Definitely squirrels and birds, though I've never seen Marley around a chicken. Oddly enough, flies. (He tries to catch them in his mouth). We think that he sees cats as potential friends rather than prey, because he had a cat in his former household. However, the cats don't see his overtures in quite the same way.
Our squirrel story today: We were eating outside at a restaurant. Normally, we put Marley in a downstay and he is extremely good... he has stayed down for a couple of hours, no problem. He usually falls asleep. But just in case, I always tie his leash to my chair. Today a squirrel ran close to our table, and it was just too much temptation. He bolted after the squirrel so hard and fast that he broke out of his collar -- we had been told that it was designed to do that, but not being able to break it ourselves, we didn't really believe it. Somehow Marley's 14.4 pounds of determination did it. Luckily nothing bad happened -- he quickly ran himself into a corner (I don't know where the squirrel went) and we got him back to the table without incident. Let's just say he was in the doghouse for the rest of the meal. (ha ha).