Birds have a muscular chamber called the gizzard, which mammals do not, and it’s full of stones called gastroliths.
Wild birds have to pick grit from the ground to keep the gizzard supplied with them. The gizzard is very muscular, and through its contractions it grinds the food up with these stones to facilitate its digestion.
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Thus, the grit in the gizzard serves the same purpose as the grinding of food by mammalian molars.
This is why cage birds such as canaries and parakeets have to be provided with grit, like coarse sand, in addition to their seed.
Alligators, crocodiles, earthworms, and a few other animals also have gizzards for this purpose.
