Presence of the cloud cover and elevation angle of the sun affect measurements of eggshell coloration and patterning obtained from calibrated digital images

Abstract

Calibrated digital photography is frequently used in studies focusing on avian eggshell appearance to measure colour and pattern features. Photographs are often taken in natural light conditions, yet little is known to what extent the normalisation process is able to control for varied light. Here, we photographed 36 blown eggs of the Japanese quail Coturnix japonica at five different elevation angles of the sun on both sunny and uniformly overcast days alongside grey standards. We normalised and processed the photographs in the MICA Toolbox software and checked how much noise was introduced by different natural light conditions to the colour and pattern measurements of the same set of eggs. Our results indicate that natural variation of light conditions affects eggshell colour and pattern measurements obtained by means of calibrated digital photography. Depending on a trait, the elevation angle of the sun had similar or even greater effect on the measurement than the presence of the cloud cover. Furthermore, measurements taken in cloudy conditions were more repeatable than those taken in sunny conditions. Based on the results, we propose practical guidelines regarding measuring colour and pattern of eggshells using calibrated digital photography in outdoor conditions.

Keywords: granularity; light; normalised digital photography; repeatability; repeated‐measures ANOVA; weather conditions.

FIGURE 1

Set for photographing eggs: custom‐made supporter on a tripod with pits for eggs, grey standards and scale bar. All set was surrounded on the sides by a box to prevent Japanese quail Coturnix japonica blown eggs from shaking due to wind.

FIGURE 2

Results of the repeated‐measures ANOVA. The same set of 36 Japanese quail Coturnix japonica eggs was photographed in sunny (orange boxes) and cloudy (blue boxes) conditions at five different elevation angles of the sun above the horizon. For brightness, red chroma, human red‐green opponency, sumPower and maxPower, non‐significant pairwise comparisons are marked. Blue lines above boxplots connect pairs of measurements taken in cloudy conditions; orange lines pairs of measurements taken in sunny conditions; and black lines pairs of measurements taken in different weather conditions but at the same elevation angles of the sun. In the case of propPower, significant pairs are shown for clarity (there were too many non‐significant comparisons), and the meaning of the colour is the same as for the rest of the plots, *p < .05; **p < .01.

FIGURE 3

Histograms with comparison of occurrences of spot size categories in different light conditions. The same set of 36 Japanese quail Coturnix japonica eggs was photographed in sunny and cloudy conditions at five different elevation angles of the sun. Numbers above facets indicate elevation angle of the sun above the horizon (in degrees), and different weather conditions are expressed as colour. Spot size category is maxFreq, value from the granularity analysis after log2 transformation to acquire linear scale of spot size categories.

FIGURE 4

LMM‐based repeatability and 95% confidence limits of measurements of six eggshells' colour and pattern traits. The same set of 36 Japanese quail Coturnix japonica eggs was photographed in sunny and cloudy conditions at five different elevations angles of the sun. Category ‘natural’ is a combination of sunny and cloudy categories. Additionally, a subset of 18 eggs was photographed twice in constant artificial light conditions (‘artificial’ category) to measure the noise that arose from putting the eggs on the egg‐holder and selecting the eggs' edges on the picture.