Common Swifts become sexually mature at the earliest towards the end of their 2nd year of life. One-year-old birds often spend the summer in the company of the breeding birds and may already occupy nesting cavities. Common Swifts form a monogamous seasonal bond with a pronounced attachment to the nest site, on which their mate fidelity is based. As a rule, a new pairing within the season only occurs if the partner is lost, but this can then sometimes happen after just 1-2 days.
The clutch of the Common Swift usually comprises 2 to 3 white-shelled, elongated elliptical eggs with unequal halves. There is only one brood per year. Incubation lasts about 20 days; both partners incubate. Incubating Common Swifts hardly lie still for a minute: they preen and shake themselves, tap at the walls, work on the nest, respond with calls to conspecifics flying past and often leave the eggs to look outside (GLUTZ and BAUER, 1980). Both breeding birds spend the nights and periods of bad weather in the nest; one sits on the eggs, the other usually right beside it or even on top of its partner.
The parents begin to feed immediately after the completely unfeathered young have hatched. During the first two to seven days they brood the young almost constantly. Food is collected in the throat pouch and formed with saliva into a ball the size of a hazelnut. In the first few days the ball is passed to the young in portions; later the young swift "engulfs" the bill and forehead of the adult bird, which forces the food ball deep into the nestling's throat.
The nestling period averages 42 days; both parents feed. The young beg intensively with singing calls and trembling wings. When older and very hungry, they also pursue the adult bird, fluttering, try to reach its bill and beat their wings wildly. If hatchlings fall out beside the nest and are unable to climb back in under their own power, the parents no longer take any notice of them despite their begging, and they starve. Feeding as a rule takes place only in the nest, even when the young are already older and crawl about the nesting cavity. When the adult bird flies in, they hurry back to the nest to receive the food; feeding at the entrance of the cavity occurs only in exceptional cases.
In freshly hatched Common Swifts the extraordinarily strong gripping reflex is striking straight away – vital for the young bird in order to hold on in the nest. When only a few days old, the still naked and blind nestling already shows preening behaviour and beats its wings in bouts. The eyes open between the 6th and 13th day and are blue during the first 4-5 weeks of life; only after that does the iris turn deep brown.
Droppings are always deposited over the rim of the nest, and from the age of three to four weeks out through the entrance hole. Activity in the nest can be observed day and night; there are no fixed sleeping phases. Even Common Swifts in the nest appear to rest only in short intervals, lying relaxed with closed eyes, the head never tucked into the plumage but often hanging down over the rim of the nest, only to start up abruptly, preen themselves or beat their wings. Soft "singing" as a contact call can be heard incessantly, even during the night, and only ceases when the young fledge.
Among themselves, young Common Swifts are extremely sociable and tolerant, whether they are true siblings or unrelated individuals placed with them. They always lie close beside and on top of one another, preen each other and lay their wings across each other. Busy exercising of the flight muscles in the form of "push-ups" and fluttering practice can be observed regularly from the 3rd week onwards, especially in the late evening hours. In bad weather and when food is scarce, the young birds mostly behave passively and devote all their energy to begging from the returning adult bird. If periods of hunger last longer, the nestlings can fall into torpor and, with their bodily functions reduced, survive several days without food.
Observations in the Swift Rescue Center on young swifts brought in almost starved have, however, in many cases revealed a pathologically heightened, feverish "activity" in the form of restless, unstoppable wandering about. This could explain why, in the course of periods of bad weather, so many young swifts emaciated to the bone are found outside the nest site: in their "compulsive wandering" they sooner or later fall out of the entrance hole instead of dying torpid in the nesting cavities.
As they grow older, the young leave the nest more and more often and move about the nesting cavity, eventually spending most of the day at the entrance hole, looking out. Even once they are fledged, many hours often pass before they leave the cavity. The young bird repeatedly stretches its head outside, fans its tail and spreads its wings before finally launching into the open. The siblings do not react to this; the parents are usually not present (GLUTZ and BAUER, 1980).
When an adult bird returns with food and finds the nest empty, LACK (1956) reports an almost bewildered behaviour – repeatedly running back and forth, pecking at the nest, courtship-like gestures with raised wings, standing upright and yawning – which he interprets as displacement activity, because the bird cannot complete the feeding of its young. In the end it swallows the food ball itself and flies off again. After the young have fledged – they do not return and are independent at once – the adult birds often roost in the breeding cavity for a few more days before setting off on their way back to the winter quarters.