Links between worlds : unraveling migratory connectivity

NH 03755-3576, USA. Migration – the regular seasonal movement of individuals, often from a breeding location to a nonbreeding location and back – is a common and taxonomically widespread phenomenon in nature, and is particularly common among birds. An important, but unresolved issue in the study of migratory birds is the extent to which individuals from the same breeding area migrate to the same nonbreeding area and vice versa [1]. We term these links between breeding and nonbreeding areas ‘migratory connectivity’. The concept of migratory connectivity is distinct from other concepts of connectivity in ecology (Box 1). Moreover, migratory connectivity can be weak (diffuse) or strong, depending on the degree to which individuals from different nonbreeding areas mix during the breeding season and vice versa (Box 2). It is generally, if somewhat implicitly, recognized that migratory connectivity has important consequences for the ecology, evolution and conservation of migratory organisms. This recognition has led to massive mark–recapture efforts in which millions of birds have been ringed. In spite of such efforts, we still have a poor understanding of migratory connectivity for most bird species, and we do not know where individual birds from high latitudes spend their winter or vice versa. Do the individuals that comprise a single breeding population all migrate to the same nonbreeding location, or do they spread out over the entire nonbreeding range of the species? This lack of understanding has a simple underlying basis: the geographical areas and numbers of individuals involved are huge, but the organisms themselves are small. Thus, the probability of identifying, on the nonbreeding grounds, a bird that was ringed on the breeding grounds is very low. This frustrating situation could soon be resolved. Exciting new advances in remote sensing, the analysis of molecular genetic markers and chemical isotopes offer a level of resolution that was not available previously. By integrating these new advances, we might be on the brink of addressing the vexing issue of migratory connectivity and its biological consequences. Here, we describe some theoretical implications of migratory connectivity for ecology, evolution and conservation, and review recent technological advances and their application to the study of migratory birds. These techniques can be applied to the study of other migratory organisms [2], and our hope is that this review will stimulate further research on the consequences of migratory connectivity.