I've had a lot of different types of spookers and the one who spook in place are the best to deal with. They jolt to a stop and look but are willing to listen to you and carry on, The horses we have now are more this type.
then there is the type that spook violently to the side then stop, if you are still on, not too bad. One Arab/appy that I got free because of his spooking was the worst I ever rode for that. He was lightening fast, very agile and no warning, by the time he thought of spooking he was already landing 15 feet over there. He didn't take off so that was good. Usually you get a split second warning but not with him.
Then there is the type that spook and bolt, this often can be a bad habit that they learned along life's path in order to get out of work or going out on a trail. I have had a few of those but if you can work thru with it they usually come around and stop the bad habit.
You were mentioning birds on the roof,, I don't think I have ever had a problem with that or don't remember it. Snow sliding off a metal roof can be cause for spooking in our area. Makes a Gawd Awful noise.