What makes a great teacher, versus simply a good teacher? Well, a good teacher knows the material well so that they can explain it to you. But a great teacher also knows what you know, so that they can teach you exactly what you need to know next. Personalizing your lesson plan is important for a number of reasons: you learn more because you stay more engaged, you remember the material better because you review older material at exactly the right time, and – perhaps most importantly – you get the most out of your time because you're not reviewing material that you've already mastered or struggling with material that's too advanced.

Duolingo recently developed a machine learning model affectionately called "Birdbrain" which learns both from and about you in order to help you learn. In this blog post we'll explain a bit about why we developed Birdbrain, what it does, and where we'll be taking Birdbrain next.


Duolingo's Birdbrain model is continuously studying both how much our learners know as well as the difficulty level of the different kinds of language material we teach. By putting these pieces together, Birdbrain makes an educated guess about whether a learner will get a given exercise right. This complements our other personalization system that figures out how well a learner knows each particular word. Birdbrain goes even further by extending what we know about our learners to all of the facets of language in our app.

title "What does Birdbrain do?" image of lesson in the app on the left labeled "exercise" and image of Zari on the right labeled "learner" and the Duo owl with between them with his brain outlined bottom of the image says "predicts personalized difficulty level"

The more exercises all of our learners complete, the more Birdbrain learns about how much you know about the language and how challenging our exercises would be for you.

So, once Birdbrain has figured all that out, how can we use it to improve your learning? Our Session Generator crafts lessons from a wide pool of prospective exercises using a sophisticated algorithm. Now that we've created Birdbrain, we can provide Session Generator with another crucial piece of information: which of all the possible exercises are at just the right difficulty level for this specific learner?

image of Duo owl holding a magic hat and a magic wand and watching a mobile phone with a Duolingo lesson jump out of the hat with stars and sparkles around it

Birdbrain is effective

Birdbrain has resulted in an improved learning experience for our learners – we're giving you more tailored lessons at exactly your learning level! We can already see Birdbrain's impact in our learning data. At Duolingo, we run A/B tests for new features before we make them available to everyone, so that we're sure we're delivering only the highest-quality learning tools. The results of A/B tests using Birdbrain show that using information from Birdbrain to construct lessons at the right difficulty level for each learner has consistently helped our learners learn more. This makes intuitive sense: if the material is too challenging, it's easy to get frustrated because you'll need more context to help you learn, and if what you're seeing is too easy, then it's probably because you've already mastered the topic!

Using Birdbrain not only results in improved learning, but also improved engagement – learners are happier with their learning experience, and so they return to their lessons more regularly! A more personalized experience translates to a better experience: Birdbrain helps you automatically skip the topics you feel good about, so you can give extra attention to more challenging material. We see that learners who had lessons constructed using Birdbrain are more likely to continue learning by doing more lessons and are also more likely to return again day after day. And that's exactly our goal. The hardest part of learning a language is sticking with it, so we want our learners to feel good about their progress!

Birdbrain: leading the flock

Birdbrain has been learning from our learners throughout 2020, and beginning in March, our learners started learning from Birdbrain themselves! Little by little, we've been incorporating Birdbrain into more kinds of lessons, with the goal of eventually using Birdbrain to personalize every activity our learners do on Duolingo.

graph with y-axis for percent of lessons personalized and x-axis for days since March 2020, the line starts low at 0% and gradually increases to about 5% in July and then spikes until it's over 20% in October

Above, you can see the percent of lessons that Birdbrain has personalized over the last half year. We're now personalizing over 20% of lessons! In 2021, we'll be improving both how well we personalize and how much we personalize so that we can make learning on Duolingo even more effective and engaging!