Nice topic....
When it comes to contemporary artwork rather than straightlaced illustration, I am a HUGE fan of Sue Coleman, whose art I first saw in a store at Olympic NP, felt driven to buy every single postcard by her with a bird in it.
She is both a true to life technical marvelous animal painter who could easyly do fieldguide illustration as well as a great graphic artist to make simple abstract brand-like forms from wildlife and she mixes those skills in different ways. She is a native woman from british columbia so all her birds are western northamerican species.
Here is the postcard thumbnail page from her website:
http://www.suecoleman.ca/artcards.htm
My favorites are those where she gets not too clever with integrating the shapes in context but just places the abstracted birds with the realistic ones, her "Kingfisher" is one of my special favorites
When it comes to famous scientific bird illustrators long gone, while Audubon sure was neat for the purely artistic aspects, I feel drawn to even earlier history. I have a special soft spot for Conrad Gessner...
This might sound strange, for his still almost of medival illuminated bestiaries reminding style often is absolutely useless for field identification to us modern spoiled people at all, yet those plump forms have something strangely compelling... There are several Gessner plates shown at
http://www.antiquariaatjunk.com/php/detail.php3?bnr=6823 .
Ulisse Aldrovandi makes me laugh, but not to mock him, his birds just have so antrophomorphic expressions at times and the stories are so wild. On the net you can see some work of him at
http://panizzi.comune.re.it/mostre/zoo/aldrovandi.htm, but allas not his Ruff I have in a book on bird illustration, I think this Ruff is the most comical depiction of a male bird in display on earth! He also painted the barnacle goose how it fell out of the shell and the pelican at feeding they young with blood... I am captivated by those old half-scientific books.... and why they did assume those bird behaviors....
There where also famous old masters where birds are not the first thing to come to mind, that got otherwise very fancyful but depicted birds so they could get identified, that cought my special attention. I have tried to do a complete species list for Hyronimus Bosch's "Garden of Earthly Delight" for fun, but for some areas of the picture I have not found suitable enlargements yet. Some of the larger depicted birds in it are truely great and the only thing wrong about them is the scale when it comes to the tiny humans around them.... an other one I tried this with is the "Air" allegory of Giuseppe Archimboldo, a human head out of many life birds of which mostly only naturalistic heads are visible...