They all seem to have something that sets them off, right? Mine is unbothered by wild turkeys, Canada geese (she tries to herd them), and other fairly large birds, but is afraid of … hens. Not roosters, but hens, especially when they get to clucking.
BTW this very same mare has proven to be an excellent “babysitter” for horses that are young and/or new to trail riding, or even those who have developed bad habits… Several years ago, she even helped keep an ill-behaved, bargey mare from dragging her rider back to the barn. (Said mare proved hopeless on trails eventually, but me and another boarder were trying to help out.) She’s been a “babysitter” at shows, too… I do not show anymore, but she’s been the horse that someone with a young or green or frazzled horse can just walk next to.
I realized later today that I pushed the mare just a bit too hard today, and I regret it. It’s part of the problem with having so much access to such great trails, finally… I want to see it all! We’re riding with company on Sunday, and hopefully Saturday too.