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 breeding birds and can already occupy nesting cavities. Common Swifts maintain a monogamous seasonal partnership with pronounced nesting site fidelity and resulting mate fidelity. As a rule, a new pairing within the season only occurs upon loss of the partner, but this can sometimes happen after just 1-2 days.
The clutch of the Common Swift usually comprises 2 to 3 white-shelled, elongated elliptical, asymmetrical eggs. There is only one brood per year. The incubation period is about 20 days, both partners incubate. Incubating Common Swifts hardly lie still for a minute, preening and shaking themselves, tapping the walls, building the nest, responding with calls to passing conspecifics and often leaving the eggs to look outside (GLUTZ and BAUER, 1980). Both breeding birds spend the nights and bad weather periods in the nest; one sits on the eggs, the other usually close by or even on top of the partner.
Feeding begins immediately after the completely unfeathered young hatch. In the first two to seven days, they brood them almost constantly. Food is collected in the throat sac and formed into a hazelnut-sized ball with saliva. In the first days, the ball is handed over to the young in portions; later, the young swift "devours" the beak and forehead of the adult bird, who pushes the food ball deep into the throat of the nestling.
The nestling period averages 42 days, both parents feed. The young beg intensively, singing with trembling wings. At an advanced age and when very hungry, they also chase the adult bird with fluttering, try to reach its beak and beat their wings wildly. If hatchlings fall next to the nest and cannot climb back on their own, they are no longer noticed by the parents despite begging and starve. Feeding usually only takes place in the nest, even when the young are older and crawling around in the breeding cavity. When the adult bird flies in, they hurry back to the nest to receive the food; feeding at the entrance of the cavity only occurs in exceptional cases.
In freshly hatched Common Swifts, the extraordinarily strong gripping reflex is already noticeable – vital for the young bird to hold on in the nest. Already at a few days old, the still naked and blind nestling shows preening behavior and periodically beats its wings. 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 edge of the nest, at the age of three to four weeks out of 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 seem to rest only in short intervals, then lying relaxed with closed eyes, the head never tucked into the plumage but often hanging down over the edge of the nest, only to start up suddenly, preen themselves or beat their wings. Soft "singing" as a contact call can be heard incessantly even during the night and only stops when they fledge.
Among themselves, young Common Swifts are extremely social and tolerant, whether they are biological siblings or foreign individuals added to the group. They always lie close together and on top of each other, preen each other and place their wings over one another. Diligent testing of the flight muscles in the form of "push-ups" and fluttering exercises can be regularly observed from the 3rd week onwards, especially in the late evening hours. In bad weather and food shortage, the young birds usually behave passively and use all their energy to beg from the returning adult bird. If hunger phases last longer, the nestlings can fall into torpidity and survive several days without food with reduced body functions.
However, observations at the Swift Clinic of young swifts admitted nearly starved showed in many cases a pathologically increased, feverish "activity" in the form of restless, unstoppable wandering, which could explain why during periods of bad weather so many young swifts, emaciated to the bone, are found outside the nesting site, because in their "compulsive wandering" they sooner or later fall out of the entrance hole instead of dying torpid in the nesting cavities.
With increasing age, the young leave the nest more and more often and move around in the nesting cavity, eventually spending most of the day at the entrance hole looking out. When they are ready to fledge, many hours often pass before they leave the cavity. The young bird repeatedly sticks its head outside, fans its tail and spreads its wings before finally jumping into the open. The siblings do not react to this; the parents are usually not present (GLUTZ and BAUER, 1980).
When a food-bringing adult bird returns and finds the nest empty, LACK (1956) reports an almost distressed behavior – repeated walking 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 feeding the young. Finally, it swallows the food ball itself and flies off again. After the young fledge, which do not return and are immediately independent, the adult birds often roost in the breeding cavity for a few more days before setting off on their return journey to their winter quarters.