Whilst crawling about the internet I’ve found a couple of great sources for public domain (United States) bird images, so I thought I’d share:
The New York Public Library’s digital collection contains hundreds of free, public domain images, such as the works of Audubon…
The New York Public Library has other bird related public domain images as well, but if you do a search be sure to check the box saying “Show Only Public Domain”.
Another great source for bird related public domain images is Vintage Printable. Under their animal section they have around fourteen pages of bird images, as well as another four pages devoted to eggs.
Note: If you live outside of the United States, make sure you check to see if these images are legal to use within your own country.