The map
bird passage: little → muchflight direction (sharper, steadier)radar and its mean directionstop stop at peak hours watch run · size = MWwind farm: mean passage of the season · size = MW
How to read it: lines and particles go where the birds fly, and are sharper the more the direction repeats night after night. Particles gather where more passage is expected. Circles are the OpenStreetMap wind zones (cells of about 25 km).
Why stop these wind farms, and only on a few nights
Four figures, all computed from the data on this page.
A broad front that crosses everythingIn autumn, 38 of the 43 radar stations on the map see the passage heading between south and south-west, 31 of them with a steadiness of 0.7 or more. It is not a narrow corridor: it is a river of birds hundreds of kilometres wide sweeping France and Iberia towards the Strait. Every wind farm along it is in its way.
A few nights decide itAt the median station, the heaviest 10% of nights carry 54% of the whole season's passage. Stopping on those nights protects a lot; stopping on the rest, little.
And they tend to be calm nightsTested on 2021-2026 at 151 zones, the rule would have stopped 7.5 nights per season and zone, for 3.1% of the night output; stopping on every very high night would cost 12.8%. Measured with the forecast itself, which flattens the peaks, it covers 8.8% of the predicted passage.
The radar confirms the chosen nightsOn the nights the rule says stop, the nearest radar measured heavy passage 67% of the time; by chance it would be 24%. That is 2.8 times more.
The forecast per zone, night by night: wind farm report; the rule tested since 2021: verification. Back to the home page.
The routes, station by station
What the white arrows draw, with all their figures.
Show the table: direction, steadiness, speed, altitude and concentration
Steadiness: 1 if every night flies the same way, 0 if they cancel out. Speed over the ground (wind included). Mean altitude above sea level, weighted by traffic. Last column: share of the passage carried by the heaviest 10% of nights.
How it is computed
From echo to flight vector
Each radar profile (vol2bird, every 5 to 15 minutes) gives, in 200 m layers from 200 m to 3 km, the density of birds and their ground speed towards the east and the north. Density × speed × layer depth × time, summed over the night, is the traffic: how many birds cross each kilometre of a front across their flight. Its direction is that night's.
Which nights count
Passage months (March to May, August to November) in which the radar published vectors. A night counts if at least 50% of its density carries a vector, if the radar saw 120 minutes or more of it and if the ground speed stays under 25 m/s: faster is usually rain or an echo carried by the wind, not birds.
From nights to a route
Per station and season, the traffic vectors of all its nights are added. The direction of the sum is the route; its steadiness (length of the sum ÷ sum of the lengths, 0 to 1) says whether the nights agree. For the direction no night weighs more than the 95th percentile of its station, so a single huge night cannot turn the arrow. It takes 15 nights. Old and renewed radars on one site are one station, each on its own scale.
From radar to map
The lines do not come from a model: they are a smoothing between radars (a 130 km Gaussian kernel on a 0.5° grid), drawn fully up to 160 km from the nearest radar and gone at 320 km. Where there is no radar, there is no line.
The coming nights
In «Coming nights» the direction is the usual one for the date (nights within ±15 days, every year) and the colour is the Sub Nocte forecast at each wind zone and city. In the record, the colour is the season mean the same model computes from the 2021-2026 weather.
Why stop here
Tap a wind farm: the night's level (relative to its own record), the forecast wind at 100 m and what stopping would cost, the route overhead, its rank among the zones by mean passage, and how well the model does against the nearest radar.
What this map does not say
- Height. The radar measures from about 200 m to 3 km; the blades sweep 30 to 200 m above the ground. The route is the air column's, not the rotor height's; on ridges, where the farms stand at 800-1,500 m, the two bands come closer.
- Track, not heading. It includes wind drift: it is where they pass, not where they point.
- A broad front, not corridors. Nocturnal passerine migration does not follow a narrow corridor; the lines give the overall sense of the flow, not a border outside which no bird passes.
- Gaps in time. The renewed AEMET radars and the French ones since 2021 hardly publish a vector on Aloft: the Spanish route rests mostly on 2018-2020 and the French one on 2016-2022. Portugal, 2019-2026.
- Gaps in space. Far from the radars there is no data and the map fades out instead of making it up. The Azores and the Canaries are off the map.
- The amount is indicative. Bird numbers depend on each radar's calibration and compare poorly between radars; the direction does not depend on it.
- Not all the victims. The documented mortality at Spanish wind farms is mostly diurnal raptors and soaring birds, and bats; this is the nocturnal migration.
Sources
Bird vertical profiles and flight vectors: Aloft (CC0), computed with vol2bird on the AEMET, IPMA and Météo-France radars (OPERA network). Wind farms: © OpenStreetMap contributors (ODbL). Weather: Open-Meteo (CC BY 4.0) and Copernicus ERA5. Coasts and borders: Natural Earth (public domain). Method: Dokter et al. (2011) for vol2bird; Horton et al. (2021) for the concentration of the passage on a few nights.
All the code and derived data (each night's traffic in data/routes/flight_nights.parquet) live in the repository.