Preface
Recent advances in our knowledge on the evolution, adaptations and costs and benefits of avian migration has been in great part due a diversification of research questions and approaches while maintaining perspective concerning the range of challenges migratory birds face. As a consequence, interdisciplinary integration has become a direct product of the relatively holistic perspective researchers have taken concerning migration. This integration is inevitable when studies employ a realistic consideration of the constraints on migrant bird survival among the whole annual cycle and how past and anticipated stresses could act to mold migratory strategies. I give an overview of recent studies on the study of migration which exemplify this holistic approach in our understanding of life on the move.
A central premise which has been advanced for understanding survival strategies in migratory bird species is that each life history stage is constrained by factors in other seasons and habitats the organism occupies throughout the year, though this paradigm has taken time in becoming established in research design. With the increasingly sophisticated techniques and technologies which allow us to probe more detailed questions, it could be easy to overly concentrate on one facet of the migratory cycle (e.g., one habitat occupied, a single migratory route, one physiological state) while ignoring the full range of conflicting demands on an organism’s survival. Apart from the prejudices researchers inevitably bring to science, logistical limits (e.g., in technology and methodologies) must also be considered to account for the seemingly narrow perspectives on a subject some studies exhibit. Not all questions can be answered. Nevertheless, the study of avian migration, while not free from limitations in our thinking, has progressed markedly in spite of logistical hindrances and in spite of the diversity of approaches researchers have followed. Indeed, for a variety of even basic questions about migration, theory has evolved more rapidly than empirical research.
The all-encompassing nature of migration in molding and being molded by survival strategies in migratory birds at different life history stages and throughout the annual cycle has demanded researchers to necessarily consider all these constraints from a variety of biological perspectives, implying a need to become interdisciplinary when attempting to answer the “why’s” of migratory strategies. The following is an overview of the types of questions being asked in migration studies, concentrating on evolution of migratory systems, connectivity among seasonal ranges, physiology and resource use during migration, highlighting some sophisticated approaches employed as well as interdisciplinary approaches taken by various workers.
Evolution of Migratory Strategies
Though still generally poorly understood, concepts on the origins of migratory systems have broadened through the use of a diversity of approaches, including inferences from observations of present day processes and migratory population dynamics. Attempting to link processes in one phase of the annual cycle to life strategies in other seasons, though not novel ¹, is useful and pervasive in the literature. Since a variety of partially migratory species exist (for which some populations migrate, others are sedentary) comparisons between resident and migratory populations have proven useful. Studying blackcaps, Pérez-Tris and Tellería² have postulated that residents are better competitors than conspecific migrant individuals due to an apparent lack of niche partitioning, suggesting that resident-migrant interactions during the nonbreeding season could influence who migrates in this species. In a similar comparison, though on a geographic scale, Bensch³ found that resident Eurasian bird species tend to have larger ranges than migrants and surmised that connectivity between breeding and wintering sites (in which individuals return to the same sites year after year) and the migratory “programs” used to travel between the two constrain the size of breeding ranges. This could potentially ultimately affect speciation patterns of migratory species, for example by promoting natal philopatry. Thus, comparing variable life strategies (migratory vs. sedentary) between species, attempts have been made at inferring past process in the shaping of migratory behavior.
Comparisons have not been limited to a resident-migrant context. After analyzing migratory patterns on various continents, Hockey 4 concluded that symmetry of habitat types about the equator within continents could be a constraint to migration and could be a product of past climatic processes. Such comparisons among migratory systems, though not common in migration studies (but see Chesser and Levey 5), demonstrate the broadening scope of thought on migration, in this case integrating past and present processes from a variety of migratory systems, and which is partially due to the increasing amount of information being compiled on different migratory systems.
Integration when assessing the evolution of migration, however, has not only been limited to linking similarities and differences among migratory systems, but also to including input from other scientific disciplines. For instance, use of data on past pollen distribution from the field of geology and simulations of wind matrices during glacial times have allowed an estimation of historical limits to breeding habitats and as a consequence inferences concerning differences between past and present migratory routes 6.
Systematics is becoming important in understanding biogeographic and evolutionary migration patters. Studies incorporating phylogenies when analyzing origins of migration are becoming more popular with the advent of advanced molecular techniques and increasingly detailed phylogenetic reconstructions, as exemplified by Joseph et al. 7. These workers postulated on the ancestral home of migration in Charadrius shorebirds by classifying breeding and non-breeding distributions as characters mapped onto a phylogeny and analyzing direction of character evolution, enabling an evaluation of the geographical origins of migration.
Connectivity
Linking an organism’s survival strategies among widely separated summer and winter territories is central to understanding the variable requirements of migrants in time and space. However, tracking the movements of organisms has traditionally proven to be a difficult enterprise (reviewed by Cohn 8), as marking an organism does not guarantee it will be observed again. The use of markers which emit a continual signal, namely radio or satellite telemetry, is useful, though selective in the environments in which a signal can be received and restricted to organisms large enough to carry a device. This situation has lead scientists to explore the existence of natural “markers”. Useful methods developed have involved the use of genetic and isotope markers. In a groundbreaking study, Marra et al. 9 were able to show, via the use of stable isotopes, that the quality of winter habitat of American redstarts (Setophaga ruticilla) affects physical condition and as a consequence date of arrival on breeding grounds. Similarly, it has been shown possible to estimate the extent in breeding area from where a group of wintering birds hail 10. The ability to document such patterns has broad applications (e.g., species management) and consequences for the formulation of more detailed questions. Thus, the advancement of such novel techniques can give investigators the ability to broaden their thinking about how limitations to migratory bird populations in one geographic area could present limitations during other parts of the annual cycle and species range. This technology also permits an evaluation of interpopulation dynamics during migration, illustrated in the leapfrog migratory pattern of Wilson’s warblers (Wilsonia pusilla) 11. In effect, stable isotope analysis has permitted researchers to surmise that northern breeding populations of this species winter further south of more southerly breeding populations in western North America.
The use of stable isotope technology represents a good example of how interdisciplinary efforts are advancing our understanding of avian migration while integrating the way we think about migrant dynamics throughout their life cycle. The combined use of genetic markers with stable isotopes is now being explored in order to increase geographic resolution in establishing population linkages 12.
Physiology
Classically, a central theme in avian migratory energetics has concerned how fuel is stored and used during migration. While previous studies have successfully documented lipids as the main fuel for migration, recent work has demonstrated protein catabolysis to occur in both flight muscle as well as organ tissue during migration 13, 14, reasons for which are still largely unknown.. Although lipids have been commonly considered the main fuel for migration, the use of protein as a fuel has raised various questions, as selection would seem to favor the conservation of muscle and organ mass during migration. Additionally, since migration is generally believed to be time-limited and as the loss of protein could negatively affect migration speed, current hypothesis on migratory strategies may require modification. Water production, anti-oxidative purposes and adaptive changes in flight muscle size are possible mechanisms being considered in support of the idea that protein loss could represent optimizing physiological flexibility 15, and require testing
Quantifying the ability of migrants to gain mass at stopover sites has and continues to be a central goal of many conservation-oriented studies, as fuel deposition during stopover is assumed to be critical in a bird’s ability to cross remaining distances and complete migration successfully. Consequently, biogeographical information (i.e. migratory routes in relation to wintering and breeding grounds) must be combined with that on stopover ecology to understand how to better conserve migratory birds 16. Such exogenous demands on migration are increasingly being coupled with endogenous, physiological constraints to fuel deposition and use, changing the way we think about migratory strategy. Adaptations to deal with increased energy demands such as dietary plasticity, mainly involving the switch from an insectivorous to frugivorous diet during fall migration, is believed to be important in a migratory bird’s ability to efficiently store lipids while cutting energy expenses 17,18. Gannes 19, for example, was able to show for some Old World Passerines that frugivores use more lipid reserves than proteins as compared to insectivores, considering it to be positively adaptive.
The link between ecological and physiological constraints is also demonstrated by the now well-documented of migratory birds to not gain mass, and to even lose some weight upon arrival at a stopover site. This is counterintuitive, as replenishing fuel stores would seem to be an immediate priority of migrants. Reasons proposed have included illness, recovery time after stressful flight, inability to locate food due to unfamiliarity with the feeding area and time required to physiologically switch from flight exercise to feeding. The phenomenon is not necessarily universal, as at least one study failed to find it 20.
The incorporation such factors as dietary plasticity along the migratory route, distances birds must travel, season, evolutionary history and available resources en route to explain such unexplained patterns as those mentioned highlights the necessarily broad nature of this work.
Resource Use
That most migratory birds during migration lack specific habitat and resource requirements during migration has become somewhat of a dogma, spurred on in part by our simple ignorance of where birds migrate through and, especially for Nearctic-Neotropical migrants (breeding in North America and wintering in the Neotropics), where they winter. Increasingly, however, selectivity in resource use by migrants during migration (though possibly more labile than during the breeding season) has been documented 21, 22, creating implications for future research directions and for management options for specific species. The fact that many migrants exhibit very different habitat preferences during the migratory period than during either the breeding or wintering seasons 23 also carries broad implications for understanding migratory bird habitat preferences throughout the year, affecting questions on the evolutionary history and how survival strategies in one season could affect those in other annual phases. Additionally, reasons for postfledgling dispersal (prior to fall migration) are still poorly known. Such dispersal by Black-throated Blue Warblers (Dendroica caerulescens) into montane spruce-fir forests, which differ from breeding ground hardwood forest habitats, may be due to competition with dominant adults 24.
Elucidating just what cues migrants use in making decisions on habitat use is exceedingly difficult, complicated by our lack of knowledge on features birds use to evaluate their surroundings as well as the wide variety of ecological, physiological and abiotic influences likely affecting a migratory individual’s priorities from one moment to the next. Due to this complexity, as well as that of resource use in general during migration, a highly interdisciplinary approach will doubtless be necessary. Moore and Aborn 21 sum up past research well: “Study of habitat selection in migratory birds has focused largely on describing habitat use during stopover rather than examination of the underlying proximate mechanisms that mediate selection”, a process which will stretch our scientific skills to the limit, requiring well-rounded, holistic approaches.
Research Trends
Overall, the study of migration has been a pattern-finding enterprise, with mention of mechanisms mainly restricted to speculations in the Discussion section. Nevertheless, as information on the basic characteristics of each species’ migration strategies continues to mount, reasons behind such patterns can begin to be investigated, especially as more refined techniques and technologies, especially those from other disciplines (e.g. stable isotope mapping by geologists) continue to advance.
Researchers are beginning to framing questions which as much as possible get at the adaptive benefits to survival of migratory bird characteristics during migration. Questions are more often framed within an evolutionary framework, formulating comparisons among and within migratory systems, studying the evolutionary history of migratory systems and trying to understand the basic mechanisms behind migratory behavior. The increasingly interdisciplinary nature of contemporary studies is evident, regardless of the question at hand. Similarly, a broader approach when thinking about a migratory bird’s requirements throughout the year and at different life history stages will be necessary to advance our understanding of the various requirements of and conflicting demands acting on these birds. As our understanding of the dynamics behind the travels of these birds grows, hopefully so too will our ability to ensure their endurance.
Literature Cited