The Truth About Wildlife:

Some animal babies may appear to be abandoned, but the animal parents may actually be close by, staying out of sight to avoid attracting attention to their young. Animal parents may also be out gathering food for the young and will return soon.

Many hatchling or nestling baby birds (those that are featherless or incompletely feathered), can be returned to the nest. Birds have a poor sense of smell. If humans touch a baby bird or nest it does not cause the parents to abandon the birds.

After a fledgling (a fully feathered young bird) leaves the nest it may live on the ground for up to two weeks. During this time the parents still provide food and protection.

The maternal instinct is also very strong in most mammal species. Mammal mothers do not willingly abandon their babies. If the mother is alive, she will, in most case, return and claim her young.