Look at your maps. Draw a line from point A to point B. If avoiding obstacles will ever involve going away from the destination for a significant amount of time, you'll probably need pathfinding. But most likely, the birds will be able to simply veer left or right to get around things, or just go over them, so obstacle avoidance will probably cover it.

But steering plays nicely with pathfinding, so you could do both. Balance the priority values so birds can be some distance from the path before it becomes a significant draw, so they can maintain their flock spacing but won't drift far enough from the path to get stuck. Following the direction of the path should be pretty high, to keep the flock headed the right direction. But then trees and things become overwhelming when distance becomes small, so birds will deviate from the path and flock spacing to avoid them (and other birds will react by trying to rebalance the flock spacing, which should be pretty cool to watch)