I live in a very urban area (a large city's downtown) - I put my 4 week old chicks into an ex-pen* with top in my backyard while I cleaned the brooder. They had only been out in the ex-pen once before and then only for 1/2 hour.
Within 15 minutes of being put outside I heard a ruckus and saw a hawk bounce off the top of the ex-pen, hit the grass next to the pen and then fly up into a tree in my yard. I went out to check to make sure that everything was OK and the hawk didn't fly away from the tree until I called my asleep-on-the-job dogs out.
Anyway - this is a very long winded way of saying that predators are everywhere and are quick to take advantage of chicken dinner.
*If you aren't familiar it looks like this:
http://www.midwestpetproducts.com/midwestexercisepens/exercise-pen-wire-mesh-top