K2 Birds
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Look at a real photo and pick the right species from four choices. The options are chosen to be plausible look-alikes, so you learn the differences that matter.
Each species you get wrong heats up and returns sooner. Every photo is credited to its photographer, with the licence, via iNaturalist.
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.