Within-clutch variation in size and shape of the Great Tit Parus major eggs

Ornithological studies on the egg characteristics of birds are important for understanding many components of their life histories.Over the past year (2019), I have been studying egg dimensions of Great Tit Parus major in a deciduous forest in Mokrice and Krušljevo Selo area, in north-western Croatia.This species is secondary hole-nesting passerine and all studied Great Tits bred in nestboxes.The effect of laying order (or laying sequence) on egg dimensions was analysed in clutches with 11 eggs (the modal clutch size in research area in study period) and only first clutches included.Average egg length, egg breadth, egg volume and elongation index (egg shape) of 110 eggs (10 clutches) was 17.35 mm, 13.47 mm, 1609.64 mm³ and 1.29, respectively.In this study, there was no general trend of increasing or decreasing length, breadth and egg volume (egg size) in relation to the laying order (all p > 0.05).However, a negative correlation between egg shape and laying sequence was significant.