Twigle Birds is a free app for birders. It claims to contain a complete field guide to birds in North America and Europe, and has photos and information built into the app so you don't need a data connection to use it. One particular function does require a data connection, and I'll get to that later.
The app lets you search for birds by color, habitat, bird size and other appearance clues. You can also listen to example bird songs from each species. The photos are zoomable so you can see the bird images in more detail.
A unique feature of Twigle Birds allows you to use your phone's microphone to capture a bird call and upload it to the web where remote software tries to identify the sound. It's a bit like SoundHound or Shazam but for bird calls instead of music.
The database built into the app seems reasonably complete. The text and pictures are sourced from Wikipedia and each entry contains a link back to the Wikipedia website. I couldn't test the accuracy of the bird call identification as I live in a desert area and things were pretty quiet this morning when I went outside to test it. I expect identifying bird calls is pretty tricky, especially from a distance, so I wouldn't have high hopes. I did try the app with some recorded bird sounds I got off the web, and the app thought what it heard was one of nine possible birds. One of the birds was the correct one, but it couldn't get more specific.
There are plenty of paid apps for birders, including the very complete Peterson Field Guide to Birds of North America (US$14.99).
Twigle Birds is a nice app, with some good ideas. It uses your GPS location to help determine birds that might be near you, which helps the app narrow the list.
Twigle Birds requires iOS 6 or later. It's not universal, and because of the amount of data that comes with the app it is a 518 MB download.