Why is the entrance hole to a bird house round?

Question

Why is the entrance hole to a bird house round?

Answer 1

so the bird wont get cut on any sharp edges? lol

Answer 2

If you observe a bird when making a nesting hole,they always make it Round,watch t.v and you will see the example of Woodpecker nest they always make them round, likewise,most birds that bore holes into trees,prefer round ones.(t*t's,like round ones)

Answer 3

Why is a regular round pizza in a square box?

Answer 4

It's easier to drill out a round hole than to cut a square or other shaped one.