Birds have tough stomachs, literally. While you tear, grind and chew food with your teeth before swallowing, birds have only beaks and stomachs to do the work. Bird stomachs have gizzards, which are tough, muscular and robust digestive organs that enable birds to break down food since they chew it first.
Birds have two-part stomachs. The first part is the proventriculus, which is where undigested food goes to start breaking down. The proventriculus produces acid that starts digesting the food -- a useful step for birds like owls and falcons that prey on live animals like mice, rabbits and other birds. After this stomach chamber starts breaking down the food, it passes on to the second chamber, the gizzard.
Inside the Gizzard
The inside of a gizzard is tough, rubber, muscular and strong. It essentially does for a bird what your jaws and teeth do for you -- grinding and crushing food so it can be digested. The gizzard doesn't do this alone, though. The organ is fortified with small rocks, gravel and grit that birds inadvertently consume over time. These embed themselves in gizzard walls, becoming hard, jagged surfaces that help the gizzard pulverize food.
Back and Forth
Because birds swallow their food in such large pieces, their stomachs sometimes have difficulty efficiently breaking it down. While the gizzard can be strong enough to crack a nut, it is frequently tasked with breaking down hearty meals filled with bones, teeth, muscle and feathers. Birds can actually pass the food back and forth between the gizzard and the proventriculus, weakening it with additional doses of acid, then physically crushing it with the gizzard, and so on.
Not everything a bird eats is meant to be digested, even after the gizzard and the proventriculus have their way with it. In such cases, the gizzard compresses all of the bones, feathers, teeth and miscellaneous matter into a firmly packed pellet, which travels back through the proventriculus and up the birds throat to be regurgitated. The size and shape of this pellet depends on the bird. Owls, for example, have relatively weak digestive acids and produce bigger pellets.