Swifts are among the most aerial of birds, spending most of the day on the wing in search of prey (Rudalevige et al. 2003) and opportunistically seeking out large hatches of air-borne insects (Dixon 1935, Knorr 1961, Marín 1999) while avoiding those areas where insects are scarce (Lack and Owen 1955). This swift forages at long distances and for long periods of time away from the nest (Collins 1998) using weather patterns to pursue pockets of concentrated aerial plankton (Udvardy 1954). At other times they opportunistically forage close to the ground or water capturing blooms of insect hatches, especially during inclement weather (Rathbun 1925, Drew 1882, K.M. Potter personal. observations).
The earliest published information on the diet of Black Swifts comes from the analysis of the stomach and esophageal contents of Black Swift specimens collected between 1882 and 1940. This information suggests they consume a wide variety of arthropods and according to Rathbun (1925) nothing in the nature of aerial insect life is rejected by this bird. Historical data sources that provide insect identifications include documentation by Dixon (1935) for one adult female collected in California, by Drew (1882) for ten adults collected over a two year period in Colorado, and for 11 adults collected by Davis (1935) and Rathbun (1925) from Washington. These studies documented that Black Swifts had consumed over 67 families of insects from 11 different orders. Bent (1940) published notes provided by Clarence Cottam summarizing the contents of 36 stomachs of Black Swifts as analyzed by the Biological Survey, Smithsonian Institution. Cottam found the food items most frequently consumed to be ants, bees and wasps of the Order Hymenoptera. Second were the flies of the Order Diptera with nine families represented in the samples. Also found were beetles of the Order Coleoptera with six families identified, and mayflies and caddisflies of the Order Trichoptera.
Information gathered from Black Swift specimens collected during the late 19th and early 20th centuries show that Black Swifts forage opportunistically for large insect hatches. Dixon (1935) found almost 100% of the crop and stomach contents of the specimen he collected to be winged ants (Order Hymenoptera). Davis (1931) found that the crops and stomachs of all five adults he collected contained 99% leafhoppers (Order Hemiptera). Drew (1882) noted that the crops of all ten specimens he collected were filled with mayflies (Order Ephemeridae). Rathbun (1925) characterized the stomach contents from three Black Swifts as 'mixed' and the other three as dominated by craneflies 70% (Order Diptera), termites 70% (Order Isoptera) and flies 60% (Order Diptera).
More recent studies analyzed esophageal contents. Black Swifts carry large masses of food items in their esophagus, enabling them to forage long distances for food from which they provision nestlings at intervals during the night (Collins and Peterson 1998). This food is compacted into several tight masses of insects bound with thick saliva and referred to as boluses. In California, Foerster (1987) analyzed two boluses, Marín (1999) analyzed ten boluses, and Rudalevige et al. (2003) analyzed three boluses. Collins and Landy (1968) analyzed the esophageal contents of two adults in Mexico. All of these studies found flying ants of the Order Hymenoptera to be the dominant food items taken. So Black Swifts may capitalize on calorically dense swarming insects such as ants and termites as suggested by Collins and Landy (1968), and Rudalevige et al. (2003).
Food items measured in studies ranged in size from 1.8-14.5 mm (Collins and Landy 1968, Foerster 1987, Marín 1999, Rudalevige et al. 2003).