are you trying to work 'live'? start with photos. Copying isnt a sin for learning artists, even Leonardo da Vinci did it. you get a subconscious awareness of form and depth, layers. I haven't seen the tutorials mentioned above by Bryon but I am sure they will do this depth by showing how to get the layers and texture right.
it is very instructive to examine anatomy as well, how tendons fix to bone and skin over and so on, you know then the functions and shapes you are bringing to life.
i am suggesting this because you are using line and outline.
i would try another approach as well, for example, work with charcoal and chalk on a grey paper and don't do lines, more try and scratch in contrast, areas of light and dark. even try it as tones, not lines.
it depends on what you are aiming for, a natural photographic style final pic, or a quick sketch of line and movement to lead the brain to a conclusion.
'birds by character' book i came across has quick line sketches that work well but may not be what you want.
play with many different mediums. have fun. i am an experienced artist in many mediums and they all have advantages and disadvantages, none are best. i do think however that pencils and crayons work less ably until you are fluid at using line quickly and only practise gets that! 'line' is hardly ever 'outline'.
another approach in the field may be to get some basic 'shapes' outlined or even as white shapes on black - look at the silhouettes in collins field guide as an example. then learn the identifiers as pattern using the sketches in Opus that Delia pointed me at earlier
http://www.birdforum.net/opus/Image:Passerine_head_1.jpg for example so you can quickly dob in the identifying markers and colours.
depends on reason, purpose for sketch and final result you want really!
