Why Are Bird’s Eggs Rounded?
The roundness of an egg allows pressures to be applied from the outside which would break it if applied from the inside. Thus a helpless chick is protected until the moment it needs to break out from its shell. It can then do this with the gentlest of tapping.
Eggs are hatched by an adult bird sitting on them, and the best container for round objects is a cup-shaped nest which prevents them rolling about. So the best shape for eggs is for one end to be smaller than the other. The normal position for eggs in a nest is to have the smaller ends pointing inwards. This means the eggs take up the minimum of room and make it easier for the sitting bird to cover them.
Birds with scanty nests, as in the case of most sea birds, have more elongated eggs. If such eggs are caught by the wind while lying on some cliff-face or rocky ledge, they will spin round instead of rolling over the edge.
Cliff-nesting birds often have highly conical eggs. They are less likely to roll off, tending instead to roll around in a tight circle; this trait is likely to have arisen due to evolution via natural selection. In contrast, many hole-nesting birds have nearly spherical eggs.
The shape of eggs varies considerably across bird species, ranging from near-spherical (such as those of the little bee-eater) to highly pyriform or conical (such as those of the common murre) with the familiar shape of the chicken egg lying in between. Early scientific investigators of egg shape suggested that the oval shape of eggs was caused by the egg being forced through the oviduct by peristalsis.
In this often-repeated but incorrect theory of egg shape formation, the contraction and relaxation of the muscles which push the egg down the oviduct cause the spherical egg membrane to distort slightly into an ovoid shape, with the blunt end caudal (i.e. furthest down the oviduct and closest to the cloaca).
The calcification of the egg in the shell gland/uterus then fixes it in this shape, and the egg is laid with the blunt end appearing first. However, this theory has been refuted by studies of egg shell formation in a number of bird species using techniques such as X-ray photography which have demonstrated that egg shape is determined in the oviduct isthmus (before shell calcification) with the pointed end caudal (furthest down the oviduct).
These observations cannot be explained by peristalsis. It has been proposed that the egg acquires its shape (with the pointed egg caudal) as it is forced through the narrow isthmus, but this assertion has not been thoroughly verified.
The shape has biological significance. A pointed egg will tend to sit on its side, with the big end tipped upward. The big end contains the air sac and its shell is pierced by a higher density of pores than the more pointed end. Tipping the big end upwards improves oxygen flow to the large head, with the physiologically demanding eyes and brain, which develops in the big end while the tail develops at the more pointed end.