Sexes alike. Blackish plumage; white rump and throat diagnostic; short, square tail and long, sickle-like swift wings. The large Fork-tailed Swift A. pacificus (18cm) has a deeply forked tail. Highly gregarious; on the wing during day, hawking insects, flying over human habitation, cliffs and ruins; strong fliers, exhibiting great mastery and control in fast wheeling flight; frequently utters squealing notes on the wing; retires to safety of nest colonies in overcast weather.