Female birds turn the end of the oviduct inside-out to lay an egg and allow the egg to emerge. A bird currently laying an egg can be identified by a rhythmic moving and opening and closing of the bill, though time varies by species from a few seconds to a few minutes. After copulation with a male bird, a female bird can store sperm in tubules.
The egg is kept clean because it never touches the walls of the cloaca. Studies have shown that birds can pick the sex of their young based on how well they can survive. If more food is plentiful, female birds tend to have more male offspring than female. According to Eastern Kentucky University, this is accomplished by the female egg determining the sex rather than the male sperm.
When it comes to the number of eggs laid, Cornell University's ornithology lab says the clutch size varies by specific species. A clutch is the number of eggs laid in one nesting attempt, and about one egg per day is laid by larger bird species. The size of eggs varies, and the popular comparison is a hummingbird egg, about 13 mm in length, versus an ostrich egg , 180 mm.
The four main parts of an egg are the yolk, albumen, shell membrane and shell. The yolk provides food, the albumen helps protect the embryo from changes in temperature and the shell is the main line of defense for the unborn bird.