SPRING WEIGHTS AND PASSAGE OF SEDGE WARBLERS ACROCEPHALUS SCHOENOBAENUS IN CENTRAL KENYA

Weight and other data were collected on over 1300 Sedge Warblers Acrocephalus schoenobaenus in central Kenya, mainly during spring (April-May). The weight, fattening rate and length of stay of spring passage birds varied from one site to another. In scrub and thicket at Kariobangi, Nairobi, passage and new arrival continued each year into May; few birds stayed to fatten, and weights over 16 g (40% above mean winter weight) were practically never recorded. At Lake Nakuru, thousands of birds were present during late April-early May 1972; many stayed to fatten for 1–3 weeks, and weights over 16 g (maximum 215 g) were not uncommon; new arrivals of lean birds continued into May. At Athi River, in 1971, many of the birds present during mid-late April stayed 1–2 weeks and fattened rapidly; 20% of all late April weights were in the range 16–21 g. Mean fattening rates at Nakuru and Athi River were 0.31 and 0.64 g/d respectively. The migration strategy and potential flight range of Kenyan spring birds is discussed. Active wing moult was rarely recorded. Most locally wintering birds and spring passage birds were judged to have renewed their plumage during the preceding October-January.