Warbler Guy, which warbler app exists for me to buy (if any)?

Excellent idea, Joannie.

I suggest you purchase the fine wood-warbler-centric app that corresponds to The Warbler Guide:

It's found at the iTunes Store.

Buying the book is also a good idea, if you wish a nice resource to complement the classic Warbler field guide in the Peterson Guide Series (Peterson Guide To Warblers, Jon Dunn & Kimball Garrett, 1997, Houghton Mifflin).