It's an app Audubonanza as Audubon discounted its nature and field guide apps to honor John James Audubon's birthday on April 26. The sale extends to both its single subject apps, now US$0.99, and its regional guides, which are $1.99.
You can find the full list of Audubon apps that are on sale on the NatureShare website. NatureShare, previously known as Green Mountain Digital, is the developer behind the Audubon apps. The sale started yesterday and runs through April 29th.