Common Swifts are gregarious all year round and during the breeding season they usually live in colonies. Nest sites may lie close together, but unlike the Alpine Swift (Tachymarptis melba), each breeding pair of Common Swifts vehemently defends a nesting cavity with its own entrance against other birds. Extremely faithful to its nest site, the breeding bird returns every year to the previous year's nesting place, and the bond with the mate also seems to be linked primarily to the return to the same breeding site, males and females not necessarily migrating together and often arriving on different days (LACK, 1956).
Aerial chases in which two swifts pursue one another a few metres apart, catch up, plunge downwards screaming and then break apart again are probably courtship flights, serving to find or recognise a mate. Since other swifts are often carried along, such chases around the colony can easily involve 10, 20 or more individuals. This "collective aerial courtship" (DAANJE, 1944) can end very abruptly and turn into foraging.
Pairing can also take place at the nest site when a swift enters a cavity. If the newcomer is the previous year's mate, the occupant of the cavity receives it with soft calls and mild threat displays, which soon give way to mutual preening. A potential new mate, by contrast, is at first met with loud screaming and vigorous threat gestures; these are followed by appeasement displays in which the birds stretch upright and present their pale throats to one another (LACK, 1956). The situation eases only slowly, and it takes a considerable time before mutual preening begins. Mating usually takes place inside the nesting cavity, but copulations in flight have also been observed.
Paired owners of a cavity behave very aggressively towards intruding conspecifics. Screams and threat displays such as raising the wings, tilting the body sideways and presenting the feet as "weapons" are intended to make the intruder withdraw, which is usually successful. If the intruder answers with the same displays, however, the opponents throw themselves onto their sides, seize each other's feet and, locked together by their claws, fight with violent wing beats and blows of the bill. Such fights, accompanied by loud screaming, can last for several hours – with intervals.
LACK (1956), who repeatedly witnessed such disputes, describes it vividly as "a painful proceeding to watch, a gladiatorial show conducted only a few inches from our eyes". The swift lying underneath pushes the upper one towards the entrance hole in order to throw it out. Attempts by the loser to escape, piping piteously, are as a rule unsuccessful, since it is held firmly. Sometimes both opponents then tumble out, still tightly locked by their claws, and even go on fighting outside the nest. Only rarely do such fights end in injury or even death, but swifts that come to the ground while fighting can easily fall prey to predators.
Around breeding colonies there are usually non-breeding individuals to be found that are searching for nest sites. LACK (1956) describes so-called "banging" as a means of investigating vacant nesting cavities: a searching swift (a "banger") strikes a potential nest site with its wing as it flies past. Breeding birds react to this as they would to an intruder and alight at the entrance hole, screaming. This behaviour can be observed throughout the whole breeding period.
As a typical social flight display, LACK (1956) describes the so-called "screaming parties": a more or less coherent flock of swifts circles at times at great height and repeatedly races past the nest sites at furious speed with shrill screaming. All members of the colony take part in these complex manoeuvres, including breeding birds and, towards the end of the breeding season, the fledged young as well as swifts from elsewhere. Often they pass directly into the ascent to roost on the wing.
The greatest concentrations, of up to several tens of thousands of individuals, can arise during weather-escape movements. If the birds are caught in prolonged cold spells, they may cling in large clusters to walls and rock faces, where they hang closely packed together, at first only to spend the night, later, from exhaustion, during the day as well. Survival time is thereby extended, since more energy can be saved than by a bird resting alone. If the weather does not improve, however, the birds – above all those on the outside – begin to drop to the ground in a torpid state.