Avian eggshell coloration predicts shell-matrix protoporphyrin content

Abstract

Avian eggshell pigmentation may provide information about a female's physiological condition, in particular her state of oxidative balance. Previously we found that female house wrens (Troglodytes aedon Vieillot, 1809) with lighter, less-maculated, and redder ground-colored shells were older and produced heavier offspring than females laying darker, browner eggs. The strong pro-oxidant protoporphyrin is responsible for this species' eggshell pigmentation, so differences in pigmentary coloration may be related to eggshell protoporphyrin content and reflect female oxidative balance and condition during egg-formation. Therefore, we tested the assumption that egg-surface coloration is related to the amount of protoporphyrin in the shell matrix. We analyzed digital photographs of eggs to determine maculation coverage as a measure of the overall ground coloration of the egg and its red-, green-, and blue-channel pixel values. Pigments were then extracted from these same eggs and analyzed using high-performance liquid chromatography. There was a strong, positive relationship between eggshell redness and protoporphyrin content of eggshells, but no relationship between percent maculation and protoporphyrin content. Thus, when older, larger females deposit more protoporphyrin in their eggshells, this may reflect a tolerance for high levels of circulating protoporphyrin or an effective mechanism for off-loading protoporphyrin into the eggshell matrix.

Keywords: Eggs; Eggshell pigmentation; Eggshell speckles; Female quality; HPLC.

Fig. 1.

Representative photographs of house wren (Troglodytes aedon Vieillot, 1809) eggs showing (A) light, (B) intermediate, and (C) dark coloration. Please refer to Figures 1 and 2 in Hodges et al. (2020) for additional details.

Fig 2.

Eggshell protoporphyrin content in relation to house wren (Troglodytes aedon Vieillot, 1809) eggshell redness. Predicted protoporphyrin concentration (back-transformed from the natural log) in relation to each eggshell’s red-channel pixel value after controlling for clutch effects, egg-laying order, and other coloration traits. See Table 1 and text for the description of the generalized linear mixed model.