The Burgess Bird Book by Thorton W Burgess (in story form with the animals as characters and illustrations) and Handbook of Nature Study by Anna Comstock are the two we are using our first year next fall. I would see if your library has them to check out if you like them for your school.
What ages are your children? Are you looking for books on particular birds such as robins or cardinals, or are you looking for books that deal more with birds in general dealing with such topics as feathers, flight, and different beaks?
Living books are usually written in story form by an author who is passionate about the subject. Look for books that make the subject “come alive.” Generally, you can tell if a book is living by reading a part of it. If it is interesting and you want to keep reading, it is probably a living book.
Libraries are good sources for books on nature topics. Check in the non-fiction sections for adults and children. I recommend looking in the adult section mainly because our library places books 4th grade level and up in the adult section. Also, sometimes librarians aren’t really sure of the age level of a book and sometimes a book gets placed in the wrong section.
I’ve used The Burgess Bird Book for Children by Thornton Burgess this year. It is a wonderful book and my children have learned a lot about different birds through it. As suggested by Tara, we use it alongside the Handbook of Nature Study. We also have a bird field guide which we refer to too so that the children can view a detailed colored photo of the birds. Ambleside Online recommend Burgess.
Look for anything by Neltje Blanchan. Some of hers are out of print but still easily available and she has had some reprinted. There was recently a reprint of her fantastic Birds Every Child Should Know by Universisty of Iowa Press. Her descriptions are enchanting.