Some birds have rings around their eyes, such as the plumbeous vireo
About.com has information on birding and discusses eye rings http://birding.about.com/od/Bird-Glossary-E-G/g/Eye-Ring.htm. It says that all families have some representatives, but that thrushes and vireos are notable for them. This means that eye rings alone will not separate out the thrushes and vireos, but I can't expect anything to do that. Since a birding guide suggests that birders look for eye-rings, it is natural to try automated detection and see if it helps distinguish birds
I attempted to go through the pictures and manually mark birds as having eye-rings or not. There is certainly a spectrum, so I gave each a number between 0 and 2, with 0 meaning there was no eye-ring, and 2 being something like the plumbeus vireo. It was a very sloppy process, and I realized that there are definitely different aspects to eye rings, as mentioned in the about.com article. Some are prominent because of color, while others are prominent by width.
It is discouraging that I have a hard time classifying them, myself. It makes me unsure of my ability to write a program to do it, and makes it seem unlikely that it is critical in how the test subjects learned.