Just a thought....
If the cat was being fed, then it is unlikely that it was a serious thread to any birds, endangered or otherwise. I currently have four cats, and while they like to "hunt" they aren't particularly serious about it since the activity isn't required for food. In over 15 years I've had to rescue one starling (shoulda let the cat eat it).
a serious thread = a serious threat
Well, I can tell you that I had two cats, one of which had been a stray (not feral - it clearly had an owner at one time) for a while. While well fed, that cat was also killing at least one bird (that we knew of) every day or two. It even managed to catch and kill a red squirrel, and those little suckers are quick.
Do you seriously think the cat has never killed without you knowing about it?