Birds are a diverse group of animals, and their diets vary widely depending on the species. However, many birds do eat worms as a significant part of their diet.
For example, earthworms are a common food source for many species of birds, including robins, blackbirds, and thrushes. These birds use their long beaks to probe the ground for worms, and then pull them out and eat them. Other birds, such as woodpeckers, eat insects that live in the bark of trees. These birds use their sharp beaks to dig into the bark and extract the insects.
So, while the answer to the question "What does a bird eat mostly?" may vary depending on the species, it is true that many birds do eat worms as a significant part of their diet.