Here are some resources we like to use while researching birds:
AllAboutBirds.org
A comprehensive guide to all things birds from the Cornell Lab of Ornithology, the creators of Merlin Bird ID, the best mobile app for bird identification.
Audubon.org
The National Audubon Society has been committed to preserving bird habitats and conducting bird research for more than 120 years. They also have an excellent guide to North American birds and a beginner’s guide to birding.