Habitat use and densities of co-existing migrant Willow Warblers Phylloscopus trochilus and resident eremomelas Eremomela spp. in Zimbabwe

摘要

Capsule Migrant Willow Warblers occupy more woodland types and occur at higher densities than ecologically-similar resident Afrotropical warblers.Aims To compare population densities of Willow Warblers and eremomelas in adjacent acacia, mopane and miombo woodlands, and assess the abundance of potential invertebrate prey in each habitat type, in order to investigate whether Palearctic migrants use more open habitats and are more flexible in habitat use than their Afrotropical counterparts in the same feeding guild.Methods Using distance sampling we carried out four replicated sets of point counts in acacia woodland and three sets of counts in miombo and mopane between December 1999 and February 2000. We noted the tree species in which we saw warblers foraging and took beating-tray samples of potential arthropod prey present on tree foliage in each of the three habitats.Results Willow Warbler density in acacia woodland increased from 1.80 +/- 0.54 (se) birds/ha in early December to 7.15 +/- 1.41 birds/ha in late January after influxes of later arrivals. Densities of Willow Warblers in miombo and mopane were much lower (1.14 +/- 0.28 and 0.38 +/- 0.23 birds/ha, respectively) and did not show significant changes. Burnt-necked Eremomelas averaged 0.74 +/- 0.34 birds/ha in acacia woodland, and in miombo densities of Green-capped and Yellow-bellied Eremomelas were 0.23 +/- 0.17 and 0.34 +/- 0.26 birds/ha, respectively. Densities in mopane were too low to estimate reliably. Willow Warblers and Green-capped Eremomelas showed some apparent preferences in tree species used for foraging but differences in tree use were not obviously related to the abundance of arthropod taxa present as potential prey.Conclusion Willow Warblers occupied more habitats at greater density than similar Afrotropical warblers. They appear to favour acacia, but their settlement patterns and the reasons for disparities between densities of immigrants and residents are unclear.