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La Tour du Valat

Institut de recherche pour la conservation des zones humides méditerranéennes

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Titre :	Egg color as an adaptation for thermoregulation
Auteurs :	David Westmoreland, Auteur ; Matthew Schmitz, Auteur ; Kevin E. Burns, Auteur
Type de document :	Article
Dans :	Journal of Field Ornithology (78(2) JUN 2007)
Article en page(s) :	176-183
Langues:	Anglais
Résumé :	Avian embryos are incubated at temperatures only 2-6 degrees C below that at which hyperthermia begins to influence survival. In habitats where sunlight directly strikes the eggs, even for short periods, heat gain may be a substantial threat to survival, and reflective pigmentation may reduce the rate of heat gain. The results of previous 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 in the near-infrared waveband that encompasses about half of incident solar radiation. We used naturally-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.

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