Coming across young birds triggers empathy and compassion in those who care about wildlife, but human intervention is not always the best thing for the young bird. The key is knowing when to intervene, when to let nature be, or when to call a licensed wildlife rehabber to assist you.
First observe the bird and take note of physical appearance, energy level, and its surroundings.
Handle the baby bird as little as possible.
Fledglings are baby birds that have almost all of their feathers but their wings and tail feathers will be short. Fledglings can usually flutter or fly very short distances, and the parents are usually aware that their baby has fledged and are still around to feed their fledgling a few times a day.
Hatchlings are very young birds who may not have any feathers or few feathers and their eyes may be open or closed. Hatchlings are usually less energetic and will not survive on their own if there is no visible nest around.
After determining the age of the baby bird, you then want to decide your next course of action. If it is a hatchling, call your local animal control officer for help contacting a licensed wildlife rehabilitator in your area.
If it is a hatchling, see if you can find the nest nearby and put the baby bird back in the nest.
If it is a fledgling, look for visible signs of injury. If you the bird is otherwise healthy, leave him or her be as the parents should still be around caring for it. If the bird is injured, gently place the bird in a box and bring him or her to a licensed wildlife rehabilitator.
If you know that a bird has been orphaned because you saw the deceased parent or if you know a nest of babies have been alone for longer than normal, contact a licensed wildlife rehabilitator.
Never feed baby birds as improper techniques can cause them to choke or become malnourished. It is always better to get them into the hands of someone who is trained in knowing what each species of bird requires in order to thrive.
There are independent wildlife rehabilitators who are listed on the WRAM website and there are a few wildlife rehabilitation centers including: New England Wildlife, Tufts Wildlife Clinic, and Cape Wildlife. Make sure to always call a rehabilitator or rehabilitation facility before you bring a wild animal to them to make sure that they have space, time, staffing, and the resources needed to care for the animal you are bringing to them.

Two orphaned hawks who were transported to Tufts Wildlife Clinic after the tree containing their nest was cut down. Mom was moving further away and had gone a prolonged period of time without feeding them. They are back in the wild being raised by another hawk!