The Common Swift obtains its food exclusively in flight. Its diet consists of insects and spiders. The prey recorded consists mainly of aphids, hymenopterans, beetles and dipterans, with spiders almost always present in small numbers; these are actively flying or passively drifting arthropods and nearly always adult, usually winged stages (GLUTZ and BAUER, 1980). The food given to nestlings consists almost exclusively of items 2 – 10 mm in length.
Given the choice, the adult bird prefers prey smaller than 5 mm, and it hunts until a certain quantity has accumulated in its throat pouch – a food bolus bound together into a ball with saliva (GLUTZ and BAUER, 1980). This ball can reach the size of a hazelnut. Over 500 insect species have been identified in such food boluses in Europe; their average weight is 0.3 g when the young are 3 – 5 days old and 1.14 – 3 g after the 10th day (WEITNAUER, 1980).
During the first days of the nestling period, a single food bolus is divided among the young and fed to them in portions; later it is always given whole. Depending on the size of the prey, a food bolus normally contains 90 – 800 arthropods, and in extreme cases up to 1,500 (GROSS, 1998). The time spent hunting varies from half an hour to several hours, depending on the weather and the availability of food. LACK and OWEN (1955) state that probably more than 20,000 arthropods are caught per day.