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 (GH2O) 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 GH2O and putative life-history correlates of adult birds, ecological nest parameters and physical characteristics of the egg itself to investigate how variation in GH2O 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 GH2O 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 GH2O 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 GH2O 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 GH2O 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 GH2O 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 GH2O 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.

The evolution of eggshell cuticle in relation to nesting ecology

“…Only one previous study has investigated the relationship between life-history parameters and the role of eggshell conductance within a comparative framework [53]. Here we report for the first time an association between the nanostructure of eggshells and nesting ecology in birds.…”

A rare mineral, vaterite, acts as a shock absorber in the eggshell of a communally nesting bird

“…The confined nest microclimate of cup nests typically leads to increased humidity, so a higher conductance ( G H 2 O) is necessary to ensure a sufficient humidity gradient between the nest environment and the inside of the egg to allow optimal water loss during incubation (Portugal et al . ,). In British birds, for example, the mean conductance value of species that build cup nests (0.28 ± 0.24 mg/day/torr) was significantly higher than that of species that build shallow tree nests (0.22 ± 0.32 mg/day/torr).…”

“…The value that we obtained for ani eggshell conductance is remarkably similar to the latter figure, supporting recent findings that the eggs of cuckoos do not show higher rates of gas exchange despite exhibiting rapid embryonic development (Portugal et al . ,).…”

“…, Portugal et al . ,) Finally, vaterite is known to play a role in visual signalling within ani nesting groups. Breeding adults use the appearance of the vaterite coating on the eggshell as a visual cue to judge the age of the egg: freshly laid eggs are coated with white vaterite, but this chalky coating is scratched during incubation to reveal the turquoise‐blue calcite underneath (Riehl , Fig.…”

Ultra-Fine Scale Spatially-Integrated Mapping of Habitat and Occupancy Using Structure-From-Motion

“…The main drivers of nest site selection (i.e. the inhomogeneous intensity of the point process β (Z i )) are likely to be associated with the hydrology of the site, as waterlogging of the nest may prevent important gas exchange over the egg shell [31], and cause hypothermia in chicks that have not yet reached thermal independence [32]. Covariates associated with the underlying suitability of terrain which were selected for inclusion in the statistical model of nesting include elevation, flow accumulation, and a travel cost metric that combines distance to the coast (where penguins haul out of the water after foraging) and slope along their commute back to the nest.…”