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

In bird migration systems, the question of coexistence and competitionof migrants with residents in the nonbreeding season and theirrole in shaping the evolution of present avian communities ismuch debated. However, conclusions are often drawn in a speculativeway. In the Palearctic-African bird migration system, many studieshave addressed the question of coexistence of Palearctic breedingbirds with Afrotropical species in the former's wintering grounds.These studies have led to some generalizations concerning habitatselection and foraging ecology about the traits that might enablemigrants to coexist with residents. Migrants were thereforeassumed to forage in more open habitats, in more peripheralparts of the vegetation and with a higher foraging speed thanresidents. Furthermore, they were also assumed to be more flexiblein foraging behavior by using a wider range of foraging tactics,but some studies revealed contradicting results. We studiedthe ecology of Palearctic migrants in Comoé NationalPark, Ivory Coast, West Africa, during three successive wintersto explore the factors of niche partitioning between migrantsand residents and to test the hypothesis of whether there arecommon behavioral traits in migrants. Therefore, we comparedthe ecology of two Palearctic breeding species: pied flycatcher,Ficedula hypoleuca, and willow warbler, Phylloscopus trochilus,with resident members of the respective guilds. With respectto use of microhabitat, foraging speed, and intake rates, wecould not confirm the above-mentioned generalizations, showingthat care has to be taken into account when drawing conclusionsfrom few studies for a whole migration system on a huge continent.However, both migrating species were more flexible in foragingbehavior than were their Afrotropical counterparts. As thereare hints that this is also the case when migrants are comparedwith residents on their breeding grounds, we suggest that thisflexibility enables migrants to partition resources with residentsand, therefore, coexist with Afrotropical species. We discuss,however, whether this flexibility is an adaptation to migratorybehavior or a prerequisite for the evolution of migration. Therole that competition plays in present communities cannot besolved with a few observational studies because of the followingproblems. First, it is difficult to detect competition in thefield. Second, there are constraints of performing field experimentsthat have not previously been performed in Africa, and third,there are several possible hypothetical scenarios about therole of competition in shaping present communities, includingfactors that might have been important in the past and are thereforeimpossible to detect at present.