Getting the night ready…
One autumn night, sunset to sunrise
The night birds: autumn migration over Romania, counted by weather radar
On the night of 29 September 2026, about half a million birds flew over Bucharest, Romania’s capital, heading south-west almost a kilometre up, and nobody saw them. The city’s weather radar counted them every five minutes. After that night comes the whole autumn, on Romania’s seven radars and on the radar nearest you.
The radar nearest youThis was the sky at 20:50. Keep going and the night starts again from sunset.19:00 · sunset
At sunset the sky is almost empty#
The sun sets at 19:00. The picture is a slice of air over Bucharest, cut along the line the birds are about to fly, with the city’s real buildings below and Sky Tower on the right. Heights and distances share one scale.
The city’s weather radar counts fewer than one bird above every square kilometre. Every mark in the drawing stands for one bird of the measured density.
20:25 · take-off
An hour after sunset the air fills up#
Between 19:29 and 20:25 the density rises from under one bird to 22 birds per square kilometre. Most do not arrive from far away: they rise from the city and the fields around it, where they spent the day, within the hour after dark.
The radar cannot tell species apart. Night migration over south-eastern Europe is dominated by small songbirds, which fly at night and rest by day.
20:50 · the peak
69 birds above every square kilometre#
At 20:50 the night is at its busiest. The slice now holds as many birds as the measured density gives, drawn far larger than they are.
Nine in ten of the birds counted fly below 2,520 metres, up to 18 times the height of Sky Tower (137 m to the roof). The busiest layer is the lowest one counted, 320–520 metres, with 28% of the birds, so many probably fly lower still. The air below 320 metres is not counted here, because echo from the ground and the buildings can drown it: that is the misty band at the bottom.
22:00 · which way
On average south-west, at 54 km/h#
The radar measures direction and speed from the Doppler shift of the echo, as an average for the whole flock: 216°, south-west, at about 54 km/h over the ground. In the drawing the birds move at exactly that speed, in real time, from right to left.
Along this line lie Bulgaria, Greece, the Ionian Sea and Libya. A bird flying without a break from 19:29 to 06:42 would be about 600 km away by morning, in northern Greece, between Kozani and Larissa.
00:00 · the moon
Night birds can also be counted against the moon#
Ornithologists point a telescope at the moon and count the silhouettes crossing its disc. That is how Pavel Zehtindjiev and Felix Liechti estimated, in 2003, that about 900 million birds pass over Bulgaria, south-eastern Romania and northern Greece on autumn nights.
That night the moon, 88% lit, rose at 20:00. From the radar profile one can work out what such an observer in central Bucharest would have seen: at 00:00, about 81 birds an hour crossing the moon, one every 44 seconds. The telescope in the picture shows them at that rate, each crossing in under a second.
02:30 · the count
Thousands of birds over every kilometre#
The counter at the top adds up the birds that have crossed, since dusk, a line one kilometre long drawn across their path. By 02:30: 19,100 birds.
Measured across their path, Bucharest is almost 22 km wide. Over the whole city, then, 22 times as many go by.
06:42 · dawn
Towards daybreak the sky empties#
The birds come down gradually all night: by about 01:15 the air held only half as many as at the peak. Towards 06:42, half an hour before sunrise, the sky is almost empty. Over the whole night, about 23,500 birds crossed every kilometre; over the whole of Bucharest, around 510,000.
The number is very uncertain. The radar measures echo and divides it by the echo of a typical songbird. If that night’s birds were larger there were fewer of them, if smaller more: between 15,200 and 51,800 over every kilometre, or between 330,000 and 1.1 million over the city. The radar measures a ring 5–35 km around it, north of the centre, and the city-wide number assumes the sky above looked the same.
Why the published numbers for Romania’s radars are almost 14 times too high#
Romania’s seven weather radars are new, from 2024, and transmit at a wavelength of about 10 centimetres. Since December 2025 their profiles have gone into Aloft, the European archive that turns weather-radar echo into birds with a program called vol2bird. The Romanian profiles were computed with the program’s default wavelength, 5.3 centimetres, the one used by the usual radars of western Europe.
It matters. From a given echo, the program works out how many birds are in the air by dividing by the fourth power of the wavelength. With 5.3 cm in place of 10.2 cm, the numbers come out 13.4–14.4 times too high, depending on the radar.
The 2016 study: 70 European radars, 20 autumn nights (Nilsson et al., 2019).
Corrected, the night of 29 September over Bucharest averages about 1,900 birds per kilometre per hour. That is above the network-wide average of the busiest night in a 2016 European study of 70 radars (1,621), though well below that study’s busiest single station (7,282). Uncorrected, Bucharest would have come out almost four times above that station.
Every number on this page is corrected. Direction and speed come from the Doppler shift and height from the beam’s geometry, so they do not depend on the correction. But the wrong wavelength also shifted the thresholds at which the program discards echo as rain, so on the busiest nights some birds may be missing even after correction.
Seven radars, the whole autumn#
Each square is a night, from 1 August to 23 October, and its colour shows how many birds crossed a kilometre. Hatched nights were only partly measured; missing ones have no data at all.
Neighbouring radars look alike: Bucharest’s big nights also show up at Medgidia, and Oradea’s at Timișoara, a sign that birds set off on the same nights across a whole region. The busiest fully measured night on any of the seven radars is the one in the story: 29 September over Bucharest, with 23,500 birds per kilometre. But Bucharest has only seven complete nights, and the night before, only partly measured, looks to have been as big.
Colour: birds per kilometre in a night. Tap a square for the numbers.
All of Europe, night after night#
41 weather radars in 12 countries have enough usable profiles this autumn; on the others, including every German radar, the night profiles are missing or flagged as unusable. On the map each circle is a radar, and how full it is shows how many birds crossed a kilometre on the chosen night. The button plays the autumn, one night a second.
Of the 934 fully measured nights on these radars, only two were busier than Bucharest’s, both at Lisca in Slovenia: 31,600 birds per kilometre on 12 October and 23,700 on 16 October, practically level with Bucharest.
Filled circle: birds on the chosen night; dotted circle: a radar with no data that night. The map can be tapped: it picks the radar nearest the point.
The radar nearest you#
Type a town or tap the map: the page picks the nearest radar with data from autumn 2026 and shows its nights. A radar counts the birds in a ring 5–35 km around it.
Pick a place: type it above or tap the map.
Nearest radar
Each bar is a night; hatched: partly measured.
This radar saw few birds: the site (mountains, sea) or the rain filters may explain it.
What the radar measures, and where it stops#
A weather radar looks for rain. Birds show up as a faint echo moving against the wind or faster than it. The vol2bird program separates this echo from rain and insects (by how evenly it moves) and writes down, every five minutes, how many birds are in each 200-metre layer, which way and how fast they fly.
The numbers assume every bird has the echo of a typical songbird, 11 cm², measured for 5 cm radars; there is no calibration of its own for Romania’s 10 cm radars. Over Bucharest, the layer below 320 metres is not counted. A night runs from the end of dusk to the start of dawn, and counts as “fully measured” when the radar has at least 80% of its profiles.
All data come from the Aloft archive (CC0 licence), downloaded on 25 October 2026. The calculations and the correction were made for this page.
Sources#
- Aloft: vertical bird profiles from European weather radars (CC0). aloftdata.eu
- Desmet, Shamoun-Baranes, Kranstauber et al., “Biological data derived from European weather radars”, Scientific Data, 2025. Radar data from the EUMETNET/OPERA network. doi.org
- The OPERA radar register (each radar’s frequency). aloftdata.eu
- Romania’s National Meteorological Administration, press release on the Bârnova and Medgidia radars, April 2024. meteoromania.ro
- Dokter et al., “Bird migration flight altitudes studied by a network of operational weather radars”, J. R. Soc. Interface, 2011 (the 11 cm² echo of one bird). doi.org
- Nilsson et al., “Revealing patterns of nocturnal migration using the European weather radar network”, Ecography, 2019 (Europe’s busiest nights). doi.org
- Zehtindjiev and Liechti, “A quantitative estimate of the spatial and temporal distribution of nocturnal bird migration in south-eastern Europe – a coordinated moon-watching study”, Avian Science, 2003 (the moon-watching count). eounion.org
- Bucharest’s buildings and city outline: OpenStreetMap (ODbL). Maps: Natural Earth. openstreetmap.org