Owl Eggs

[waspur]

Why are they spherical ?

[heathfield]

Owls are very wise old birds, and know that a spherical shape will enclose the maximum volume using the least amount of shell.

Some birds lay many eggs, and oval shapes will fit closer together in the nest, allowing the bird to sit over all of them. Owls don't lay a large number of eggs, so there's no need for theirs to be oval.

Some birds lay few, or a single egg, but the nest is in a precarious position, like on a cliff. In this case, the egg might be very pointy, so there's less chance of it rolling out of the nest - instead of rolling, it just turns around. Owls nest on flat surfaces, or in hollows, so there's no need for theirs to be pointy.

[Clanad]

My copy of Owls of the World: Their Lives and Behavior states the following: "... most birds lay eggs that are oval, like the familiar chicken's eggs. Owls, in contrast tend to lay round eggs. And though bird eggs come in a rainbow of colors, all owls lay plain white eggs.
These two traits are related to the owl habit of nesting in enclosed spaces. Round eggs pack closer together than oval eggs, so they fit better in a small, enclosed space. Oval eggs have the advantage of rolling around less than round eggs, however, and Snowy Owls and Short-eared Owls, which nest on the ground, lay eggs that are more oval shaped than those of most owls. ... The female lays one egg every one to three days, until the clutch is complete. The number of eggs in an average clutch varies from species to species, but five is typical..."

[Clanad]

Well, heathfield, a glitch again... you weren't there when I began looking for my desk reference. Glad to see you though!

[heathfield]

Hi Clanad! Glad to see you'r keeping the flag flying!
I query the 'round eggs pack closer together' statement. Some birds that lay oval eggs keep them with the small end toward the centre of the nest. If the eggs were spherical, the gap in the centre of the clutch would be greater. The pointier the eggs, the closer they'll be. The European Lapwing, common hereabouts, is an example. See here

[kwicky]

Mostly the reason is for shell strength but other factors dominate. See

http://www.royalalbertamuseum.ca/vexhibit/eggs /vexhome/sizeshap.htm