Research paper: Variation in egg size, shell thickness, and metal and calcium content in eggshells and egg contents in relation to laying order and embryonic development in a small passerine bird
http://sci-hub.cc/10.1642/AUK-16-16.1#Quote: "As the presence of embryonic development and laying order are critical factors responsible for variation in some features of egg morphometry (primarily egg volume and mass) and the levels of elements, they should be taken into account in studies of the chemistry and morphology of avian eggs."