In the cool mist of a waterfall, several black arrows dart back and forth to the recesses behind the falls. Behind this veil of cascading water in the damp moss-laden cliffs, hidden and safe from predators, are nests of the northern subspecies (borealis) of the Black Swift. Restricted to the mountains and rugged coastlines of the western United States, they can be found foraging over a wide area, often far from their nests. However, most people are only lucky enough to see them at their traditional nesting grounds.

These sites are widely scattered because the waterfalls and seacliffs that provide suitable nest locations are rare and local throughout much of the subspecies' range. However, in coastal British Columbia nesting opportunities abound and the swifts are often abundant in this population stronghold. (The two other subspecies breed in Mexico and the West Indies.) The nests, made of moss bound with mud or simply a cushion of grass or bare mud, are often built on small ledges with overhanging moss or grass. Unlike their North American relatives, these swifts prefer to nest near water and choose waterfalls in the mountains and sea-sprayed crevices on the coasts. Normally the female lays only one or two eggs. These eggs are enormous in size, about 3 times larger than the egg of the White-throated Swift—a bird nearly the size of a Black Swift.

We may take for granted our knowledge of the eggs and nests of the Black Swift, but these were undiscovered until 1901. Pursuing one of the last mysteries of North American oology (study of eggs), A.G. Vrooman found and confirmed the first nest along a seacliff in Santa Cruz, California. He was collecting the eggs of a Pelagic Cormorant and while climbing down the cliff on a rope, accidentally flushed a swift off of its nest. It was not until 1905, though, that he found another nest and, with specimens in hand, was able to convince the skeptical ornithological community.

One of the rare and unpredictable phenomena during migration along coastal California is the sighting of flocks numbering dozens if not hundreds of Black Swifts flying north in late May. Often traveling in front of storms, these birds fly swiftly, covering vast stretches of coastline enroute to British Columbia. Autumn migration yields a few sightings of individuals and small flocks, yet never the grand spectacles seen in the spring.

The wintering range is thought by some to cover much of Central America, but sightings are rare and only suggest that perhaps at least some of the Black Swifts winter there. Identifying a high-flying swift, not only to species but to race, is a daunting task that may never be accomplished with certainty. Maybe some day its wintering biology may be as well known as its breeding range.

Often called the "cloud" swift, these birds feed on aerial insects and hunt in the rising air masses that sweep large numbers of insects into the sky. No one really knows how far these birds venture from their nests in search of food, but legend has it that it is hundreds of miles each day. Perhaps this is a mystery that may never be solved.