You place the crow in your map, put an entity called path_corner in your map and in properties type a name e.g. start1 then type start2 in next path bit.
Copy it to where you want the bird to fly and hammer is clever enough to put a line between the two path_corners you just made.
You then type the name of the first path_corner to end the path that the crow flies in so its start1 in next path so it makes a circle that the bird uses to fly around.
When you start the map the bird sits there unless you scare it with a grenade near it.