EDIT: More is the pity… these stopped being supported around iOS-9 and no longer work on my iPad
At least for the iPhoneAnd iPad versions: I have, and use, several of these Audubon Field Guides on my devices. I find them both accurate and easy-to-use. Include all of the usual things you would expect on description, range, but they also include things like birdsongs. The “Birds and Butterflies” costs $1.99, but the others are going for only $.99… that is only 20% of their usual price.