Evolution of eggshell structure during rapid range expansion in a passerine bird

Abstract

Summary1. Environmental factors such as temperature, humidity and partial oxygen pressure can affect avian eggshell structure because gas exchange across the shell must allow sufficient water loss while preventing dehydration of the embryo. Studies of species with known chronology of colonization of novel environments provide a powerful insight into the relative importance of ecological factors shaping the evolution of eggshell structure. 2. Here, we examined changes in eggshell structure that accompanied rapid range expansion of house finches (Carpodacus mexicanus) across North America. We analysed thickness and pore density in eggshells from three ecologically distinct populations: the native desert population in southwestern Arizona and two 30-year-old populations in the northwestern (north-west Montana) and southeastern (south-east Alabama) parts of the species' range. We also conducted cross-foster exchanges of freshly laid eggs within and between the northwestern and southeastern populations to examine consequences of population differences in eggshell structure on embryo development. 3. Eggshell structure was most distinct in a recently established population inhabiting higher humidity environment (southeastern Alabama), where eggs were the largest, eggshells the thickest and pore density the lowest. Populations that experienced highly distinct ambient temperatures (southwestern Arizona and northwestern Montana) nevertheless had similar eggshell structure. These results were corroborated by experiments where humidity differences between cross-fostered nests had twice the effect on embryo survival compared to the effect of change in ambient temperature. Correspondingly, experimental egg exchanges between southeastern. Alabama and northwestern Montana populations were associated with fourfold increase in embryo mortality compared to within-population egg exchanges. 4. We document rapid evolution of eggshell structure in response to colonization of novel environments and establish the relative importance of environmental factors on avian eggshells. We discuss these results in relation to population variation in incubation behaviour and its ability to shield eggshell structure from the selection exerted by novel environments.

Effects of environmental conditions on the egg mass, yolk antioxidant level, eggshell thickness and eggshell spotting patterns of Great Tits (Parus major)

“…This foraging flexibility of Great Tits could possibly be the reason why we found that environmental conditions had no significant effect on eggshell thickness. Our results are in concordance with previous findings, as eggshell thickness also did not vary with laying date and ambient temperature in other bird species (Eeva and Lehikoinen 1995;Massaro and Davis 2004;Hargitai et al 2011;Stein and Badyaev 2011).…”

Variation in eggshell traits between geographically distant populations of pied flycatchers Ficedula hypoleuca

“…Hence, the rate of egg water loss during laying and incubation might not be determined by ambient temperature in these populations, but most likely by the female's behaviour (Higham and Gosler 2006). Further studies are needed to determine whether the relationship between shell thickness and temperature is due to immediate environmental constraints (as found in previous poultry studies; Hempleman et al 1993) or to adaptive evolution of shell structure (as suggested by Stein and Badyaev 2011).…”

“…Likewise, less porous shells should be adaptive in these environments (Davis et al 1984, Davis andAckerman 1985). Previous findings in wild birds show either a positive effect of temperature on shell mass (Ojanen 1983) and thickness (Arad et al 1988) or no effect at all (Stein and Badyaev 2011). Bourgault et al (2007) found that blue tits Cyanistes caeruleus laid eggs with heavier shells in a colder deciduous forest compared with a coniferous forest; yet, the effects of temperature and habitat type could not be disentangled.…”

Eggshell porosity covaries with egg size among female House Wrens (Troglodytes aedon), but is unrelated to incubation onset and egg-laying order within clutches

“…Ricklefs and Smeraski (1983) cross-fostered eggs among nests of the European Starling ( Sturnus vulgaris ) shortly after laying, and found that the length of the incubation period depended on the incubation behavior of the foster parents, and also on the nest from which the eggs originated. They found that differences in incubation periods were associated with variation in the composition of eggs laid by both the original and foster mothers, as revealed by both nest-of-origin and nest-of-rearing effects, leading them to suggest that the formation of eggs may be correlated with maternal incubation behavior (see also Stein and Badyaev 2011). …”