K2 Nature teaches you to recognise wildlife from real photographs. Start with birds: 200 common species from around the world.
It uses the same K2 engine as the language apps, so the species you miss come back until you know them on sight. Photos are credited to their iNaturalist photographers.
Every card has a temperature. Get it right and its chance of coming back halves; miss it and it doubles. K2 picks the next card at random, weighted by that chance, so a session keeps returning to whatever you are getting wrong and lets what you know fade into the background.
When the cards you are working on have cooled, a new batch joins them, so you always study at the edge of what you know. There are no review dates and no daily queue to fall behind on. Every drill is free and works without an account, and everyone gets 20 free AI uses a day; past them, signed-in AI practice runs on your Bike4Mind credits.