Latitudinal variation in biophysical characteristics of avian eggshells to cope with differential effects of solar radiation

Abstract

Solar radiation is an important driver of animal coloration, not only because of theeffects of coloration on body temperature but also because coloration may protectfrom the deleterious effects of UV radiation. Indeed, dark coloration may protectfrom UV, but may increase the risk of overheating. In addition, the effect of coloration on thermoregulation should change with egg size, as smaller eggs have highersurface-volume ratios and greater convective coefficients than larger eggs, so thatsmall eggs can dissipate heat quickly. We tested whether the reflectance of eggshells, egg spottiness, and egg size of the ground-nesting Kentish plover Charadriusalexandrinus is affected by maximum ambient temperature and solar radiation atbreeding sites. We measured reflectance, both in the UV and human visible spectrum, spottiness, and egg size in photographs from a museum collection of plovereggshells. Eggshells of lower reflectance (darker) were found at higher latitudes.However, in southern localities where solar radiation is very high, eggshells are alsoof dark coloration. Eggshell coloration had no significant relationship with ambienttemperature. Spotiness was site-specific. Small eggs tended to be light-colored.Thermal constraints may drive the observed spatial variation in eggshell coloration,which may be lighter in lower latitudes to diminish the risk of overheating as a resultof higher levels of solar radiation. However, in southern localities with very high levels of UV radiation, eggshells are of dark coloration likely to protect embryos frommore intense UV radiation. Egg size exhibited variation in relation to coloration, likelythrough the effect of surface area-to-volume ratios on overheating and cooling ratesof eggs. Therefore, differential effects of solar radiation on functions of colorationand size of eggshells may shape latitudinal variations in egg appearance in the Kentishplover.