Fruit also gives robins energy to fuel their long migration. Cold weather keeps fruit fairly fresh, but eventually it gets moldy just as food left too long in a refrigerator does. And old fruit slowly ferments, which means that the some of the sugar changes to alcohol. If robins eat too many fermented berries, they can become confused and clumsy, and sometimes crash into things or get killed by predators. So as soon as any fresh food becomes available in spring, robins desert the old berries and concentrate on worms and any early bugs until the trees leaf out and new fruits start to grow.
Worms have lots of protein, so are nutritious for robins.
But just as a person needs a balanced diet with many different kinds of food, robins round out their diet with other things, too. When they're running on a lawn hunting for worms, they're also searching for other tiny animals. They eat beetles, caterpillars, spiders, and even snails and slugs, all rich in protein, and some with other important nutrients that robins need for growing strong bodies and new feathers. Use binoculars to carefully study a robin on a lawn. Make a list of the different animals it takes.