Peterson's famous Field Guide to Backyard Birds [App Store link] has come to the iPhone/iPod touch, and in many ways it is a natural fit with the iPhone multimedia features. The field guide, which is a 92 MB download (!), contains hundreds of bird species, as well as the sounds of their calls, and of course illustrations and information about each bird.
I gave the app a try in my Arizona backyard. First, you enter the first two digits of your zip code, then you are provided a list of birds that should be local to your area. The quail that were sitting on my back wall were on the list, as well as the pesky road runner that peeks in the window every so often. I also learned that the roadrunner is part of the cuckoo family. Who knew?
The guide has some quizzes that can test your knowledge of our feathered friends, and tests to see if you can identify bird calls. You can also zoom in on the bird illustrations to see more details.
Some users have reported bugs and crashes of the app, but in my testing it was quite stable and I can't report any problems. The developer does have a note on the App Store web page saying a new version will be coming out soon with bug fixes and new features.
I think the app needs a search mode so you can type in the name of a desired bird. The information about each bird is pretty thin, and when the lists of local birds is displayed it doesn't seem to be in any order that I can fathom. I'd also like to see the program work in landscape mode.
Even with those criticisms, I found the app useful and informative. The app is $2.99US. Birders will also want to take a look at iBird Explorer Plus [App Store link]. It is pricey at $19.99US but it has a far more expansive catalog of birds, and does allow for searches.