Variation in Bird Eggs—Does Female Factor, Season, and Laying Order Impact the Egg Size, Pigmentation, and Eggshell Thickness of the Eggs of Capercaillie?

SIMPLE SUMMARY: Birds eggs are unique in the animal kingdom thanks to their different shapes, colours, sizes, and maculation patterns. Generations of people have been fascinated by their variety, thus, egg collecting by scientists and collectors has, in the past, been quite a popular pursuit. Nowadays, this activity is illegal in many jurisdictions, but egg variation has not lost its fascination. Despite extensive research, scientists are yet to determine why birds eggs are so varied, not only between species, but in one species. There are many possible sources of intraspecies egg diversity, such as female factor, laying order, season, and many others. In the presented work, we investigate egg variation and its sources for Capercaillie. We found that size, shape, and pigmentation were not connected to laying order, nor season, but egg traits were highly consistent for individual females. This conclusion indicates that, in the case of Capercaillie, visual identification can be useful in identifying the eggs of different females. ABSTRACT: Despite numerous studies, intra-species variation in bird eggs is still not well explained. In the presented studies, we investigated the possible sources of this variation: female factor, laying order, and season, using the following traits of Capercaillie eggs as an example: egg size and shape, eggshell lightness, and thickness. Samples were collected for three years from three Capercaillie breeding centres located in different parts of Poland, where birds are kept in conditions close to their natural habitat and have a similar diet. The obtained results showed no significant impact of laying order on egg size, shape, pigmentation, nor eggshell thickness. This indicates that the provided nutrition ensures an adequate supply of minerals for the entire laying period. Most results did not show statistically significant differences between eggs from different breeding centres, but in one breeding centre, eggshells had lighter pigmentation. We assume the observed differences may result from females’ individual features or local environmental conditions. Egg traits were highly consistent for individual females, proving that visual identification can be useful in identifying the eggs of different females.