Common swift (Apus apus) in flight, 18 July 2025. Photo: Alexis Lours, CC BY 4.0, Wikimedia Commons.
The common swift · a year in the air
City swifts and their year in the air: some don’t stop flying for almost ten months
Thirteen swifts tracked with loggers in Sweden were airborne more than 99% of the ten months they spent away from the nest; three never stopped for two hours or more. No tracked swift from Romania is known yet, so the year of the swifts that scream over Romania's tower blocks is pieced together below: dates and winter quarters from the nearest tracked birds, in the Czech Republic and Italy, and the way they fly from the Swedish ones.
Pick a Romanian county. The dates and distance in the story follow it.
Pitești, a town in southern Romania, 13 May 2025. Recording: Radu Teodoreanu, CC BY 4.0.
In summer, swifts nest in cracks, under eaves and in the holes of tall buildings. By day they hunt insects above the roofs in screaming flocks.
Nesting adults sleep in the nest at night. Birds that are not nesting usually climb at sunset and spend the night in the air; before migrating, in bad weather, some young birds occasionally rest in trees or on buildings.
When swifts are seen in Romania
1,871 sightings from all of Romania, by ten-day period, all years together.
These are sightings people sent in (eBird, Observation.org, iNaturalist and others), not the birds' exact arrival and departure dates: they depend on how many observers go out, and when.
On radar in Sweden, swifts spending summer nights on the wing climbed to 1,335 metres on average. Up there they face into the wind and alternate wingbeats with glides; sometimes the wind carries them backwards.
Other studies have recorded climbs at dusk and dawn as high as 2,500 metres. The block below is drawn to the same scale.
The swifts tracked from the Czech Republic left around 14 July, those from Italy around 15 July. In Romania, SOR, the Romanian Ornithological Society, says they leave in August, and sightings thin out towards the end of August.
From here on, the date on screen follows the clock of the Czech birds.
From Bucharest to where the Czech and Italian swifts spent the winter is about 6,300 km in a straight line. Real routes are longer: in autumn the tracked Swedish swifts flew 9,769 km on average, more than half as far again as the straight line.
The dotted line is imagined: no tracked swift from Romania is known so far.
Each dot is the average November–December position of one swift tracked from 11 European populations. Those from Sweden and Finland wintered in West and Central Africa, as far as the forests of the Congo basin.
Those from the Czech Republic and Italy, the nearest to Romania, mostly went further, to south-east Africa, from Tanzania to Zimbabwe and Mozambique; one Czech bird stayed in the Great Lakes region. Romania's swifts probably winter in central or south-east Africa too: birds that nest further south in Europe winter further south in Africa.
In the 13 Swedish swifts, the loggers showed flight more than 99% of the time during the ten months away from the nest. Three never stopped for two hours or more.
For one, the data suggest about 314 days aloft, with a single recorded stop, of two hours. The others stopped now and then, at night, usually briefly.
Nobody knows for sure. Researchers think swifts sleep in the air, perhaps while gliding at night, but a swift's sleep has never been measured.
It has been measured in frigatebirds, large ocean birds, with sensors on the brain: in flight they slept about 41 minutes a day, sometimes with only half of the brain.
In spring they hurry. Six tracked Swedish swifts flew 9,208 km home on average: 469 km a day on flying days (from 274 to 650) and 336 km a day including stops.
The tracked Czech swifts came back around 4 May, after 294 days away, the Italian ones around 27 April.
In Romania, in the 15 years since 2010 with enough sightings, the first swift of the year has been seen between 1 and 27 April, and by 14 April in half of those years. More sightings come from May than from any other month. A pair that has chosen its nest returns to it every year.
Anders Hedenström and colleagues at Lund University caught adult swifts at two colonies in southern Sweden and fitted them with activity loggers in 2013 and 2014. In the following years, when the birds returned, they recovered loggers from 13 swifts. A logger records flight activity and body position. The authors counted as a stop a period with no movement while the bird was near-vertical, that is, clinging.
The result, published in Current Biology in 2016: the swifts were airborne more than 99% of the ten months when they are not nesting. Three never had a stop of two hours or more between September and April. In the bird that rested most, all stops together came to 0.64% of the time. The stops were at night, and four swifts settled for a whole night at least once.
The longest record so far belongs to a young swift caught at a colony in Ghent, Belgium, when it was at least a year old. Its logger recorded 19 months of continuous flight: out of more than 12,000 hours, only about 48 were flagged as possible rest. It then nested, in a nest box. It is one bird, and its exact age is unknown. The popular idea that young swifts stay aloft for two or three years until they first nest has not been tested by measurement.
A swift over Gennevilliers, France. Photo: Alexis Lours, CC BY 4.0.
Nobody knows for sure: no swift from Romania is known to have been tracked with a geolocator. What is known comes from 102 adult swifts tracked from 11 European populations, from Lapland to Spain (Susanne Åkesson and colleagues, Evolution, 2020). The rule they found: the further south a population nests in Europe, the further south it winters in Africa.
Swifts from the Czech Republic and Italy, the nearest to Romania among those tracked, spent November and December mostly in south-east Africa, from Tanzania to Zimbabwe and Mozambique; one Czech bird stayed in the Great Lakes region. SOR, the Romanian Ornithological Society, says the species winters in the southern half of Africa. The distances below are straight lines from a main town in each Romanian county to the middle of the Czech and Italian dots; the route flown is longer, by 43–53% in the Swedish birds.
Czech Republic and Italy (nearest to Romania) · 7
Sweden and Finland · 60
UK, Netherlands, Belgium, Germany · 26
Spain · 9
Romania (white dot: Bucharest)
Straight-line distance to where the Czech and Italian swifts wintered
SOR says the swift “arrives generally from April and leaves for its wintering grounds in August”. Sightings people send to eBird, Observation.org, iNaturalist and other platforms agree: since 2010, in the 15 years with at least 20 sightings, the first swift of the year was seen between 1 and 27 April, and by 14 April in half of those years. Sightings peak in mid-May (11–20 May), and 95% were made before 17 August.
Sightings from August, September and October may be young birds, northern birds passing through, or pallid swifts mistaken for common ones. They do not show that the swifts nesting here leave late. The counts are per ten-day period, after keeping one sighting per place and day.
When swifts are seen in Romania
These are sightings people sent in (eBird, Observation.org, iNaturalist and others), not the birds' exact arrival and departure dates: they depend on how many observers go out, and when.Swift sightings by county (one per place and day)
It weighs about 40 grams: 39.5 g on average across 1,160 adults weighed by British ringers. The oldest swift on the European list of ringed birds, from Sweden, was at least 21 years and one month old when it was found; one in Switzerland was at least 21 when a ringer caught and released it.
Anders Hedenström estimated that a swift that lives to 20 flies, in total, the equivalent of seven trips to the Moon and back. That is his calculation, not a measurement.
The fastest speed measured in a flapping swift, as of 2010, is 31.1 metres a second, about 112 km/h, in the screaming flocks that chase each other in summer. The “over 200 km/h” in some Romanian articles has no source.
Leave their nest sites. Swifts nest in cracks, under eaves and in the ventilation holes of apartment blocks. SOR says building renovation is the main threat to the species in Romania, and that new or renovated buildings no longer have cavities to nest in. SOR gives an estimate of 15,000 to 60,000 pairs nesting in Romania, and nobody knows whether their number is rising or falling.
Put up a box. Swifts nest successfully in boxes fixed to façades. SOR's advice: at least 5 metres above the ground, with open air in front, on a shaded side of the building. Once a pair has chosen its nest, it returns every year.
If the block is being renovated. In the Bucharest-Ilfov Regional Programme 2021–2027 (Bucharest is Romania's capital), renovating residential and public buildings requires a bird and bat survey by an expert before work starts; where birds are found, either postponing work until the chicks have fledged or relocating them under a legal derogation; and artificial nests where nests were removed. Whether Romania's national block-insulation programmes have a similar rule has not been checked, and no count of lost nests was found. Ask the owners' association whether such a survey was done and whether façade work avoids May to August.
If you find a swift on the ground, it is probably injured or a chick that fell from the nest. SOR advises calling a specialist and giving it neither food nor water. Among the volunteers who care for grounded swifts in Romania is the Inițiativa “Drepneaua Neagră” (Common Swift Initiative).
Swift boxes on an apartment block in Prague, Czech Republic. Photo: Ivana Cabrnochová, CC BY-SA 3.0.Two swift chicks a few days before their first flight, in a nest box in Greifswald, Germany. Photo: Tonio Schaub, CC0.
Measured: the flight of 13 swifts from southern Sweden for ten months (activity loggers); the winter positions of 102 swifts from 11 populations (light loggers, with an error of 100–200 km); the routes and speeds of six Swedish swifts; the height of night flights on radar in Sweden; 19 months of flight of one young swift from Belgium; sleep in flight, in frigatebirds only.
Imagined: the dotted line on the map and the swift's progress along it. No tracked swift from Romania is known, and the departure and return dates on screen are the medians of the three tracked Czech swifts. Swifts sleeping on the wing is the researchers' inference, not a measurement.
Observed: the county calendar counts sightings sent to eBird, Observation.org, iNaturalist and other platforms (through GBIF, downloaded on 2 October 2026), one per place and day. A county gets its own calendar from 30 sightings up. 69 sightings with no county count only in the national total.
