The next time you make an omelette, take a moment to think about the shell that you have just destroyed. In addition to protecting the young within, the remarkable structure provides calcium for development as well as permitting essential gases to leave and enter. ‘The avian eggshell is a complex bioceramic’, says Steven Portugal from The Royal Veterinary College, UK. But the structure can also teach us about the factors that influence biological diversity. Portugal and a team of international colleagues explain that eggs are laid under a wide range of different circumstances – in locations from burrows and cliffs to high in trees and low on the ground – to parents whose lifestyles range from the semi-aquatic to those that are almost permanently on the wing, and many of these factors could dramatically affect the passage of water vapour through the shell. But it wasn't clear which of these factors might have contributed to the evolution of the shells we see today. Portugal and his colleagues decided to find out what influences have shaped the eggs of modern British birds (p. 3326).
However, instead of heading out to reserves and wildernesses, the team converged on the egg collection of the UK's Natural History Museum (Tring), where they could access samples of eggshell from over 150 different species – ranging from the common starling (Sturnus vulgaris) to the capercaillie (Tetrao urogallus) – to find out how well they lose water vapour. Fastening a segment of each eggshell across the mouth of a tiny test tube containing a 200 μl droplet of water, the team measured how much water was lost each day by evaporation. The team explains that under natural conditions, the water loss rates are likely to be similar across species; however, they say, ‘Only under standard laboratory conditions will differences due to structural adaptations of the avian eggshell…become apparent.’ Then the team compiled a comprehensive database of parental lifestyle factors – including breeding range, nest type, diet, habitat and whether the parent returned to the nest with wet feathers – before building a family tree incorporating each of the species to investigate which lifestyle factors most affected the rate of water loss.
However, when the analysis was complete, the team was surprised to see that factors that they had thought would influence eggshell water loss rates did not. ‘We did not detect an effect of clutch size and developmental mode as significant main predictor variables of gas transfer’, they say. Instead, eggs that are incubated by parents that routinely return to the nest with wet plumage have higher water loss rates than eggs incubated by dry parents, to compensate for the humid conditions and maintain optimal water loss rates over the course of incubation. Likewise, eggs that are incubated in confined and humid conditions, such as cup nests and burrows, tend to have higher water conductance than eggs reared in more open environments on the ground and in crevices.
‘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 team concludes.