If it appears that the baby bird fell out of a nest, then simply locate the nest nearby and carefully place the bird back in the nest. Despite persistent fallacies, the parents cannot smell your odor on the baby bird, and will usually accept the young back into the nest. Young birds often fall from nests after wind storms, when the nesting sites are disturbed by construction, predators, or children, or when an older sibling kicks them out of the nest. If you cannot locate a nest, keep the bird in a suitable warm container, and contact your local SPCA or a certified wildlife care facility, such as the ones below, immediately:
Wildcare (San Rafael)
Peninsula Humane Society (San Mateo)
Wildlife Rehabilitation Information Directory