Incubation and embryonic development affect the color of eggs of Mountain Bluebirds

Abstract

Technological and theoretical advances over the past two decades have allowed researchers to quantify eggshell color in ways that were not previously possible. However, differences among studies in the timing of color measurements during laying or incubation and inclusion of data from inviable eggs may affect the results of these studies as well as conclusions based on comparisons among studies. To determine the effect of the timing of color measurements, we compared the color of the eggs of Mountain Bluebirds (Sialia currucoides) during both laying/early incubation and late incubation. We also assessed the influence of egg viability on eggshell color by comparing viable and inviable eggs from the same clutch. We found that all color metrics investigated (blue-green and UV chroma, brightness, and hue) were significantly different between early and late incubation, and viable and inviable eggs. However, color metrics of eggs measured during early and late incubation and of viable and inviable eggs in the same clutch were correlated. Our results suggest that the timing of color measurements and the viability of eggs have important effects on eggshell color and, therefore, in future studies, investigators should always provide information about the timing of measurements and the viability of eggs measured. Our results also suggest that comparisons among studies where eggshell color was measured at different times and/or the viability of eggs was not determined or reported are possible, given that the color metrics of viable and inviable eggs and of eggs measured at different times in our study were correlated, but those comparisons should be interpreted with caution.