Where the storks go

I followed 228 tagged birds across Europe, Africa, Asia and the Americas: young white storks from eight populations, North American ospreys, cuckoos from the Baltic and marsh harriers from Belgium. Watch them go, see where the routes squeeze through narrow straits, who crosses the open sea and who will not, where they spend the winter, and how many never come back.

Every autumn white storks leave the roofs and chimneys of Europe for Africa, and they do it in a strange way. They are big, heavy birds that travel by soaring, circling up in rising warm air and gliding to the next thermal. Thermals do not form over the sea, so storks go round it. The ones from Spain and Germany funnel through the Strait of Gibraltar, fourteen kilometres wide; the ones from Poland, Greece and Russia go the long way round, over the Bosporus and down the Levant into Africa. Other migrants have other ways. Ospreys flap and fish and cross the Caribbean; cuckoos fly through the night and cross the Mediterranean and the Sahara in a few days.

These routes are known from bird ringing and, since the 1990s, from tags that report positions by satellite. Some of the tracking data is published openly. I took four such data sets, all free to reuse, cleaned them in the same way and put 25,236 daily positions on one hand-inked map. Press play to watch a year of migration: every bird from every year is folded onto one calendar, from 1 July to 30 June. Click a bird for its story.

01 · The mapA year of migration

02 · Two roads to AfricaThe great divide of the storks

The young storks in this study were tagged in their nests in 2013 by a team led by Andrea Flack at the Max Planck Institute for Ornithology, in eight populations from Spain to Uzbekistan, to compare the costs of their different journeys. They had no parents to follow: storks migrate in loose flocks, and the young set off on their own schedule.

Their routes split along a line through central Europe. Of the 18 storks from Germany and Spain that left home, 13 crossed the Strait of Gibraltar into Morocco; the rest stayed in Spain or France for the winter, or died before reaching the strait. Of the 23 from Poland, Greece and Moscow that left, 21 crossed the Turkish straits, the Bosporus or the Dardanelles, and 20 flew down the narrow strip of Israel and Sinai into Africa. None of the eastern birds crossed at Gibraltar, and none of the western birds at the Turkish straits.

The Tunisian birds had the Sahara in front of them and nothing else: 7 set off straight across it. The Armenian storks went south through Turkey, Iraq and Iran; none reached Africa. And the 6 young storks from Uzbekistan did not migrate at all: none went more than 141 km from home in their first year.

The winter quarters fan out from the Sahel, the belt of dry savanna south of the Sahara where 18 of the 34 storks with a winter position spent December and January. 5 went on into East and southern Africa, and 5 crossed the equator. The farthest traveller, DER AR445 from Moscow, ended up 9,186 km from its nest, in Zimbabwe. 4 German storks stopped for the winter in Morocco, and 1 in France.

03 · Around the sea or across itSoarers, flappers and night fliers

For each species I measured how much of its flying was over open sea: the share of the straight lines between consecutive positions whose midpoint lies off the coast (for the GPS tags of storks and harriers, positions at most an hour apart from tracks thinned to one every ten minutes; for the satellite tags of ospreys and cuckoos, which give a few positions a day, lines up to 24 and 72 hours long).

The storks flew 0.6% of their distance over water, most of it in short hops across bays and straits; the longest stretch any stork spent over the sea was 82 km. The ospreys flew 22% over the sea, about 36 times the storks' share. 4 ospreys made crossings of 300 km or more, typically from Cuba or Hispaniola straight over the Caribbean to Venezuela or Colombia, a median 728 km. One, KD, set off south from Hispaniola, spent 82 hours over the sea and turned back to Haiti, 859 km later.

The cuckoos are the boldest. They migrate at night and their satellite tags fix them only every day or two, but 4 lines run from southern Europe across the Mediterranean to Africa. One young cuckoo left Poland on 27 September 2017 and was in Chad 4.8 days later, 4,347 km to the south, having crossed the Ionian Sea and the whole Sahara in between. The marsh harriers, hunters of reed beds that also soar but can flap for hours, crossed at Gibraltar and along the coast of the Western Sahara; the longest harrier crossing was 123 km, over the Bay of Biscay.

04 · How fast, how farThousands of kilometres a year

A migration here runs from the last day a bird was near its summer home to the first day it was near its winter quarters (and back in spring), and only migrations with a position on at least half the days count. A typical stork autumn migration covered 4,829 km in 61 days, 79 km a day including the days spent resting and feeding. The ospreys covered 5,301 km in 23.5 days, 170 km a day, and the marsh harriers 4,639 km in 51 days.

On their best days they go much faster. The best single day of the median stork that migrated was 371 km, and the best of all 558 km. Ospreys reached 730 km. The record is a marsh harrier, Lilla, who on 11 September 2019 left central Spain at midday, crossed the Strait of Gibraltar in the evening and flew on into the night, and by the next noon was 871 km away in Morocco.

Over a whole year the totals are startling. 17 storks were followed for a year; the median covered 12,724 km between daily positions, and the young Moscow stork DER AR445 at least 23,680 km, 31,180 km measured between hourly positions. Daily positions miss the zigzags of a day's flying: for the storks the hourly track is 41% longer.

05 · Winter quartersThe same bush, year after year

The marsh harriers show what the short tracks cannot: the same bird, year after year. Lilla, a female from the reed beds of the Scheldt near Antwerp, was followed for 3.0 years and made 5 complete migrations. Each winter she went to the same place near Dakar in Senegal: her winter centres in three winters were within 2.6 km of each other. Lillo, a male from the same marshes, wintered twice in Guinea, 0.8 km apart. Their autumn journeys took between 31 and 86 days; in spring both hurried home in about five weeks.

The ospreys spread out over the Americas. Of the 65 that were tracked south of 25°N in autumn and winter, 38 reached South America, most often Brazil (14), Venezuela (8) and Colombia (7). 13 went no farther than Mexico, 9 of them from the western population along the Columbia River and in Oregon.

06 · The cost of the journeyMany young birds never come back

Of the 72 young storks, 30 (42%) were confirmed dead within the year of tracking. Many died before they had gone anywhere: 16 never went more than 300 km from home, in the first weeks out of the nest, and 15 of the deaths came before 1 October 2013. The route mattered too. Of the storks that left home, 3 of 23 on the long eastern route were found dead, 4 of 18 on the western route, and 5 of the 7 Tunisian birds that set out across the Sahara.

The osprey records note 8 deaths. 3 probably died on floating debris in the Gulf of Mexico or the Caribbean, birds that came down at sea and never reached land. One was shot in Mexico, one was found dead in Cuba and one near its nest. Two more, X2 and X3, were found in 2016 still wearing transmitters that had gone quiet many years before; X2, electrocuted on a power pole, was at least 22 years old.

07 · The cuckoo experimentLost, and finding the way

The cuckoo data come from an experiment. Young cuckoos are raised by other birds and never meet their parents, so how do they find Africa? Kasper Thorup and colleagues caught cuckoos at Rybachy on the Baltic coast, and tagged them; some were let go at once and others were carried about 1,800 km east to Kazan before release. 23 birds released at home and 19 displaced birds have usable positions. 8 of the 9 displaced birds whose first 500 km I could follow headed south-west, a median bearing of 201 degrees, as if correcting for the move; the birds released at home headed a median 176 degrees, almost due south. The small tags did not last: the median control bird was followed for 53 days and the median displaced bird for 26, and 11 cuckoos in all were tracked south of 20°N.

08 · How I did thisMethods and caveats

Data. Four open data sets: the MPIO white stork lifetime tracking data, GPS fixes every five minutes in daylight (1,584,192 rows); the Martell osprey satellite tracks (86,383 rows); the Rybachy cuckoo satellite and GPS tracks (8,498 rows); and the INBO marsh harrier GPS tracks (393,122 rows; the one common buzzard in that set is left out). All are CC0.

Cleaning. Satellite (Argos) positions were kept only in the three best location classes, with a stated error under about 1.5 km (19.1% of the osprey rows); positions flagged as outliers were dropped, and any position that needed a speed over 120 km/h from the previous one (150 for cuckoos) over more than 5 km was removed. GPS tracks were thinned to one position an hour, and every bird to one position a day, the one nearest noon (UTC).

Measures. Distances are great-circle distances between positions, so they are lower bounds. A migration runs between a summer and a winter anchor (the median position in June and July, or the nest, and in December and January) at least 500 km apart. Funnels are counted when a bird has a position on both sides: Iberia then Morocco for Gibraltar; Thrace then Anatolia for the Turkish straits; any position in Israel, Sinai or western Jordan for the Levant. Sea is Natural Earth's 1:10m land outline.

What this is not. A few hundred birds, mostly young storks of one year and ospreys of the late 1990s, are not whole populations. Satellite tags give a few positions a day, sometimes none for days, so the osprey and cuckoo tracks are straight lines between sparse points. A tag that stops does not mean a bird died: only the deaths the data owners confirmed are counted as deaths.