Pressure on the inside of the foot causes the toes to lock around a perch. That is how chickadees can sleep without falling off the branch. Swimming birds like ducks, puffins, and loons have webbed feet for paddling in water. Grebes are also swimming birds, but their feet are lobed. Tree climbers have wide-spread toes with curved claws.
Some woodpeckers have two toes in front and two toes in back.
Three-toed woodpeckers have two toes in front and one in back. Birds of prey like hawks, owls, and eagles have strong legs and feet with sharp, hooked claws to grasp and carry prey. Ground feeders like sparrows and juncos often scratch the ground for food. Their feet have three forward toes and one hind toe. Both wing shape and flight patterns are clues to identification.