Egg

All birds lay amniotic eggs[2] with hard shells made mostly of calcium carbonate. Non-passerines typically have white eggs, except in some ground-nesting groups such as the Charadriiformes, sandgrouse and nightjars, where camouflage is necessary, and some parasitic cuckoos which have to match the passerine host's egg. Most passerines, in contrast, lay coloured eggs, even if, like the tits they are hole-nesters.
The brown or red protoporphyrin markings on passerine eggs reduce brittleness and are a substitute for calcium when that element is in short supply. The colour of individual eggs is genetically influenced, and appears to be inherited through the mother only, suggesting that the gene responsible for pigmentation is on the sex determining W chromosome (female birds are WZ, males ZZ).
The eggs are laid in a nest, which may be anything from a bare cliff ledge or ground scrape to elaboratey decorated structures such as those of the oropendolas.