iOS Pick: iBird Ultimate
Leo Laporte's app cap on iOS Today 299 is iBird Ultimate Guide to Birds. This is widely agreed by birders to be the best bird software available. It's an extensive guide to 940 species of birds in North America. It has great illustrations and photos of birds, along with descriptions and information. The app also gives you field marks which tells you what to look for to identify birds properly, and has audio of bird calls that you can listen to.
iBird Ultimate is one of many iBird apps available. There's also iBird Pro, iBird Lite, iBird Plus, iBird Yard+, iBird Journal, and other apps with guides to birds in other countries. There's also The Ultimate Birder bundle which includes iBird Ultimate, iBird Hawaii Palau, and iBird Journal for $19.99. Purchased individually, those apps would total up to $45.