Hi!
I just relocated 2, 5ft black akes! Last week! The first was one I kept finding in our nest boxes. I was trying to be a good neighbor to it and let it go out back in our hay shed which is crawling with mice, but he kept going into our coop. I think it was eating eggs. I found that it was living in the soffits above. It would come down, have a nice egg snack and then head back up to the soffit to digest. I had to rip apart the soffit of the coop to get the snake out. I didn't take any chances with the possibility of it coming back, I drove it 20 miles away! LOL And believe it or not, when I got back home from removing that snake I walked back to the coop and found a SECOND 5 ft black snake on the ground laying parallel to the nest boxes! I was so mad! I reached down, snatched that snake up by the tail, stomped back over to my car, grabbed the bag that was still inside and stuffed the snake in and drove that one off to another section of uninhabited woods.
The snakes will eat anything that it can get into its mouth. They can unhinge their jaws and stuff anything inside that is a few times grater in diameter than its head. So I would say, eggs...yes, chicks...yes, it may even try to eat bantam types of chickens if its head is big enough, but I would say that regular sized chickens are probably safe.
I am sad that you killed it. I know how mad you must have been, but everyone's first thought is to kill them. They are great predators of rats, mice, voles, moles and even baby squirrels and chipmonks. They keep the local rodent population in check. They will even eat songbird eggs, toads and frogs. That's why you sometimes hear bluejays and mocking birds squawking in the treetops....snake alert! I had a big discussion on the subject of snake killings with my in-laws last night. They were yelling "Why did you let them go?" "You shoud have chopped it up!" And the quite surprising "I even run them over on purpose if I see them in the road!" Made me really sad and perplexed. I was like, "Why kill them?" "They serve purpose, they keep the mouse/rat population down!" They just didn't get it...