I think bird in flight are the most difficult to do and require many hours of study to get them right. Of course you could just rely on photos but this can lead to unusual distortions and odd angles. I prefer to do as many sketches as I can trying to get a feel for the bird - of course this is not always possible when a passing bird only leaves a fleeting image in your mind. But I have used that too and with a bit of work, can lead to some convincing studies like this study of a sub-adult Bald Eagle.
This Black Vulture for example came from some pencil and pen & ink drawings I did a while ago in my sketchbook. I liked the final version so much that I decided to do a watercolor study of the bird in a similar pose.
Like the vulture, this juvenile Northern Harrier was flying away from me - as most of them usually are! But I think I did capture something of its presence. Hopefully the next one I do will be in a landscape setting.
Although this adult Bald Eagle also look to be flying away, it was actually soaring quite low down and almost directly overhead (as they sometimes do). Admittedly, I was close to its nest, so for a while I got some amazing views of this most spectacular raptor. All the watercolor studies shown here are about 6 x 9 inches and done in my sketchbook.