THE SOUTHWARD MIGRATION OF PALAEARCTIC BIRDS OVER NGULIA, KENYA

Since 1969 remarkable numbers of night migrants have been attracted during misty conditions in November and December to three 1 kW floodlights at a game viewing lodge on the northeastern side of the Ngulia ridge, a small range of hills in the Tsavo National Park (West), southeast Kenya. The main species involved have been Palaearctic passerines, principally the Marsh Warbler Acrocephalus palustris, the Whitethroat Sylvia communis and the Sprosser Luscinia luscinia. Data were collected at the Lodge in the late autumns of 1969–71, and in particular between November 1972 and early January 1973, when over 2500 Palaearctic passerines were caught and ringed. Large falls have depended on mist or rain during the latter part of the night, at any time during the month except around full moon. Highest numbers have occurred in late November and the first half of December. In 1973 falls continued into the second week of January. Grounded birds move on quickly, extremely few having been retrapped. During 1972–73, the species prominent in falls at the Lodge were abundant as transit migrants in Tsavo only from mid-December to early or mid January, at which time retrap rates were highest. The high weights and considerable fat deposits of many birds caught suggested they were grounded some distance north of their destination. Forty-two migrants analysed had a mean lipid content of 12·9% of their live weight; none was appreciably dehydrated. In 1972–73, highest weights were found at the beginning and end of the season. Individual species are discussed, and in several cases their African status reviewed. Several species were encountered at Ngulia in numbers far larger than those previously reported from elsewhere in Africa. In 1972–73, for instance, over 1000 Marsh Warblers were caught and many thousands of others seen, hundreds of River Warblers Locustella fluviatilis occurred, and White-throated Robins Irania gutturalis, Basra Reed Warblers Acrocephalus griseldis, Rufous Bush Chats Cercotrichas galactotes and Olive-tree Warblers Hippolais olivetorum were caught regularly. Most of the Basra Reed Warblers, Upcher's Warblers Hippolais languida and Olivaceous Warblers H. pallida, and many of the Whitethroats (apparently all of the eastern race icterops) handled during late December and early January were in fresh plumage, although these species are not known to moult north of the Sahara. They are presumed to have renewed their plumage in northeast Africa earlier in the autumn. In many Basra Reed Warblers and Whitethroats moult was only partly completed; in almost all such cases it was arrested. Itinerancy south of the Sahara is discussed. It seems clearly established that a regular southward migration, in the usually accepted sense of the word, occurs across Tsavo, of Palaearctic species which have already been in tropical Africa up to three months. Most species involved in this migration cross the equator on a remarkably narrow front, and are rarely recorded in Kenya west of Nairobi.