WEIGHTS OF SOME PALAEARCTIC MIGRANTS IN SOUTHERN UGANDA

Abstract

SUMMARYBetween March 1966 and May 1968 Palaearctic passerines were mist‐netted in thick bush and lightly wooded savannah habitats near Kampala, on the northern shore of Lake Victoria. This paper reports weights of the seven principal species involved.Most migrants appeared to be in a lean condition during the winter months, when weights were relatively low and varied little in each species. Birds were not particularly light on arrival. In fact, autumn Garden Warblers Sylvia borin and Willow Warblers Phylloscopus trochilus were sometimes markedly heavy, and for the former species there was some evidence that the individuals concerned were passage migrants. Autumn weights of Swallows Hirundo rustica, Reed Warblers Acrocephalus scirpaceus and Yellow Wagtails Motacilla flava were similar to those recorded in winter.The mean weight of all species rose during late March or early April. Although most Garden Warblers and Willow Warblers trapped at the time of spring migration were within the normal winter weight range, many Acrocephalus warblers and the majority of Sand Martins Riparia riparia and Yellow Wagtails were rather heavy. Spring weights 40% or more above mean winter weight were not uncommon in the Sedge Warbler Acrocephalus schoenobaenus, but were recorded only occasionally in other species. Although most passerine migrants evidently left Kampala with substantial fat reserves, it was concluded that a considerable number of warblers departed at rather low weight.High spring weights were mainly confined to a period of two or three weeks in each of the warbler species. Locally wintering Acrocephalus warblers must have attained full premigratory weights within three weeks, and a number of spring retraps showed substantial gains at minimum mean rates of between 0–1 and 0–35 g per day. Most heavy Garden Warblers were probably on passage.Significant correlations between weight and wing‐length were obtained for all species investigated, regressions of weight on wing‐length being in the range 011‐0‐25 g/mm.Spring weights are briefly compared with data from Nigeria, and the northward migration of passerines from Lake Victoria is discussed.