Everything comes from an egg

Every animal starts life as an egg, but only birds have evolved to enclose their egg in a hard shell, to stock it with massive amounts of nutrients, and to guard, incubate, and cradle it in a nest.Many interesting stories lie below the stunning surfaces of eggs and nests. Each egg becomes a package of often enormous size relative to the mother, and laying it is the equivalent of a birthing event. There is a tradeoff between producing more than a dozen eggs per "clutch" (the number of eggs laid or incubated at a single time), as in most rails, quails, and ducks, and laying just one egg, as in the extreme case of the kiwi. As with vertebrate neonates, there is another tradeoff between the developmental stage of the young hatched from the egg and its size, though some birds with large clutches have young that can run some after hatching. The egg requires continual parental care until a second birthing when it hatches. Birds whose young are unable to walk, run or fly shortly after hatching must prepare an often elaborate nest before the eggs are laid. Although many animals build nests, nowhere in the animal kingdom have both egg construction and nest building evolved to produce such lavish, beautiful, and diverse creations as among bird species.

Excerpt by Bernd Heinrich Harvard University Press