I’ve down-loaded 8 birding apps on my iPhone 4 so far (not counting Angry Birds
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), including 5 field guides (Sibley Birds, iBirdPro, Audubon USA, NG Handheld Birds & North European Birds). The 3 other apps are BirdTunes & ChirpUSA (N. American bird songs) & BirdsEye (a bird “finder”). Of the 4 North American field guides, the iSibley is the best overall & the iAudubon (by far) the worst, though all have their strengths & weaknesses. The Sibley range maps, for instance, though not bad, are not as good as those in iBirdPro. All 4 of the American field guide apps include sound files, Sibley leading the pack with a very comprehensive set of songs & calls, all carefully labeled with locality & (often) function as well. I haven’t had an occasion to use the North European field guide app in the field, but I’ve looked it over & have liked what I've seen.
Of the 2 bird song apps, BirdTunes is much superior to Chirp, its library of bird songs & calls being comparable in size, quality & documentation to the Sibley’s, & is the one I tend to use most often in the field because of various convenience features which it has & the Sibley lacks. I haven’t played enough with BirdsEye--the bird finder app--to give it a fair trial, but my initial impression (FWIW) is not favorable.
As far as I’m concerned, the great feature of these apps--the field guides & the specialized song/call apps--are the sound files. These have no equivalent in paper field guides & are extremely useful in the field, both for reference & for luring birds in for closer views. When it comes to the core function of the traditional field guide--the identification of birds in the field--however, the apps are less successful & screen technology in particular will have to improve a lot before I’ll be tempted to leave the paper versions behind.
Anyway, I hope this helps. But to really evaluate these apps you’ll need to download them yourself & put them through their paces. And, of course, I don’t have an iPod (though I intend to get one in the near future) so don’t know how the apps perform on the much larger screen of that device.