It's been quite a while since I've given an update on this project, and I'm happy to say that I've just released a version of the app that includes the ENTIRE eBird taxonomy (at least, all species with audio recordings).
This is a monumental upgrade for this application, and now you're able to fully customize and build your own quizzes, rather than having to rely on prebuilt ones. There are also now two modes, one for learning, in which you're able to see the name of the species and listen along, and the classic quiz mode, where you can test your abilities.
In addition, I've totally reskinned the entire application, and you are now able to optionally show or hide the spectrogram. I've also added the feature to save your custom quizzes, and suggest them for review to be added under the 'curated' section. Finally, I've removed the leaderboard, as I don't believe that it is in line with my vision for the app as a learning tool, not competitive one.
Here's the link: http://earbirder.com/