Last week I announced their Birds app as 99 cents, the price has gone up but many other related apps have dropped in price. See the Audubon apps by Green Mountain Digital.
Audubon Birds Field Guide for iPad and iPhone is only $2.99 cents! The second best price ever they’ve ever had. This is their 606 MB huge book (app) that’s is normally $19.99. 
Another interesting choice is the deluxe Audubon Guides – A Field Guide to Birds, Mammals, Wildflowers, and Trees for only $14.99 this is normally $39.99 (but occasionally on-sale for $24.99-$19.99) and this combines their four main apps (books?). But this huge multi-book app weighs in at 1.3 GB (that’s a big chuck of space if you’ve only got an 8GB iOS device).
If you want to see what these apps are like, the North American Owls app is free!
If you’re looking for Butterflies, Insects or Fish (or Tropical Fish), those are on-sale too!
This is one of those deals you should pick up even if you’re into birds/nature/etc. and thinking of getting an iPad or iPhone in the future (assuming you’re interested in birds/nature at all). It’ll be hard to beat these prices.
FYI, I think most (except the Owls?) run natively on the iPad and iPhone, but please double-check before buying!
And the 5 Audubon apps/books available for Android on on-sale at Amazon too.
