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First, Welcome! A great place to learn and share.
Second, Ben pretty much covered the issues.
I shoot birds with a crop sensor camera and the 100-400L. It aint easy. Find some larger subjects to practice on. Ducks, cormorants. Head to a zoo and shoot some flamingos. Turkeys. Large animals. Then return to the greater challenge of smaller birds. Hold off on birds-in-flight practice until you gain more mastery over static subjects. And shoot alot, throw out alot, review it all.
Third, backgrounds are important. And often overlooked, especially while learning. Seek out clean, uncluttered backgrounds. Double check thru the lens to make sure you dont have extraneous poles, wires, cars, lights, etc.
Finally, if you really are into learning bird photography, pick up Artie Morris' ebook The Art of Bird Photography II. Go to www.birdsasart.com for a link to his store and to see astonishing bird photography that will cause you to Wow and despair. Or take a look at our own Nature forum for similar quality viewing and reactions.
Hope to see more of your work.