That's a tern photographed with a high speed camera. She was caught during an extremely complex movement which involves most parts of her body, and that's the kind of motion which makes a tern look like a master of flight and not a chicken. They are probably the most skilled fliers in the world along with hawks and owls.
How do you even begin to study poses like this one?
These were my early attemps to extract LOAs from photos of birds (last part of the batch of sketches with crappy text, I swear!):
When birds are perched the tail is often an important line.
I tried to reduce each pose to a minimum number of lines (in red, the red number is the lines count). But the resulting stick figure has nothing to do with the original, if you look at the red lines alone you could never tell they represent a bird during a wing beat...
Often the most visible lines of birds in flight are the edges of wings and in fact birds are often simplified like this:
That's easy to recognize but also very anonymous. The supreme elegance of the tern's wingbeat is gone.
These are new from yesterday, tried again with the tern photo:
The right one is the best I got so far, I'd start from a scheme like this to design a tern in flight. It's very different from a classic LOAs scheme, but it's just three lines and it keeps the lines and proportions I see as most evident in the photo. The left one looks odd because the wing line on the right is too parallel to the body line; both the left one and the center one show the body too large so it doesn't look like a tiny bird at all.