I recommend the new warbler app from Princeton University Press.
The one I like is complementary to The Warbler Guide.
I copied and pasted from the Press's web site the following information:
The Warbler Guide App is the perfect companion to Princeton’s revolutionary and widely acclaimed book The Warbler Guide, by Tom Stephenson and Scott Whittle. Taking full advantage of the Apple iOS® platform, the app allows you to identify birds by view or song, quickly and intuitively.
Exciting new 3D graphics enable you to view a bird from the exact angle you see it in the field. And the whole range of warbler songs is easily played, compared, and filtered. Whether for study or field use, this innovative app delivers the full power of The Warbler Guide in your pocket, built from the ground up for the Apple iOS® platform, and complete with unique new app-only features.
Breakthrough features from The Warbler Guide book that are included in the app:
- Rapid and confident two-step ID process using visual finders and comparison species
- The first complete treatment of warbler songs, using a new objective vocabulary
- An intuitive visual finder that includes side, 45 degree, and undertail views
- Master Pages with detailed ID points
- Complete guide to determining the age and sex of warblers with photos of all ages and sexes
- Annotated sonograms showing song structure and key ID points
- Complete songs, chip calls, and flight calls for all species
- Comparison species for making confident visual and audio IDs
- Many additional photos to show behavior and reinforce key ID points
- Highlighted diagnostic ID points
- Color Impression Icons for narrowing down ID of warblers from the briefest glimpses
- Behavior and habitat icons
Unique new app-only features:
- 3D models of birds in all plumages, rotatable and pinch-zoomable to match field experience of a bird
- Intuitive, visual, and interactive finders with filters for possible species based on audio and visual criteria chosen by the user
- Playback of all songs and vocalizations with sonograms makes study of vocalizations easy
- iPhone® and iPad® versions let you take these useful tools into the field
- Selectable finder sortings grouped by color, alphabetical order, song type, and taxonomic order
- Interactive song finder using objective vocabulary for fast ID of unknown songs
- Simultaneous visual and song finders makes identifying an unknown warbler even easier
- Half-speed song playback allows for easier study of song structure
- Comparison species with selectable side, 45 degree, and undertail views
- Features 75 3D images
- Covers 48 species and 75 plumages
- Includes 277 vocalizations, 156 songs, 73 contact calls, and 48 flight calls
- Requires iOS 7.0 or later. Compatible with iPad 2/iPhone 5 and above.