Common Swifts live socially year-round and usually in colonies during the breeding season. Nesting sites can be close together, but unlike the Alpine Swift (Tachymarptis melba), each breeding Common Swift pair vehemently defends a nesting cavity with its own entrance against foreign birds. Extremely faithful to nesting sites, the breeding bird returns annually to the nesting site of the previous year, and the bond to the partner also seems to be primarily linked to the return to the same breeding site, whereby males and females do not necessarily migrate together and often arrive on different days (LACK, 1956).
Flight games in which two swifts chase each other at a distance of a few meters, catch up, dive down screaming and separate again are probably courtship flights and serve to find or recognize a partner. Since other Common Swifts are often swept along, such chases in the colony area can easily include 10, 20 or more individuals. This "collective flight courtship" (DAANJE, 1944) can end very suddenly and transition into foraging.
Pair formation can also occur at the nesting site when a swift enters a cavity. If the newcomer is the partner from the previous year, the cavity owner greets them with soft calls and weak threat gestures that soon transition into mutual preening. A potential new partner, however, is initially met with loud screaming and violent threat gestures, followed by appeasement gestures in which the animals stand up and present their light throats to each other (LACK, 1956). The situation relaxes only slowly; mutual feather care takes considerable time. Mating usually takes place in the nesting cavity, but aerial copulations are also observed.
Paired cavity owners behave very aggressively towards intruding conspecifics. Screams and threat gestures such as raising the wings, tilting the body sideways and displaying the feet as "weapons" are meant to cause the intruder to retreat, which is usually successful. However, if the intruder responds with the same gestures, the opponents lunge to the side, grab each other"s feet and fight clawed together with violent wing beats and beak strikes. Such fights, accompanied by loud screams, can last several hours - with breaks.
LACK (1956), who witnessed such confrontations several times, vividly calls it "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 to throw them out. Attempts by the defeated to escape, pitifully chirping, are usually unsuccessful as they are energetically held. Sometimes both opponents fall out, still firmly clawed together, and even continue fighting outside the nest. Such fights rarely end in injuries or even death, but Common Swifts that fall to the ground while fighting can easily become victims of predators.
In the vicinity of breeding colonies, non-breeding individuals looking for nesting sites are usually found. LACK (1956) describes so-called "banging" as a means of exploring free nesting cavities: A searching swift ("banger") strikes a potential nesting site with its wing while flying past. Breeding birds react to this as they would to an intruder and settle down screaming at the entrance hole. This behavior can be observed throughout the entire breeding season.
As a typical social flight game, LACK (1956) describes the so-called "screaming parties": A more or less closed swarm of Common Swifts temporarily circles at great height and repeatedly flies past the nesting sites at racing speed with shrill screaming. All members of the colony participate in these complex maneuvers, including breeding birds and, towards the end of the breeding season, the fledged young as well as foreign conspecifics. They often transition directly into the ascent for aerial roosting.
The largest concentrations of up to several tens of thousands of individuals can occur during weather escapes. If the animals encounter prolonged cold, they can cling in large clusters to walls and rock faces, where they remain densely packed, initially only for overnight stays, then from exhaustion also during the day. Survival time is thus increased since more energy can be saved than with the individually resting animal. However, if the weather does not improve, the birds, especially the outer ones, begin to fall torpid to the ground.