I agree with Sea Wanderer, Busby's book is a must, especially as it has now been updated. He is awesome in his ability to portray a bird in just a few lines.You might find the following also useful:
'The Right way to Draw' by Mark Lindley(Paperfronts ISBN 071600805)
'Learn to Draw Wildlife' by Peter Partington (Harper Collins ISBN 000412746-3)
'Learn to Paint Birds in Watercolour' by Peter Partington (Collins ISBN 000412343-3)
Have fun and let us know how you get on. It's not easy to sketch real,moving birds. Buy lots of cheap sketchbooks!