I will say that birds and birds in flight are very challenging. In flight you have some 5-8kg of glass in hand and you're waving that around following the birds path, trying to grab focus... but even a bird sitting in a tree is a challenge, mainly due to branches and leaves and DISTANCE!

Often you will hear accomplished photographers saying that "the camera/lens does not make the photo/photographer" but in the instance of birds, you simply must bring a fairly long lens to the party or you'll end up with pictures of bushes/trees.