The shape of the bird was hard. I wasn't sure what to draw first, the bird or the branches. And if the bird, what part of the bird?
For me, I really started to notice details on the wing. The folded primaries and secondaries I may not have noticed unless I was asked to draw it. Unfortunately, birding in the field is hard because the birds are fast and rarely sit still. With drawing I think we can pay more attention, check other sources, and study the birds rather than just see enough field marks or hear the song to get an ID.