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Home , Gelochelidon, Gull

-billed 30. nilotica

It was 5 October 1988. In the desert north of Iwik on the Banc d’Arguin, someone looking down from above would have seen the Mauritanian landscape take on a fl ickering quality, but at ground level the speckling resolved itself into a transient phenomenon that is one of Nature’s fi nest – butterfl y mass migration across a treeless landscape. Innumerable Painted Ladies Vanessa cardui had been passing on their southward migration in an uninterrupted stream for three hours; as far as the eye could see, butterfl ies galore. To get some idea of the scale of this happening we came up with the following fi gures: over a band 50 m wide, an average of 231 butterfl ies passed every 5 minutes. This tranquil scene suddenly changed, however. The wind direction swung round to the north, the sky turned inky- black and we were buffeted by gusts of wind, harbingers of a heavy thunderstorm. Once the storm had passed, fi rst a few and then abruptly huge numbers of Desert Locusts drifted with the desert wind towards the Atlantic , and beyond. On this occasion, the cliché, ‘the sun was literally blanketed’, was deserved. So many locusts! The beaches and tidal fl ats of the Banc d’Arguin were smothered by living, dying and dead insects, piles littering the tide line. It took some days for numerous Whimbrels and Turnstones, and the occasional Grey Plover, to begin to exploit this profusion of easy prey. Other ignored it, sticking to their usual diet of tiny worms. But to the Gull-billed , this was manna from Heaven! Without hesitation, they abandoned the pursuit of their usual prey, fi ddler crabs, and began immediately to crush and swallow locusts as if these had always been their day-to-day preference! In West

3364 Livvinng onn thee edge Africa, Gull-billed Terns are flexible in their prey and habitat choice. In Guinea-Bissau they feed on fiddler crabs as long as the tidal flats are exposed (Stienen et al. 2008), but at high water they will patrol above tidal creeks in the mangroves, fishing or taking insects from the air, vegetation or bare ground. If a tern were defined as a fish-eater, the Gull-billed Tern would fail that test in its African wintering quarters and on its breeding grounds, but even such adaptable feeding behaviour cannot prevent a steady decline in numbers over much of its range.

Breeding range Migration

The world population of the Gull-billed Tern was estimated at 55 000 Ringing recoveries and observations of visible migration both sug- pairs (del Hoyo et al. 1997), of which 10 000 – 12 000 pairs in Europe gest that breeding from SW Europe winter in West Africa, and and Africa between 1980 and 2000 (Sánchez et al. 2004, correcting that birds wintering in East Africa originate from the Balkans and for the recent decline on the Banc d’Arguin, Isenmann 2006). The eastern Europe (Møller 1975c; Cramp & Simmons 1983). There are species almost disappeared from its northernmost breeding range very few recoveries from sub-Saharan Africa to confirm such a di- in Denmark and Germany (50-60 pairs), but has shown increasing chotomy, namely, a recovery of a Danish from the Casamance numbers in South Europe: Italy (325-450 pairs), southern France (S Senegal) and another one from Mauritania. An ornithological ex- (250-300 pair) and Spain (3000-3500 pairs). These, the 180-1000 pedition to Guinea-Bissau (Wymenga & Altenburg 1992) captured Mauritanian pairs and the much smaller numbers elsewhere in NW 112 Gull-billed Terns in 1986/1987, of which four had already been Africa, form the western population (4660-4850 pairs). The eastern ringed. Three of these had been ringed as nestlings on 1 July 1986 at population in Greece, Turkey and Ukraine may be estimated at a single site (Lucio del Cangrejo in the Coto Doñana, Spain), their 5200-6800 pairs. recapture being to the utter surprise of their ringers! The fourth

Gull-billed Tern with large fiddler crab, its main prey while wintering along the West African coast.

Gull--bbilled Terrn Geelloccheelidonn nilotica 365