As in the case of the Kordofan Lark, this blog-post deals also with another potential vagrant to the Western Palearctic region: the Horus Swift (Apus horus).
In January 2018, Frédéric Bacuez and Bram Piot discovered about 18 Horus Swifts at the Doué river, located in northernmost Senegal. The identification, however, was not easy at the beginning.
Initially, the observers thought about the White-rumped Swift (Apus caffer), because this species along with the Little Swift (Apus affinis) were the only swifts with a white rump and already known in the Senegal Valley area. A hybrid Little Swift x White-rumped Swift was also initially suggested, but apart from their characteristics, seeing 18 hybrids together is almost impossible. The answer came after a detailed study of the photos, especially those obtained in mid-February. They were Horus Swifts. The ID was also confirmed by people with experience in the matter (see the links).
So, why this is important? Because, hitherto, the closest known site where the Horus Swift occur is located hundreds of kilometers to the east. So, the Senegalese site is the westernmost for the species. It’s probably also the northernmost site as well (although it’s only marginally more north than some sites in Sudan). Here is a direct quote from Bram Piot’s text:
Why do we get so excited about this one? It’s always exciting of course to find an addition to a country’s species list, but in this case we have a highly unexpected record since it comes down to a range extension of no less than 1,600 km, and because the species may even breed here. Plus, one can now safely assume that Horus Swift also occurs in Mauritania, Mali and Burkina Faso, and why not in northern Cote d’Ivoire and NE Guinea too (and for any WP listers out there: it may well make its way across the biogeographical border!)
So, if the Horus Swift also occur in other sites in Senegal and Mauritania, and especially if they are breeding there, there is no reason why we won’t see them north of that invisible ‘biogeographical border’. Here is a reminder (from the same bird family!) that we still know very little about so many species: the wintering of Plain Swift (Apus unicolor) in equatorial West Africa was totally unknown, until a recent geolocator study revealed their movements.
For more details about this discovery (context, structure and plumage of the birds, their behaviour,…) see the following links (as you can see, one in English and the other in French, but I recommend you reading both of them, because they were written from two different perspectives, and they contain different information):
Piot, B. (10/04/2018). Those Mystery Swifts: Horus, New to Senegal. Senegal Wildlife.
Bacuez, F. (11/04/2018). Le Martinet horus, une espèce de plus à la liste des oiseaux du Sénégal. Ornithondar.
Horus Swift / Martinet horus (Apus horus), Doué river, northern Senegal, 12 Feb. 2018 (Frédéric Bacuez).