roost question.

[canesisters]

Hello. I saw a thread about a 'kit box' for pigeons and since I know NOTHING about them I was curious. I couldn't open the link so I just googled 'pigeon kit box' to see what I could find. I found dozens of pictures of nice little coops - all with funny little roof top looking perches or roosts.
So the question is - why are pigeon roosts shaped like that?

 

[larrylofts]

THE POOP FROM THE BIRD ABOVE DOES NOT FALL ON THE BIRD BELOW

 

[canesisters]

Ahhh, of course!
Thank you.