Distribution ecology of Palearctic migrants in the humid Guinea savannah in West Africa

Abstract

Declines in breeding populations of most migrants across much of the Palearctic have been linked to environmental conditions in their African non-breeding grounds. Studying winter distribution dynamics for these species is necessary to understand how factors in these areas may influence their overall population dynamics. This thesis explored in detail the distribution ecology of migrants in the Guinea savannah, the region from where wintering migrants currently show the greatest breeding population declines. In particular, I investigated some prevailing but hitherto little tested ecological hypothesis concerning impacts of geographical, vegetation and anthropogenic characteristics on the densities and winter distribution of migrants in Africa. Migrant distribution seemed to fit a pattern where decisions leading to winter habitat choice and association were hierarchical and jointly influenced by factors extrinsic and intrinsic to the habitats at large and finer scales respectively. Migrants were distributed in reasonable densities across a wide range of habitats. There was also evidence for an independent effect of latitude on densities and distribution, even after controlling for habitat characteristics. There was no evidence of large changes in latitudinal density patterns within a given winter season and site density patterns were generally consistent over the study duration. Migrants and taxonomically-related/ecologically similar Afrotropical residents showed similarities in habitat requirements and utilization, although migrants utilized habitats over a wider latitudinal range. Some migrants tended to show correspondence in site occurrence between consecutive winters but less so within a given winter season and there was an overall low transferability of habitat models for Palearctic migrants between sites in Nigeria. Collectively, the results describe distribution mechanisms typical for ecologically flexible species that can best be described as habitat generalists. As generalists, migrants are expected to show some resilience, especially in dealing with local and small scale changes on their wintering grounds such that these are unlikely to be the primary limiting factor in their population dynamics. However, the scale of ongoing habitat change across much of Africa is perhaps contributing to overcome the resilience engendered by their generalism. Conservation efforts for these mainly generalists species may therefore aim to preserve habitat on a large scale, perhaps through the promotion of sustainable land use practices.