Question: Wh do the eggs of different birds have different shapes?
Answer: The type of nest often determines the shape of the egg.
For instance a Screech Own egg, near right, is almost round. Screech Owls nest in cavities where the egg can not roll out.
A Killdeer nest is a shallow scrape commonly done in open spaces on the ground, so a Killdeer egg, far right, has a "pointy-round" shape so if it does roll, it won't go far from the nest.
Note-- the sizes of the eggs in this illustration are not in proportion to each other.