Just do it and dont think too much about the way it should look as opposed to the way it turns out. With field sketching the subject will stand at odd angles as often as not and it can be frustrating at first when you feel that the results are not what you feel they should be. Often subjects move and frustrate you, you end up with pages of bits of birds, unfinished as such, heads, outlines, feather details etc. Often though these are most valuable drawings of all in the long run and eventually it all falls together, with time it becomes much easier to sketch whole birds quickly as their features and shape become more burnt into your memory. It takes time and mostly practice...
Try to start with cooperative birds like Cormarants, resting Waders, Gulls, Wildfowl etc. Helps to concentrate one a few species too, whatever is plentiful locally. You can then become famliar with these birds and derive a lot more confidence from that as you improve.
There is no substitute for practice and hours spent drawing what is in front of you. You will learn a lot about the birds you sketch regardless, drawing is the best way to understanding birds appearance and plumage and is very rewarding. Hope this helps a little...
Get out there and enjoy it!