor, compared with the G_H2O for the same species measured in eggs from multiple different donors. Analysis showed the values of G_H2O to be highly repeatable for a species, and that egg donor origin was not a significant factor in the determination of average G_H2O for a species. Values of G_H2O for the three segments are combined (blunt end, equator and pointed end).

Fig. 2.

Phylogenetic tree of the 151 species of British breeding birds. Those species which are classified ‘wet parents’ (habitually return to the nest wet) are coloured blue. The remaining species (coloured black) are ‘dry parents’. The branch length are proportional to the rate of diversification in G_H2O, which was ~5.5 times greater in the wet incubating parents group when compared with those species that return to the nest dry.

Fig. 3.

Mean (±s.e.m.) water vapour conductance (G_H2O) for six nest types and nest locations, measured in 151 species of British breeding birds. Nest types/locations indicated with filled squares had significantly higher G_H2O values than those indicated with open squares.