They are terrible here year round. Saw a bunch circling over the pasture our fall calving cows are in today. When you see that, you drop what you're doing and get your butt out there ASAP. We haven't lost any yet, but fortunately we have a pretty good view of the whole place. We don't really go anywhere during calving season.
One thing that has helped us is killing some hogs or varmints and dumping them on the opposite side of the ranch. Actually, I think it has made a tremendous difference. Keeps them occupied. That, and we have great neighbors that will come hunt you down if you have buzzards circling anything. If you have 40 or so (normal size group here) circling a pair, they can do a lot of damage fast, and there's no way the cow can fight off that many. Our heeler enjoys chasing them off..."bowling for buzzards!"