Niche partitioning of two Palearctic passerine migrants with Afrotropical residents in their West African winter quarters

摘要

In bird migration systems, the question of coexistence and competition of migrants with residents in the nonbreeding season and their role in shaping the evolution of present avian communities is much debated. However, conclusions are often drawn in a speculative way. In the Palearctic-African bird migration system, many studies have addressed the question of coexistence of Palearctic breeding birds with Afrotropical species in the former's wintering grounds. These studies have led to some generalizations concerning habitat selection and foraging ecology about the traits that might enable migrants to coexist with residents. Migrants were therefore assumed to forage in more open habitats, in more peripheral parts of the vegetation and with a higher foraging speed than residents. Furthermore, they were also assumed to be more flexible in foraging behavior by using a wider range of foraging tactics, but some studies revealed contradicting results. We studied the ecology of Palearctic migrants in Comoe National Park, Ivory Coast, West Africa, during three successive winters to explore the factors of niche partitioning between migrants and residents and to test the hypothesis of whether there are common behavioral traits in migrants. Therefore, we compared the ecology of two Palearctic breeding species: pied flycatcher, Ficedula hypoleuca, and willow warbler, Phylloscopus trochilus, with resident members of the respective guilds. With respect to use of microhabitat, foraging speed, and intake rates, we could not confirm the above-mentioned generalizations, showing that care has to be taken into account when drawing conclusions from few studies for a whole migration system on a huge continent. However, both migrating species were more flexible in foraging behavior than were their Afrotropical counterparts. As there are hints that this is also the case when migrants are compared with residents on their breeding grounds, we suggest that this flexibility enables migrants to partition resources with residents and, therefore, coexist with Afrotropical species. We discuss, however, whether this flexibility is an adaptation to migratory behavior or a prerequisite for the evolution of migration. The role that competition plays in present communities cannot be solved with a few observational studies because of the following problems. First, it is difficult to detect competition in the field. Second, there are constraints of performing field experiments that have not previously been performed in Africa, and third, there are several possible hypothetical scenarios about the role of competition in shaping present communities, including factors that might have been important in the past and are therefore impossible to detect at present.