The Black Kite is among the earliest of spring migrants and certainly the most abundant, tens of thousands crossing the Strait of Gibraltar from Morocco into Europe (below). The first ones appear in January and passage is substantial in February, building towards a peak in the first days of March.
View of the Strait of Gibraltar from the north. The mountain on the right is Jebel Musa in Morocco. The Rock of Gibraltar (in Europe) stands out on the left.
Having already undertaken the arduous crossing of the Sahara Desert, these soaring birds have to run the gauntlet of the Strait. It may not be a huge distance (only 14km at the narrowest) but flying over the sea is never good for a soaring bird. Additonally, unpredictable winds add to the hazard.
Travelling in flocks helps with keeping on the right track and offers protection. Flocks descend to roost together at night. But the downside is that wherever they stop there is unlikely to be sufficient food to feed them all. So they tend to fast during the journey, catching prey opportunistically when the chance arises, and simply get on with the migration as quickly as possible.
The early birds are the ones from the south. They will breed in southern Iberia. As the season advances birds heading further north arrive. These may be going all the way to Switzerland, Germany and France. One the breeding adults have passed, a second movement - of immatures - takes place between late April and June. All in all, almost six months of Black Kite migration!