Biology of Swifts

Swifts are migratory birds that overwinter in Africa. They nest and raise

their young here during the summer. A swift clutch usually contains 2–3 eggs, which produce an average of two fledglings. Swifts only nest once a year.

Swift Portraits

The Common Swift is the smallest and most common of our swifts. With a length of 16 cm and a weight of about 50 g, it is larger than a swallow and more than twice as heavy. Its plumage is brownish-black, with a pale throat patch that is only visible at close range. The Common Swift has a wingspan of about 40 cm. Its call is a pure, piercing "sriiii".

(Image: S. Volpi, Rümlang.)

The Alpine Swift is rarer than the Common Swift. At 22 cm long, it is also larger. It weighs up to 100 g and has an average wingspan of 55 cm. It is easy to recognize by its white belly, crossed by a brown breast band. Its call is a high-pitched trill.

(Image: S. Volpi, Rümlang.)

In Switzerland, the Pallid Swift is only found in Ticino. With a wingspan of about 45 cm, it is barely larger than the Common Swift. Its plumage is slightly paler. However, distinguishing between the two species remains a matter for specialists. Their calls are also very similar.

(Image: G. Boano)

What makes Swifts so special?

Swifts fly fast and skillfully. Most of their lives are spent in the air. Their morphology is specially adapted for long-duration flight. Their wings are particularly long. Once the swift lands, its wings cross 5–8 cm behind its back, like two sickles. Their very small legs are not visible on a bird on the ground. Swifts cannot walk or hop. After landing, they laboriously crawl with the help of their wings. It is very difficult for them to take off from the ground. This is why they never voluntarily land on the ground but only on steep places, from which they can take off by simply dropping. They catch their prey, insects and small spiders, in flight, collect nesting material, and drink water from rivers and lakes.

Young Common Swift: its long wings show it is close to fledging.

(Image: S. Volpi, Rümlang.)

As early as 1945, it was known that Common Swifts could spend the night in the air (Weitnauer, 1983). To do this, they ascend to an altitude of 1000 to 3000 m, into warmer air layers. Early in the morning, they return to their colony without getting lost, even if currents have displaced them by more than 50 km during the night. During their nocturnal flight, they only need to maintain their altitude and can even save energy by slowing down their wing beats. During incubation and when the chicks need to be kept warm, they obviously stay in the nest overnight.

Nocturnal observations in large colonies of Alpine Swifts have shown constant coming and going, especially after the young have fledged. It seems that they often visit other colonies. Recent research has shown that outside the breeding season, Alpine Swifts also spend the night in the air, just like Common Swifts.

In cold and wet weather, adult swifts stay in the nest or move away from a cold front over long distances. Compared to other species, nestlings are quite resistant to cold. From about twelve days old, juveniles can enter a state of torpor, a kind of hibernation, during cold weather spells. Their body temperature drops, which reduces energy consumption. When the adults return, the juveniles are warmed and fed normally again. The time spent in the nest is around 6 weeks, or up to 8 weeks in poor conditions. Alpine Swifts require approximately 8 weeks, or more depending on poor atmospheric conditions.

High-Up Nesting Sites

Swifts need to reach a certain speed to fully display their aerial abilities. Their nesting sites must therefore be sufficiently high up. This allows the birds to drop down and immediately gain speed. In the past, they nested in high cliffs. Today, the vast majority of nesting sites are found in buildings. The Common Swift prefers rather small and dark cavities, whereas the Alpine Swift also appreciates more spacious places, such as unused attics, which several pairs can occupy. Both species are considered colonial breeders, even if solitary nesting pairs are found.

Pairs looking for a nesting site usually explore the immediate vicinity of other nesting pairs. They then particularly inspect corners and edges, looking for dark, not too large openings.

The circles show places often prospected by Swifts when looking for a nesting site.

Swallow or Swift Nests?

Swifts and swallows nest on our buildings and have similar lifestyles. It's therefore not surprising that they are often confused. Unlike swifts, which nest in dark cavities, swallows attach their nests to walls or beams, in visible but rain and wind-protected locations. They build their nests with mud pellets collected from the ground. Swift nests, on the other hand, are made of small feathers, grasses, light leaves, and similar materials, collected in flight and glued together with saliva to form a cup-like shape.

Natural House Martin nest, built with mud pellets between artificial nests.

(Image: M. Werder.)

This swift nest in the under-roof has been cleared. The cup is firmly cemented with saliva. The material partly comes from sparrows