In the Common Swift, food acquisition takes place exclusively in flight. Its food consists of insects and spiders. Mainly aphids, hymenoptera, beetles and diptera have been detected, spiders almost regularly in small numbers; these are actively flying or passively floating arthropods and almost always adult, usually winged stages (GLUTZ and BAUER, 1980). The food for nestlings contains almost exclusively objects 2 - 10 mm in length.
When the adult bird has the choice, it prefers prey that is smaller than 5 mm and hunts until a defined amount has been collected in the throat sac - a food ball stuck together with saliva into a ball (GLUTZ and BAUER, 1980). This ball can reach the size of a hazelnut. The average food quantity of such food balls, in which over 500 species of insects have been identified in Europe, is 0.3 g on the 3rd - 5th day of the young and after the 10th day 1.14 - 3 g (WEITNAUER, 1980).
In the first days of the nestling period, a food ball is divided among the young and fed in portions, later always as a whole. Depending on the prey size, normally 90 - 800 arthropods are contained in a food ball, in extreme cases up to 1,500 (GROSS, 1998). The hunting time varies, depending on the weather and food availability, from half an hour to several hours. LACK and OWEN (1955) state that probably more than 20,000 arthropods are caught per day.