UV completed his third Autumn migration yesterday. An unusual view of it.
His total distance travelled was… let Paul explain:
“How far has UV flown during his migration?
You’d think this would be a fairly simple question – and it can be provided with a simple answer. Problem is: there are rather a lot of different answers…
The reason is that “how far” is capable of being measured in several different ways – all of which give the “correct” result according to their method. For example, do we include all his changes of altitude, while flying? Should we take into account the height of the terrain he was flying over at various points? He covered a significant amount of distance while foraging to and fro at his stopover locations – should this be included as well? Does it even count if he was flying backwards?
Not so simple after all, this “how far” business.
My usual way of arriving at a total distance is to take the recorded track and (using computer software) lay it flat onto a mathematical model of the Earth’s surface known as the WGS84 ellipsoid. This tends to give a result that is the distance made good over the ground, but ignoring transient changes of height (in thermals, and so on.) I also take out any local movements during stopovers. Applying these criteria, UV’s overall migration distance works out at 5354 km. (3327 statute miles)
At the other extreme, I can load ALL his individual recorded points, take the distance between each one (vertically as well as horizontally) and tell my PC to add them all up. This give a result (stopovers, thermals, mountains AND flying backwards, all included) of 5741 km. (3567 statute miles)”
UV’s migration took him 38 days. In 2016, his ‘record’ Autumn migration lasted 32 days. Both are significantly slower than his Spring 2017 migration time of 19 days, and that included 4 days stopover.
In Autumn 2016, UV had 2 stopovers totalling 18 days. This Autumn the same number of staging posts lasted 20 days.
Lots of numbers!
Let’s go back to UV’s last couple of days leading up to arrival in Senegal. The last detailed update saw UV out of true migration mode on 13 October, as he made his way slowly south. On 14 October, he had about 260 km to fly in a direct line to the Langue de Barbarie. He travelled 236 km in the day, but ended up just short of his final destination as night fell.
The first part of his day consisted of multiple short stops along the coast.
UV reached the coast by 10.20 UTC, but flew slowly south before the period in the image above. At 13.42, he headed offshore and took advatage of a thermal to gain height and speed. He kept up his pace once back over land. By 16.44 he was about 1900m above the terrain as he approached the Senegal River. He stopped at the first irrigated land just north of it for around an hour. At just after 18.21 UV crossed the river into Senegal. He roosted at twilight near the Diama dam.
UV was in no rush to reach his wintering grounds yesterday, travelling west to the sea initially, then perching at several points. He was across the river boundary and back in Mauritania during that time, but at 11.43 he returned to Senegal and was soon over the city of Saint-Louis.
UV spent the rest of the day refamiliarising himself with the northern part of the Langue de Barbarie.
UV perched overlooking the sea to the west, the river to the east.
The Google Earth image was taken in April, just after UV had begun his Spring migration. Now, it is likely the land will look rather different because of the rapid erosion – and sand accretion – in the area.
Comparing the two images, you can see the small island just SE of UV’s perches has now merged with the spit. Filau trees have been planted to try and reduce the speed of erosion. Their growth is visible. On Doune Baba Dieye, the island to the right of the image, the filaus may prevent the separation of the northern tip from the rest of the island.
Later in the afternoon, UV perched at the southern end of the spit before heading inland to a familiar roost area used at times last winter.
There were few fixes in a late morning email today – the atmosphere has been thick with dust, affecting the charging of the battery. UV was still inland, although he had probably foraged early in the day and returned with a meal. Some lazy days ahead – probably!