Cliff Swallows, as their name suggests, will build nests out of mud along cliffs or under bridges. You won't find any here, though there are some nesting under the Croton River Bridge.
You are searching for a medium-size lock-n-lock container at the foot of the rock formation a few steps from the trail. If you can't see the trail, you've gone too far. The cache is covered by some bits of bark and can get covered with leaves in the fall. If in doubt, poke it with a stick.
Choate Sanctuary Info and trail map
This cache is hidden with permission. See: SMRA Geocaching Guidelines