Summaries of Selected Studies from the Northern Hemisphere

The study of bird migration has been one of the oldest endeavors of ornithologists, as the phenomenon is obvious and often spectacular. Investigators working in North America and Europe have been the main developers of research techniques for centuries. Thus, the Nearctic-Neotropical and Palearctic-Paleotropic systems are the two best-studied avian migration systems on the planet. Virtually all theories and research on the ecology and evolution of migrant birds concentrate on these two systems. This fact is clearly evident from a perusal of contributed chapters in the proceedings of major symposia on migrant birds (e.g., Keast and Morton 1980, Hagan and Johnston 1992, Martin and Finch 1995) and in overviews on migration ecology and evolution (e.g., Gauthreaux 1982, Rappole 1995, Alerstam and Hedenström 1998).

While migration within Africa and South America has been mostly ignored, these migratory systems potentially hold answers to many poorly known biogeographic, population and community processes of birds at those latitudes. Documentation of partial migration (species in which some populations are sedentary and others migratory), for example, is an open door for evolutionary and basic ecological research into why only some individuals of Neotropical species migrate.

Because data on austral migration are so scant, we present sketches of 6 research programs on Nearctic-Neotropical migration. Our goal is twofold: (1) to present theoretical frameworks successfully being used by North American ornithologists, and (2) to showcase novel techniques recently applied to migratory birds. In the context of this proposal, these goals take on significance when one ponders their applicability to the study of austral migration. South American ornithologists have tremendous potential for rapid progress because they can take advantage of refinements in theory and methodology championed by their Northern Hemisphere colleagues. Of course, these advances are already known and largely available in South America. Despite the opportunities, however, progress in applying them is slow. We believe that the underlying problem is one of critical mass -- the few South American ornithologists interested in migration are widely scattered and face enormous challenges because even the most basic information on migratory birds is lacking. The symposium-workshop brought together researchers on avian migration from across the Western Hemisphere to generate greater enthusiasm for austral migration, set research goals and establish collaborative ties across countries and continents.

For each of the six sections that follow, we first name research topics that are revolutionizing the study of Nearctic-Neotropical migration. These topics are in italics and underlined. We then “bullet” one or two recent studies that exemplify such lines of research. Finally, we indicate (with arrows) how such research is directly applicable towards furthering our understanding of the austral migratory system.

1. Dietary and morphological plasticity

Migratory birds are faced with a wide variety of environmental conditions and physiological demands throughout the year. Our ability to predict requirements and constraints associated with migration depends on understanding the interaction between physiological adaptations to migration and their ecological consequences (McWilliams and Karasov 2001).

• Parrish (1997) studied dietary plasticity of fall migrants on Block Island (Rhode Island), a stopover site. Practically all passerine species, including those typically considered purely insectivorous, consumed relatively large amounts of fruit. Because fruits are more energy dense, easier to capture, and more predictable in abundance than insects, Parrish hypothesized that migrants’ ability to shift to a frugivorous diet is a key adaptation for fueling migration (Parrish 2000). However, there may be a physiological price to pay for such a dietary shift, if sufficient nutrients are not acquired in a timely fashion. McWilliams and Karasov (2001) , for example, argue that digestive organs, though highly plastic in response to increased food intake during migration (e.g., larger gut size), can also be severely reduced during times of food deprivation (e.g., during long migratory flights). The resulting decrease in efficiency of nutrient utilization can severely compromise the speed of migration (Dekinga et al. 2001).

The take-home message of such studies is that a deeper insight into the ecological and physiological requirements during migration fundamentally changes our view of the nature of avian migration. In particular, migratory birds are not simply non-breeding versions of the birds studied by the vast majority of ornithologists in breeding season. The challenges they face are as unique and predictably changing as the seasons they chase.

→ How has the diversity of unique obstacles faced by austral migrants molded physiological adaptations necessary to complete migration in a timely fashion? Work on causes and consequences of dietary plasticity in North American migrants is providing insights into the relationship of migrants with their environment during migration. Knowledge of what resources austral migrants depend upon during different stages of migration will require a concerted, continent-wide sampling effort. Yet, understanding use of such resources is key to formulating management plans. Well-formulated hypotheses and easily-testable predictions on migrants’ physiological and dietary plasticity are waiting to be tested on a migratory system independent of those that generated the hypotheses (McWilliams and Karasov 2001).

→ More generally, we stress that essentially all hypotheses and theories of migration have been tested on the same migratory systems that generated them (e.g., Levey and Stiles 1992). The only way to break free of this circularity is to apply the same hypotheses and theories to another migratory system. As far as we know, this has been accomplished only once (Chesser and Levey 1998) and the theory being tested was extensively revised as a result .

2. Annual Survival

The period during which migrants are migrating has been virtually a black hole in our knowledge of challenges faced by migratory birds throughout their annual cycle. This point is best illustrated by a consideration of survivorship. Survival estimates have been generated for passerine migrants primarily on the breeding and wintering grounds, mainly because it’s most feasible to recapture birds during these periods. In contrast, estimates of mortality during migration are exceedingly difficult to establish.

• Using modeling, information generated from stable isotope analyses and color banding records, Sillett and Holmes (2002) estimated mortality of Black-throated Blue Warblers (Dendroica caerulescens) throughout the annual cycle. Their results indicate that mortality during migration is approximately 15 times that during stationary periods and that the migratory period can account for 85% of annual mortality. Such information carries broad implications for how we view the cost/benefit balance of migration. Sillett and Holmes (2002) emphasize the need to understand habitat-related survival in both demographic and geographic terms. This lesson underscores a central tenet in the study of migratory birds: to understand the ecology of migratory birds, we must first understand how events during any one season impact processes in other seasons.

→ With the growing recognition of migration as a major selection pressure on migratory birds, it is becoming a priority among researchers to understand the basic constraints (e.g., adequate habitat availability) that migration imposes on migratory birds. Identifying the period(s) when austral migrants face the highest challenges to survival will permit a better understanding, within an evolutionary context, of the forces molding migratory patterns in South America. In a more applied sense, it will streamline future conservation efforts for these species on a continent characterized by accelerating and widespread habitat alteration.

3. Stopover Ecology

Scientists are beginning to understand that migrants, even conspecifics, perceive their surroundings differently. These differences are a result of physiological conditions, experience, and weather that place various combinations of conflicting demands on each bird. Given that migration typically takes weeks and is probably the most energetically stressful period of a bird’s life, events during stopover are thought to be critical to the fitness of individuals and the conservation of species (Moore 2000).

• Moore and Aborn (2000) tracked Summer Tanagers (Piranga rubra) during stopover following trans-Gulf migration. They found that lean individuals showed differences in foraging behavior and habitat use relative to fat individuals. Their results suggest that, depending on energetic condition, birds exhibit different requirements during stopover. It is still not understood what cues birds use to assess habitat quality; indeed we are just beginning to understand basic migrant-habitat relations during stopover.

→ By understanding what strategies austral migrants employ under different environmental and endogenous conditions, we can approach the study of austral migrant ecology at understudied localities and habitats under a predictive framework. This necessarily requires a broad range of information on environmental and migrant bird conditions across seasons and locations in South America. More generally, stopover ecology is a topic that can draw in researchers from practically anywhere throughout South America because few places lack migrating birds.

4. Migratory Routes

Identifying migratory routes is a basic prerequisite for understanding migrant ecology, evolution, and conservation. For many species, migratory routes vary among individuals, age classes, and populations.

• Fuller et al. (1998) used satellite telemetry to trackPeregrine Falcons (Falco peregrinus) and Swainson’s Hawks (Buteo swainsoni) between breeding grounds in North America and wintering grounds in South America. The technology they applied not only allowed them to document a major migratory route, but also provided average speed and distance of migration, and wintering sites occupied. Using similar technology, Martell et al. (2001) followed Ospreys (Pandion haliaetus) and were able to document migratory route differences among breeding populations, as well as different migratory strategies among sexes (females migrated further).

→ Information gained from satellite telemetry would be invaluable for documenting spatial patterns of austral migrants during migration. This technology, however, is relatively new and is underutilized in South America. Satellite telemetry (and its future combination with other technologies, such as stable isotopes) is the only tool that easily permits collection of data as subtle as that of variation in migratory patterns among populations, ages and sexes. Collection of such basic information as how the timing of migration and migratory distance traveled varies demographically within a species could be greatly aided by the use of newly-available techniques. Application of this technology in South America will be aided by the strong intellectual and financial collaborations we aim to establish.

5. Connectivity of Populations/Stable Isotopes

Any given species of migrant bird is likely to lead different lives in different places. Although it is relatively easy to study a species’ ecology on its breeding and wintering grounds, it remains particularly challenging to link the impacts of population processes in one season on strategies and processes in other seasons. Yet, such links are important to establish if, for example, we are to pinpoint causes of population decline and implement effective management strategies. Stable isotopes offer a new means towards this goal.

• Marra et al. (1998) used stable isotopes to examine consequences of dominance among American Redstarts (Setophaga ruticilla) on their wintering grounds. Dominant males exclude females and first-year males from resource rich (mangrove) habitats in Jamaica and are easily distinguished from conspecifics in other habitats by the unique stable isotope signature of their tissues (which is generated by the marine food chain that predominates in mangroves). Marra et al. (1998) found this stable isotope signature in the first birds arriving on the breeding grounds in spring. Because early arrival is a strong predictor of reproductive success, this study unambiguously linked habitat use in winter to breeding success (i.e., fitness) in summer. Such links between seasons and the conflicting demands that may result have broad consequences for how we view costs and benefits of different strategies employed by migrants.

→ This point emphasizes the need to understand the ecology of austral migrants throughout their entire annual cycle, something only accomplished by integrating research foci throughout each migrant’s annual range. To do this, researchers must build on each other’s work and maintain open lines of communication. Our symposium-workshop is a first step in that direction.

→ Stable isotopes can also be used in a different way to link breeding and wintering populations of a migrant species. Because isotopic signatures in the environment vary predictively with latitude and/or longitude, and because these signatures are incorporated into consumer tissues, one can use tissues from a migrant to determine approximately where it was living when the tissue was produced. Thus, for example, one can analyze a wing feather (grown on the breeding ground) of a wintering bird and match its isotopic signature to the isotopic signature of potential breeding areas (or to isotopic signatures of birds collected in breeding areas) (Kelly et al. 2002; Rubenstein et al. 2002). This technique provides a powerful means of figuring out one of the most basic mysteries of migration: where migrants come from and where they go.

6. Partial migration

Within a species, the tendency to migrate is not absolute. In fact, many species of migratory birds have non-migratory populations, a phenomenon known as “partial migration.” Partial migration has received much recent attention because it is thought to be an intermediate step in the evolution of migration (Levey and Stiles 1992, Berthold 2001). Thus, identifying the factors that underlie the behavior may reveal the origins of migration.

• In Spain, where migratory and non-migratory Blackcaps (Syliva atricapilla) coexist in winter, Pérez-Tris and Tellería (2002) showed that competition between these groups resulted in a tradeoff. Resident (non-migratory) individuals were dominant, occupying preferred habitat almost exclusively, whereas migrants were found in many sub-optimal habitats. Furthermore, residents were typically larger than migrants. The authors suggest that migratory individuals trade competitiveness on the wintering grounds for improved migratory performance (i.e., smaller body size), while residents are better competitors (but would perform relatively poorly on migration).

→ Such comparative studies offer a promising and virtually unexplored means of discovering mechanisms underlying austral migration because they easily control for phylogenetic influences. Most austral migrant species have overlapping breeding and wintering ranges, in part due to the geography of the continent (Chesser 1994). Thus, partial migration is probably common in the system and could be used to gain important insights into the evolutionary history of the system.

Literature Cited

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