My favourite app.

The Smart Search and Compare functions on this app make it worth the price by themselves. I find the search works much better than iBird Explorer or even Peterson. Always the first eGuide I reference, it has all the best features of the paper version (like arrows point out field marks) and songs and calls for almost every bird. In "compare" you can even listen to both birds in question at once.

Dwayne Biggar about The Sibley eGuide, v1.6