SPRING WEIGHTS OF SOME PALAEARCTIC MIGRANTS AT LAKE CHAD

Abstract

SummaryA visit was made to Malamfatori, on the western shore of Lake Chad, Nigeria, from 22 March to 13 April 1967, with the principal aim of studying Palaearctic migrants in relation to the environment. About 2,400 Palaearctic migrants of 29 species were mist‐netted in beds of bulrush Typha australis and thickets of saltbush Salvadora persica. Some 300 of these were collected for fat analysis; the remainder were released after measurement and ringing, and provided 275 retraps during the course of the study. Data were supplemented by further netting by A. J. Hopson in late April and May.Yellow Wagtails Motacilla flava, Sedge Warblers Acrocephalus schoenobaenus and Whitethroats Sylvia communis were abundant and were studied in greater detail than other species. Yellow Wagtails fed almost exclusively on midges, particularly the abundant Tanytarsus spadiceonotatus. Sedge Warblers fed on small insects and spiders, and Whitethroats on Salvadora fruits. Sedge Warblers and some other chiefly insectivorous species turned to a diet including berries shortly before emigrating.Changes in weight during the course of the day were difficult to investigate, but were probably bimodal. Good correlations of weights with wing‐lengths were obtained for species for which many data were available. Pre‐migratory fattening did not occur synchronously in all populations of a species, but once it started in an individual it proceeded at a constant rate of 0.2 g/day in Sedge Warblers and 0.6 g/day in Whitethroats. Predation pressure probably ensured that individuals emigrated immediately they attained their maximum pre‐migratory weight, although some Sedge Warblers and Whitethroats left the area before attaining maximum weight.Differences between the mean weight curves of first‐caught and retrapped birds are discussed, and it is concluded that in some species there were both through‐migrant and temporarily‐resident populations at Malamfatori.The mean weights per day of Whitethroats, Sedge Warblers and also Reed Warblers A. scirpaceus were falling in late March, and slight changes in the weather, particularly temperature, may have been responsible. In mid‐April there was a “rush” of lean Whitethroats, which are thought to have originated to the SW in Nigeria.Yellow Wagtails, Sedge Warblers and Whitethroats deposited up to 40% fat/live weight; Sand Martins Riparia riparia had up to 28%, and a small sample of Ruffs Philomachus pugnax up to 17%. In the case of Sedge Warblers, fat reserves were sufficient for crossing the Sahara both to the north and to the northeast.Contrary to the findings in some previous lipid studies, the fat‐free dry weight and water content increased during fattening. The fat‐free dry weight increase was about 34% in Yellow Wagtails, 18% in Sedge Warblers, and 35% in Whitethroats. The tissues involved in this increase were not investigated, but a study of Yellow Wagtails at Malamfatori in 1968 (in prep.) suggests that the pectoral muscles hypertrophy during fat deposition sufficiently to account for nearly all of the increase in fat‐free dry weight and water fraction.