Here are some abandoned (at the time photographed) cliff swallow nests. These were spotted under a bit of an overhang in an area we explored off the Notch Trail. According to the Park Service, cliff swallows are considered abundant in the park. So the chances of you spotting one or at least a nest is pretty good. There are 215 species of birds that have been recorded in the park.
This bird averages 13 cm (5 inches) long with a tiny bill. The adult has an iridescent blue back and crown, brown wings and tail, and buff rump. The nape and forehead are white. The underparts are white except for a red face. The tail is square-ended. Young birds are essentially brown above and whitish below, except for the buff rump and dark face.
Cliff Swallows breed in large colonies. They build conical mud nests and lay 3-6 eggs. The natural nest sites are on cliffs, preferably beneath overhangs, but as with the Eurasian House Martin, man-made structures are now the principal locations for breeding. Female Cliff Swallows are known to lay eggs in and move previously laid eggs into the nests of other birds within the colony.