This article is from the Birds FAQ, by Lanny Chambers with numerous contributions by others.
If you come across a nest full of nestlings with no parent in sight,
do not assume that the nest has been abandoned. In fact, the best way
to ensure that the nest does not become abandoned is to leave the area
at once. Birds do not like large animals of any kind near their
active nests, and may cut their losses at any time.
If you find a nestling that has fallen out of the nest, consider
placing it back in the nest only if the task can be done quickly and
with a minimum of disturbance. You may also consider placing it in a
nest of the same species. In either case, make sure that your attempt
is unobtrusive and rapid. You should not feel guilty if, after
examining the situation, you decide not to replace the nestling; no
nestling's survival is guaranteed, in or out of the nest.
By placing a nestling or egg back into a nest, or even by observing
the nest for the necessary length of time, you may be helping
predators find the nest.
If you find a fallen nestling which you cannot replace in a nest, or
if after several hours of unobtrusive observation you determine that a
nest full of nestlings is abandoned, do not attempt to rescue the
birds yourself unless you are prepared to commit to dawn-to-dusk
feedings, keeping them close by you at all times. See _The Bluebird:
How You Can Help Its Fight for Survival_, by Lawrence Zeleny (Indiana,
1976), for an account of hand-raising Eastern Bluebirds (Sialia
sialis). Contact a wildlife rehab center for assistance.
If a bird can perch on a branch by itself and is covered with
feathers, it is a fledgling, not a nestling, and should be left alone.
Note that hand-raising birds without authorization may be a violation
of the law.
For more information about rehabilitation, see