The "Download Now" link directs you to the iTunes App Store, where you must continue the download process. You must have iTunes installed in order to open the link, and you must have an active iTunes account to download the application. This download may not be available in some countries.
From CleverMatrix Ltd:
The Backyard Birds of America app contains 115 identification guides of birds found in America. It will help all nature lovers identify birds whilst out and about. The app is a great reference guide for the novice bird watcher or experienced twitcher, giving detailed information on each bird and a gallery of high quality photos and some great bird sounds and calls. Birds can be marked as spotted, storing location data and date spotted automatically. You can save multiple notes to each spot along with your own photos, to create your own birding journal! These spots can then be shared with your friends on twitter.The app has the following features: 115 bird guides, listed by A to Z or by Family 477 photos to accompany these birds, assisting you with identification 68 songs/calls Mark birds as spotted Ability to Tweet your spottings directly from the app Automatically records all birds spotted including location data and date spotted Automatically plots all spots on a map, which can be edited Family screen where by the birds are split down into families Favorites screen where you can store your favorite birds Search screen where you can search by either common or latin name Detailed information on each bird listed Scroll through pictures with a simple swipe of the finger Fast, simple to use and regular updates Once downloaded, no internet connection needed Make notes for each bird spotted and upload your own photo, like a mini journal, with the ability to upload this to twitter Coming soon; Ability to backup all spotted information and upload to a new online bird community.If there is a bird that you think we have missed and you would like to see it listed, then please drop us an email at firstname.lastname@example.org and we'll be glad to include it.