Both Amazon (Kindle) and Barnes and Noble (Nook) used Google's Android OS (which is available free) as the basis of their readers, BUT both of them wrapped a superstructure around them. They may have wanted to deter their customers from buying the other company's books, but it looks like mostly what they were doing was to produce a reader for technophobes. And they seem to have done a pretty fair job of it.
The superstructure interferes with many or even most of the available Android apps running on them because the apps need to access the root of the Android OS to run properly. Since the quality of both Nook and Kindle was high and the price low relative to the generality of android tablets, people developed ways around the superstructure, "rooting" the machines. This made them incapable of working as Nook or Kindle. In the case of Kindle, most attempts to root the machines result in "bricking" them, i.e., turning them into a brick, so far as electronics go. I don't think this was a consequence of deliberate design, it just happened that way. Nooks root fairly easily, and a commercially available mini SDHC card, the N2A card, actually allows you to switch back and forth, and by removing the card, to return the machine to a plain Nook.
Some android apps that don't need to access the root will run on Nook or Kindle (more of them on Nook) and most can be modified to run on the Nook or Kindle superstructure, apparently more easily for Nook than for kindle, if the programmer thinks it's worth the trouble (many don't).
Of course, there are many birding books and some field guides that are available as Kindle Books, I've got 11 or 12 on my modified Nook right now, including the new edition of Hawks at a Distance.
Will
xda-developers is the way.
