Swifts feed almost exclusively on the aerial plankton of flying insects and airborne spiders of small to moderate size. They mainly feed at around 50-100 m, but sometimes weather conditions force them down to lower levels.
Turbulence can sweep insects far higher in the air, and swifts have been observed following these swarms to about 1,000m.
They are opportunistic feeders, and exploit swarms and hatchlings wherever possible. They are selective with what they take, and generally go for the largest available prey that they are able to swallow. They avoid stinging insects, which are recognised by means other than the insects warning colours, enabling them to exploit foods such as hoverflies.
As the bird is hunting, insects are collected in the back of the throat in a special food pouch and bound together with saliva into a ball called a bolus, which is periodically eaten or taken to the nest. These food balls can contain thousands of insects.
In rough weather, large numbers of swifts feed over water where insects are easier to catch. Swifts tend to avoid low pressure centres and other areas of bad weather. They fly into the wind in search of better weather and to get round an area of rain, and can thus fly over 800 km a day.
Swifts drink by catching raindrops in the air, or by flying low over water, skimming a mouthful from the surface.