If you can’t get enough birds killed before the guards attack you and the train stops, you can always drive the train up to speed up and jump onto the coal pile in the car right behind the engine and shoot, at any birds flying nearby and if you slow down before you shoot enough birds repeat until you get your 5 birds, I found 2 areas that work best, the swampy area north of saint denis and the bard’s crossing bridge, usually a ton of birds fly off from the bridge when the train comes