Introduction

This swift occurs widely throughout western North America in summer, with its breeding range extending as far north as southeastern Alaska, as far east as central Colorado, and south through Mexico and Central America to Costa Rica, with additional populations in the West Indies. Despite this extensive distribution, only about 80 specific nesting localities have been documented; most nesting sites are associated with sheer cliffs and waterfalls. Nowhere in this range is it considered to be an abundant summer resident.

The Black Swift is the most incompletely studied North American swift. The first nest of this species was not located until June 1901, when A. G. Vrooman collected a nest and its single white egg on a seacoast cliff near Santa Cruz, California (Vrooman 1901, Farrand 1990). This location and single-egg clutch, so unlike what was known for any other swift at the time, caused many people to feel Vrooman’s identification was incorrect and perhaps he had found the nest of a storm-petrel. It was not until 1914, when Vrooman was able to show a nest to W. L. Dawson, that skepticism over his discovery was overcome (Dawson 1915). Subsequent to Vrooman’s discovery of a seacoast nest, the Black Swift has been found to use inland sites with nests placed in cool, dark, damp sites near or behind waterfalls in mountains; the first such inland nesting was found near Banff, Alberta, in September 1919 by C. E. Chapman (Bent 1940).

Even today much remains to be learned about this enigmatic swift. The South American wintering range of North American breeders has only recently been recognized. The winter range of Central American and a portion of the West Indian breeding populations has yet to be defined, although it is probably also in South America.

Other unique aspects of the breeding biology of the Black Swift are its single-egg clutch, long incubation and fledging periods, and apparent specialization in foraging on the nuptial-flight swarms of fat-rich, winged reproductive ants. The small clutch, long development period, and long intervals between chick-provisioning visits to the nest are more typical of some small procellariiform birds than non-cypseloidine swifts.

The rarity of known nest sites has attracted interest in this species. Charles and Enid Michael (C. Michael 1927; E. Michael 1926, 1933) provided early published records of nesting biology, and O. A. Knorr seemed to make a career in finding nesting sites in Colorado and nearby states (Knorr 1950, 1961, 1962, 1993b; Knorr and Knorr 1989). Recent studies on behavior and breeding biology have been undertaken, particularly in southern California (Foerster 1987; Foerster and Collins 1990; Collins and Foerster 1995; Marín 1997a, 1997b, 1999b; Collins and Peterson 1998) and Costa Rica (Marín 1999a, Marín and Sánchez 1998).