Migratory species depend on several habitats - often separated in space and time - to complete their annual cycle. Consequently they are especially vulnerable to the impacts of habitat loss. Long term population declines of Afro-Palearctic migratory birds have been linked with loss of suitable breeding habitat and conditions on the non-breeding wintering grounds, but relatively little is known about the winter ecology and its influence on migrants’ overall ecology.
Ecological theories suggest that habitat use and distribution patterns are potentially influenced by a sequence of hierarchically ordered and scale-dependent decisions. Where geographical and/or stochastic processes have greater impacts at the larger scale, and habitat quality has a larger effect at the smaller scale. Furthermore, theory also predicts that migrants should largely be habitat generalists considering their dependence on several habitats separated in space and time and the high potential for stochastic processes to affect initial site selection on a large scale, especially for juveniles on first migration.
We thus explored in detail the distribution ecology of Palearctic migrants in African Guinea savanna habitats - the region from where wintering migrants currently show the greatest breeding population declines. In particular, we investigated some of these prevailing but hitherto little tested ecological hypothesis concerning the impacts of geographical, vegetation and anthropogenic characteristics on the winter densities and distribution of migrants in Africa. Fieldwork was done in humid Guinea savanna habitats in Nigeria over three northern winter seasons from 2011 to 2014 during which a total of 629 unique circular plots of 50 m radius were repeatedly surveyed for birds and to assess vegetation and habitat quality both within and across all three winter seasons. These plots were spread in a North-South gradient between latitudes 6°N – 10°N in Nigeria.
Along with several Afrotropical resident species, some 23 Palearctic migrant species from 8 families were recorded during this study. The six commonest and most widely occurring migrants were: Whinchat (Saxicola rubetra), Pied Flycatcher (Ficedula hypoleuca), Willow Warbler (Phylloscopus trochilus), Common Whitethroat (Sylvia communis), Tree Pipit (Anthus trivialis) and Wood Warbler (Pylloscopus sibilatrix). The occurrence of these six insectivorous migrant bird species were assessed on a large scale in relation to geographical latitude, elevation and vegetation productivity. And on a small scale with vegetation density and anthropogenic habitat disturbance.
The predictions were that latitude and vegetation productivity on a large scale would be key predictors of occurrence whereas finer scale vegetation characteristics would only weakly predict occurrence. Their distribution and degree of habitat specialization were also assessed and compared with taxonomically and ecologically similar resident species by calculating the relative occurrence of the species along habitat gradients - where wide occurrence would indicate generalism and narrow occurrence specialism - with the aim to confirm whether migratory Palearctic birds wintering in Africa were relatively generalist compared to taxonomically similar resident species.
In line with ecological theories and our predictions, we observed distribution patterns where geographical characteristics appeared to have the most influence on the probability of migrants’ occurrence. Although, their responses to habitat characteristics especially those relating to vegetation density and anthropogenic disturbance and habitat quality were species-specific and quite varied, they were generally weak as would be expected for generalist species. These species were also distributed in reasonable densities across a wide range of habitats.
However, migrants when compared to taxonomically-related and ecologically similar Afrotropical residents showed similarities in habitat requirements and utilization, even though migrants utilized habitats over a wider latitudinal range. 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. However, the scale of ongoing habitat change across much of Africa is perhaps contributing to overcome the resilience engendered by their generalism further indicating that conservation efforts for these mainly generalists species should best aim to preserve habitat on a large scale, perhaps through the promotion of sustainable land use practices.
Researchers: Sam T. Ivande (1) & Will Cresswel (1,2)
1. A.P. Leventis Ornithological Research Institute (APLORI)
2. University of St Andrews, Scotland, UK
1. Ivande, S. T and Cresswell, W. (In prep.) Factors influencing distribution and habitat use of Palearctic migrants wintering in humid savannahs in Nigeria, West Africa
2. Ivande, S. T and Cresswell, W. 2016. Temperate migrants and resident bird species in Afro-tropical savannahs show similar levels of ecological generalism. Ibis 168:496-505. doi: 10.1111/ibi.12371
3. Cresswell, W. 2014. Migratory connectivity of Palaearctic–African migratory birds and their responses to environmental change: the serial residency hypothesis. Ibis 156:493-510