So I've been living in my city for 30 years, and never saw any kind of bird of prey. Then I get chickens, and several months later, I find a dead chicken and a giant hawk looming over our chicken pen. He killed our almost fully grown Buff Orpington. This happened in the middle of a city of 100,000 people. Zoom in to see this monster up close. https://goo.gl/photos/fkgfYfCF452Du6qw5 So the moral of the story is that you do need to cover your chickens. We put a large piece (25x25) of aviary cloth over the pen to prevent this in the future, which has its own challenges, such as, how to keep the leaves from piling up on top and how to coax a chicken down who happens to get on top of it.