Why We Should Adopt a Broader View of Neotropical Migrants

Abstract

Our worldview dictates our approach to science. Sometimes this influence is obvious; often it is not. In the latter case, we are at risk of intellectual stagnation because of unconscious biases. Here I argue that the study of Neotropical migrant birds still suffers from largely unacknowledged temperate-zone biases. My purpose is to illustrate that, for both conservation-related and scientific reasons, we need to adopt a broader, less traditional view of Neotropical migrants. The most widely held view of a Neotropical migrant is a species that breeds north of the Tropic of Cancer and spends the nonbreeding season to its south (MacArthur 1959, Hagan and Johnston 1992a, Stangel 1992). This definition excludes Austral and intratropical migrants, an exclusion that is more than a problem of semantics. First, a narrow view of any topic usually defines a narrow scope of study. It is not coincidental that research on Neotropical migrants remains tightly focused on species that fit the above definition (hereafter referred to as Nearctic-Neotropical migrants). Despite recognition that Austral and intratropical migrants are also Neotropical migrants (e.g. Hagan and Johnston 1992a), they are rarely studied as such. In fact, they are rarely studied at all (Loiselle and Blake 1991, Chesser 1994, Powell and Bjork 1994). One reason is a relative paucity of Neotropical ornithologists (Short 1984). Another reason is that lack of awareness of these migrants may breed lack of interest. Second, the typical definition of a Neotropical migrant disregards the evolutionary connection between intratropical and Nearctic-Neotropical migrants. The former probably gave rise to the latter and presently the two groups are practically indistinguishable in terms of taxonomy, diet, and habitat use (Dixon 1897, Mayr and Meise 1930, Rappole et al. 1983, Ramos 1988). Their ecological and evolutionary parallels are especially apparent while Nearctic-Neotropical migrants are in the tropics; they become well integrated into tropical communities (Levey and Stiles 1992, Rappole and Tipton 1992). Thus, to understand the ecology of Nearctic-Neotropical migrants we need to understand the dynamics of tropical communities from which they came. This necessitates, for example, a broad view of interactions between resident and migrant birds and how their populations are linked (Ricklefs 1992). These points are not new. They crystallized at a 1977 symposium (e.g. Rappole and Warner 1980, Stiles 1980) and, although eloquently repeated since then (Ramos 1988, Greenberg 1992a), have been ineffective in guiding current research. An exception is work on shorebirds, some of which integrates resident and migrant ecology and encompasses the temporal dynamics of tropical species assemblages (e.g. Van Dijk et al. 1990, Hockey et al. 1992). More recently, Young and Morton (1994) took a broad view of Neotropical migrant landbirds. Third, the narrow view of Neotropical migrants restricts the types of questions we ask, which consequently may rob us of fresh insights. Tropical habitat requirements of Nearctic-Neotropical migrants provide a clear example. Contrary to the notion of tropical stability, many tropical -bird communities are highly dynamic (Davis 1945, Beebe 1947, Karr and Freemark 1983, Ramos 1988, Loiselle and Blake 1992). In Costa Rica, for example, a large proportion of species show evidence of seasonal movements (Stiles 1983, Levey and Stiles 1992). Presumably, many of these movements are driven by resource fluctuations (Loiselle and Blake 1991, Rosselli 1994). Nearctic-Neotropical migrants also experience and likely respond to spatial and temporal variation in their tropical resource base. Indeed, many species display movements in the tropics that are analogous to the seasonal movements made by closely related tropical residents (Morton 1971, 1980, Levey and Stiles 1992). Despite the dynamic nature of Neotropical bird communities, most habitat studies on nonbreeding Nearctic-Neotropical migrants are short term. This practice may reflect a temperate-zone bias, since the