Sedge Warbler migration and reed aphids.
Sedge Warblers Acrocephalus schoenobaenus breed in western Eurasia and winter in Africa south of the Sahara (Vaurie I959, Moreau I972). They occur in marshland especially on migration and in winter, when they are less catholic in their habitats than in summer (Lack I97I). Recent ringing recoveries reported to the British Trust for Ornithology, from Senegal (three), Liberia (one), Sierra Leone (one) and Ghana (one), suggest that British breeders winter in West Africa, though they have not been widely reported there Bannerman I953, Mackworth-Praed and Grant I955)· Both the spring and the autumn migrations require long flights over the Sahara (Moreau I96I); like other long-distance migrants, Sedge Warblers prepare for these flights by accumulating large deposits of body fat (Gladwin I963, Fry et al. I970, Sitters 1972). The extent of fat accumulated by them in southern England in autumn led Gladwin to suggest the possibility of a single direct flight to the winter quarters, a feat comparable to that of the Blackpoll Warbler Dendroica striata crossing from New England to the West Indies or Venezuela (Nisbet et al. I963). Until now, however, there have been no critical studies of the scale of weight gains or the feeding conditions that make them possible.
This paper reports the results of studies extending over three autumns (I973-75) at Radipole Lake, Dorset. Few Sedge Warblers breed there, but many from north-west Britain pause on migration; intensive ringing has been carried out since I972 (Pepler and Pepler I973, Pepler I976) and observations have been made simultaneously on feeding ecology and the availability of food. Aphids have received special study, as they appear to form an important part of the diet (Green and Bibby I973)·