My boyfriend and I have noticed similar friendships between sheep and crows. I dunno what the crows get out of it, but we have seen, more than once, that the crows kinda babysit the lambs and follow mother and lamb around the landscape. I managed to take a few pics of one such trio on one of our outings once:

I haven’t kept count on how many times we have seen this stuff, but it’s happened enough times at this point that I don’t think it’s a coincidence. The crow will literally walk on the ground next to the lamb at all times and be a lookout.