There's probably no real reason for this. There are other things in the Pokemon games that don't make sense or are just Game Freak logic, and this is one of them. Until Game Freak tells us why a silhouette of a Fearow is used, we may never know.
Alternatively, you can look at it as a crudely drawn Moltres, as Sky Attack was Moltres' signature move at one point. It doesn't look too much like a Moltres, however.
Hope I helped!