Egg color as an adaptation for thermoregulation

Abstract

Avian embryos are incubated at temperatures only 2-6 ! C below that at which hyperthermia beginstoinfluencesurvival.Inhabitatswheresunlightdirectlystrikestheeggs,evenforshortperiods,heatgainmay beasubstantialthreattosurvival,andreflectivepigmentationmayreducetherateofheatgain.Theresultsofprevious studies suggest that light-colored eggs acquire heat slower than dark eggs, but artificial pigments were used to create differences in egg coloration. This approach is problematic because natural eggshell pigments have low absorbance inthenear-infraredwavebandthatencompassesabouthalfofincidentsolarradiation.Weusednaturally-pigmented eggs to measure the influence of egg coloration on heat gain. Triads (N = 18) of eggs from Brewer's (Euphagous cyanocephalus), Red-winged (Agelaius phoeniceus), and Yellow-headed (Xanthocephalus xanthocephalus) blackbirds were crossed with six nests of each species and either exposed to full sunlight or placed under a diffusing umbrella. Thermisters recorded internal egg temperature every minute until an asymptotic temperature was reached. Eggs in full sunlight acquired heat more rapidly than eggs in the shaded environment, but heat gain did not vary with egg color in either environment. Eggs placed in Yellow-headed Blackbird nests took longer to reach asymptotic temperature, but there was no significant egg-by-nest interaction. Thus, it appears that differences in reflectivity of eggshell pigments in the visible range (400-700 nm) do not result in different rates of heat acquisition. The thermoregulation hypothesis was not supported.