A common swift catches insects from the air (predominantly aphids, cicadas, bugs, flies, ephemerids, mosquitos, ants, butterflies) as well as spiders which it forms into a pellet sized like a hazelnut and weighted about 1g using saliva inside its pharynx (the ball may consist of 90-800 individuals).
Then it carries the food within its throat pouch over to the nest.
Feeding takes place, as the young swift appears to swallow the head of the older bird with its beak wide open, while that one chokes the food ball directly into the throat of the nestling.