437 ?? NEW APPROACHES TO OLD QUESTIONS The study of bird migration has fascinated scientists since the early days of ornitho- logical research. Our aim in assembling this book was to provide readers with a sense of renewal in all aspects of the study of bird migration from evolution and ecology to physiology and morphology. The major questions that drive research on migra- tion systems continue to dangle out there like brass rings eluding our grasp: How did long-distance migration originate? How do different events and seasons shape the life history and adaptations of migratory species? How do migratory species interact with resident species? Although we await final answers to these and other questions that may never come, new approaches have been brought to bear on these time-hon- ored issues. As witnessed within this volume, researchers are increasingly integrat- ing studies of migratory birds with other seemingly unrelated disciplines of biology such as biogeochemistry and remote sensing. The Evolution of Long-Distance Migration Probably the single most significant improvement in our understanding of the evo- lution of migration has been the use of molecular genetics to determine the phylo- genetic relationships between migratory and resident taxa. Several studies have been published to date (see Joseph, Chap. 2 [All such references herein are to chapters in this volume.]) and it is clear that birds have shown surprising evolutionary lability in R U S S E L L G R E E N B E R G A N D P E T E R P. M A R R A The Renaissance of Migratory Bird Biology A Synthesis migratory behavior. Although migration has deep roots in the major taxa involved, migratory patterns appear to be quite responsive to environmental change. The develop- ment of direct (e.g., fossil evidence) and indirect (e.g., iso- topes analyses of climate change) approaches to establish what the paleoecosystems might have been in the regions where migration might have evolved provides a powerful backdrop for understanding these evolutionary studies (Steadman, Chap. 1). The use of new phylogenetic tech- niques, combined with traditional approaches within histor- ical biogeography, provides a fresh breath for testing specific hypotheses about when and where migration evolved (Out- law et al. 2003). These same techniques also provide better resolution than morphological analysis for how migratory and nonmigratory taxa are related ( Joseph, Chap. 2). The Diversity of Migration Systems The diversity of avian migration systems is just becoming apparent. There are six major migration systems around the world: Palearctic-Afrotropical, Nearctic-Neotropical, Palearctic-Asian, Austral-African, Austral-Neotropical, and Austral-Asian (Hayes 1995). Only recently has the extent of migrations from Southern Temperate regions into the trop- ics (austral migration) been appreciated. Probably because of the limited landmass at high latitudes and the less conti- nental climate of these regions, the diversity of austral mi- grants has been thought to be low. Also, the state of bio- geography of Southern Hemisphere avifaunas still lags behind their boreal equivalents. However, upon close ex- amination, we now know that some of these austral sys- tems are quite diverse. For example, over 220 species of birds from the Southern Cone of South America are now classified as complete or partial migrants (Chesser, Chap. 14). Furthermore, we now have a much better understand- ing of some of the basic ecological and biogeographical fea- tures of at least the South American austral migration sys- tems (Chesser, Chap. 14). We can begin to perceive finer patterns within austral migration systems, similar to those made for Northern Temperate Zone birds. Joseph (Chap. 2) makes a compelling argument that what we have consid- ered to be the austral migration systems involves two very distinct strategies with respect to response to climate. Fur- thermore, the diversity of movements within tropical re- gions is receiving increased recognition. Such tropical movements include north/south movements confined to tropical areas?some of which involve crossing the equator to take advantage of different seasonal regimes. Intratropi- cal movements can also involve altitudinal movements of birds (Loiselle and Blake 1992), particularly omnivorous species, and migrations between wet and dry habitats. Given that migration may have originally developed in mul- tiple lineages under more tropical conditions during the early Tertiary (Steadman, Chap. 1), the study of present-day tropical migration systems may provide insight into the eco- logical prerequisites of migration (Chesser and Levey 1998). Among landbirds, Old and New World migration sys- tems are dominated by distantly related taxa. Hemispheric comparisons of migratory traits can be quite robust be- cause a shared phylogenetic history is minimal. Such com- parisons of unrelated taxa allow for the determination of which features are general and which are responsive to present-day ecological and historical differences between temperate and tropical ecosystems. For example, current conditions in northern Africa are considerably more arid than those in the northern Neotropics, although both re- gions showed periods of distinct aridity through the Pleis- tocene. We also know that the vegetation of Europe was more extremely affected during the glaciations than that of North America. The geography of barriers to migration dif- fers as well. How these large-scale and the myriad more sub- tle differences shaped migration in the two hemispheres is just beginning to be explored in a comprehensive way. Comparisons between Old and New World patterns is a pervasive theme in many of the chapters in the book. Pat- terns of distribution of migratory birds across the Holarc- tic are compared by M?nkkonen and Forsman (Chap. 11). The difference in habitat distribution of migrants in the Western Palearctic and Nearctic is strongly related to differ- ences in habitats in Africa and the Neotropics (B?hning- Gaese, Chap. 11). However, differences in the response of vegetation to Pleistocene events need to be factored into future comparisons between the Old and New Worlds. Greenberg and Salewski (Chap. 26) find similarities in the degree of sociality in migrants in the two hemispheres, with telling differences (such as the lack of highly social and om- nivorous Old World tropical migrants). Some of the differ- ences between the New and Old Worlds are unexpected and demand further research to develop explanatory hypothe- ses. An example of this is the lower frequency of extra-pair fertilization in Old World migratory passerines (Stutchbury et al., Chap. 24). It should be noted that most comparisons focus on Western Palearctic/African migration versus North America (often just eastern North America). In many ways, the Eastern Palearctic is more similar to North Amer- ica in its present-day habitat and Pleistocene vegetation his- tory. We hope the next synthesis will involve more compar- ative work with this region as well. The Nonbreeding Season Thirty years ago, the Tropics formed a mysterious realm into which migratory birds disappeared for long periods. Perhaps nothing has changed more dramatically than the accessibility of the Tropics and the increase in our knowl- edge of the natural history of tropical avifaunas, both mi- gratory and nonmigratory. Basic information on the natu- ral history of migratory species on their tropical wintering grounds has increased but this period of the annual cycle is still seriously understudied (Rohwer, Chap. 8; Stutchbury et al., Chap. 24; Froehlich et al., Chap. 25; Greenberg and Salewski, Chap. 26; Runge and Marra, Chap. 28). For some 438 ? ? ? ? ?? ? ? ? ? short tropical regions around the world, we have information on the geographic distributions of winter populations of mi- gratory birds and some rudimentary habitat use data for many species, along with anecdotal observations of social and foraging behavior. We are just now beginning to see signs of greater attention to individual species and specific demographic classes within species during the nonbreeding season. However, quantified data have been garnered for only a few well-studied species. Information on the ecology of migrants wintering in East Asia, in particular, is still al- most nonexistent. The Macro-Distribution of Migratory Birds MacArthur?s (1959) focus on breeding landbirds of North America was the first attempt to find patterns in the relative abundance and diversity of migratory and resident birds. As more information has become available, researchers have looked across the different continents to establish general rules governing the distribution of migrant and resident taxa (Newton and Dale 1996). These approaches have gen- erally been applied in accounting for patterns of distribu- tion during the breeding season in the North Temperate Zone, but several authors in this volume have synthesized what we know about the relative importance of climate, habitat structure, and food resources across most of the ma- jor migration systems and periods of the annual cycle. Other chapters in this volume introduce innovative ap- proaches to the question of the relationship between the distribution of long-distance migrant and resident birds in both the Temperate Zone and the Tropics. Various studies (Hockey, Chap. 5; M?nkk?nen and Forsman, Chap. 11; B?hning-Gaese, Chap. 12; Gauthreaux et al., Chap. 15) each integrate, in their own way, the broad distributional pat- terns with the ecological processes that might mediate the relationships between migrants and their environment. This new integrated approach to the distribution of mi- grants is based on an array of concepts, some old and some new: evolutionary origins, global patterns of climate (temperature and wind patterns in the Temperate Zones, rainfall in the Tropics), the properties of ecosystems that contribute to climatic buffering, the seasonality of particu- lar resource types, and the competitive and cooperative in- teractions between migrant and resident species (and how these change between climatic regimes). These concepts go a long way toward explaining the relative abundance and di- versity of migratory and resident species, particularly in the Temperate Zone. Studies of ecological interactions between migrants and residents in tropical areas have become less popular in re- cent years. It is increasingly clear from such analyses that our ability to investigate large-scale questions seeking to ex- plain the distribution and abundance of species depends upon a more complete knowledge than we now have of the population ecology of individual species and specific guilds of species. The Micro-Distribution of Migratory Birds Avian ecology has traditionally focused on the role of re- sources (particularly food) in determining the distribution and abundance of species. Evaluating the relationship be- tween migratory bird populations and their food supplies requires sound information on the abundance and season- ality of food and its variation between years and habitats. Furthermore, the data are more interpretable if data on diet are used to help select a resource-sampling scheme that fo- cuses on the relevant prey items. With new attention to em- pirical evidence, Sherry et al. (Chap. 31) have established a prima facie case that food supply is often the most impor- tant limiting factor in the distribution and abundance of nonbreeding migratory birds in the Tropics. On the breed- ing ends of things, annual fluctuations in food supply also underlie variation in reproductive success and this variation appears to have important consequences for population size in subsequent years (Sillett et al. 2000). Our understanding of population dynamics also requires detailed knowledge of the effect of different predators and parasites (avian brood parasites, ectoparasites, disease- causing microbial agents). The role of nest predators and brood parasites has long been incorporated into breeding season studies of migratory birds. However the effect of predation and disease on adult birds has been more elusive. Recent studies of incidence of human disease (e.g., West Nile virus) in migratory birds underscore their potential as long-distance vectors (Ricklefs et al., Chap. 17) and the topic is certain to receive much more research attention. The analysis of morphological and behavioral adaptation has become increasingly sophisticated, allowing us to use the traditional comparative approaches with increasing rigor. The multivariate models necessary to handle complex and interrelated traits are far more robust than they were two decades ago, enabling researchers to simultaneously ex- amine the effect of many different types of variables. More importantly, comparative analysis has seen the advent of techniques to tease apart correlated characters that are a re- sult of shared phylogenetic history from those that are adap- tive responses to similar environments. These statistical tools have resulted in our ability to draw out emergent pat- terns of behavior and morphology that distinguish migra- tory taxa from ecologically similar or related resident species. For example, adaptations that promote long-distance flight clearly provide constraints on trophic adaptations and related bill and hind-limb morphology (Winkler and Leisler, Chap. 7). The patterns that emerge from these broad comparative approaches should certainly inform the development of more focused experimental tests of function. Demography and Life History Demographic data, in particular, are crucial to answering key questions such as when in a migratory species? life cy- cle do population limitation and regulation occur and what The Renaissance of Migratory Bird Biology 439 short are the life history trade-offs involved in the evolution of migration (Runge and Marra, Chap. 28, and Sillett and Holmes, Chap. 32). Life history data are also becoming in- creasingly important to address the conservation needs of migratory species. Our understanding of the population biology of migratory birds has grown from an increase in studies of marked populations as much as?perhaps more than?from any particularly theoretical development. These studies, particularly those carried out over many years, provide necessary fodder for the development of the- ory in the field. For example, the intriguing question of why birds migrate as far as they do (to the point that northern populations ?leap-frog? over the winter range of more southerly breeding populations) has inspired some sophisti- cated models (Bell, Chap. 4) that require much more de- tailed demographic data to test. We are beginning to see studies of populations of mi- gratory species with uniquely marked individuals during the nonbreeding season and a few studies (too few) of species that encompass populations at both ends. The estimation of survival and reproductive output is increasingly gaining prominence in studies of migratory bird ecology. New sta- tistical techniques for estimating survivorship and repro- ductive success have moved the field forward in the past decade. Estimating and untangling juvenile survivorship and natal dispersal remain the greatest methodological challenge to developing a complete picture of the demog- raphy of migratory species. Winkler (Chap. 30) reviews a number of promising approaches to the study of dispersal in migratory birds; most evidence still appears to support the notion that migratory birds tend to have longer disper- sal distances and that this results in more open and less dif- ferentiated local populations. NEW PERSPECTIVES Cross-Seasonal Approaches Are Necessary We believe that the single most profound epiphany in the study of migratory birds is that their life histories and adap- tations simply cannot be understood through increasingly detailed studies in a single season. Although this idea is not completely new, the degree to which it has permeated the thinking of ornithologists studying migratory species rep- resents a clear paradigm shift that is reflected in all of the contributions to this volume. The contributions on mor- phology, cognition, foraging behavior, population biology, macroecology, and evolutionary processes (such as specia- tion) all underscore the fact that we need to integrate the ef- fects of the different phases in the life of migratory birds to achieve a holistic understanding of each of these topics. For example, early research examining the role of natu- ral selection in shaping life histories and adaptations in mi- grants analyzed it as two competing hypotheses: either the breeding season is critical or the winter is. It should be clear that for a migratory bird to reproduce successfully?the hallmark of evolutionary success?it must possess a pheno- type that functions in all seasons. This will almost invariably result in a convex fitness set, where traits are not specialized for one particular season or habitat, but instead function successfully at all seasons. This generally requires adaptive compromises for any one season that can only be under- stood by examining how things work at all seasons. Price and Gross (Chap. 27) provides an intriguing example of how the ecological niches of Phylloscopus warblers are constrained across seasons. Adaptive compromise is a critical theme in understand- ing various specific adaptations in migratory birds. For ex- ample, studies in this volume of external morphology (Winkler and Leisler, Chap. 32), physiology (McWilliams and Karasov, Chap. 6; Holberton and Dufty, Chap. 23), en- ergy storage and management (Rogers, Chap. 9) are begin- ning to show how we can integrate selection through the annual cycle to understand adaptation in migratory species. Of course, the need to be successful in the face of changing conditions across the calendar is not unique to migratory birds, but can be found in all birds facing seasonal environ- ments. Migratory birds are, therefore, a model system for studying adaptation in seasonal environments?an issue of general biological significance. Webster and Marra (Chap. 16) provide clear evidence that it is critical to understand the contribution of selection in different seasons and that events in one season can carry over to another. The importance of carry-over effects is modeled by Runge and Marra (Chap. 28) and show that competition for resources during the win- ter expresses itself in differential reproductive success on the breeding grounds thousands of kilometers away. It is completely clear that ecological decisions made in one season are integrated into the selective factors shaping other necessary activities at other times in the annual cycle. The tools of cognitive ethology and comparative psychol- ogy are just beginning to be applied to learning how indi- viduals acquire and use information during and between migratory excursions (Mettke-Hoffman and Greenberg, Chap. 10). A more holistic approach to the annual calendar must go beyond considerations of winter and summer events (two worlds) to include all time-sensitive activities. Constraints on where and when birds must migrate to op- timize resources and flying conditions along the way cer- tainly affect the choices birds make during the more sta- tionary periods of their annual cycle. Data are beginning to confirm that migration is a time of year that accounts for much of the mortality (Sillett and Holmes, Chap. 32) and in- fluences the condition of individual birds as they face the breeding season (Moore et al., Chap. 20, and Piersma et al., Chap. 21). As a consequence, we are beginning better inte- gration of migration studies with our understanding of life history and demography as a whole. However, because in- dividuals are hard to follow for extended periods, migration research requires additional innovative approaches to make major advances. As little as we know about how the period of migration affects the overall life history of migratory birds, the transition periods?such as the period immedi- 440 ? ? ? ? ?? ? ? ? ? ately following breeding?are even more mysterious. Rohwer et al. (Chap. 8) reveal the richness of strategies in- fluencing where and when to complete molt, arguably the most critical of the post-breeding activities. A thorough un- derstanding of when and where birds molt is not only im- portant in its own right, but is critical for using isotopes in feathers (see below) to study migratory connectiveness. Migration, Speciation, and Population Connectivity The role of migration in influencing evolutionary diver- gence of populations has long been an area of speculation (Salomenson 1955). Generally, this thinking has focused on the role of migration in increasing dispersal distances and gene flow between breeding populations?thus inhibiting genetic divergence. Additionally, selection during the non- breeding season may counter any selection for local adap- tation that occurs when populations segregate in the breed- ing season. Such are the classic areas of inquiry into the evolutionary significance of migration. However, recent re- search has revealed other ways in which migration can in- fluence the fundamental process of speciation. The study of migratory divides, which is exemplified in chapters by Irwin and Irwin (Chap. 3) and by Smith et al. (Chap. 18), suggest that geographically imposed migratory divides may select against hybrids and hence hasten the rate at which species form on either side of the divide. Migration may also con- tribute to speciation through a hypothesized ?migration dosing,? as is proposed by Bildstein and Zalles (Chap. 13). They develop the argument that migrants blown off course (i.e., vagrants) may form the nucleus of populations that colonize islands and other isolated landforms. In the rare circumstance that these birds form stable reproductive pop- ulations, speciation may eventually occur. In summary, mi- gration appears, at least theoretically, able to either impede or facilitate speciation depending on the situation. Such processes of speciation acting primarily on breed- ing areas have consequences on the distribution of migra- tory birds during the nonbreeding season. Webster and Marra (Chap. 16) outline a broad and important research agenda on population connectivity that needs to be ad- dressed as we learn more about how breeding populations map onto winter distributions and vice versa. Because such mapping will allow researchers to pursue many different kinds of questions, we spent considerable space in this vol- ume discussing the various tools for the job. These tools in- clude the use of genetic markers, stable isotopes, and trace elements in bird tissues (Smith et al., Chap. 18, and Hobson, Chap. 19). With the tools available to characterize popula- tions, we may soon be able to link performance between particular areas of the breeding and wintering grounds (given that such structure exists). Sz?p and M?ller (Chap. 29) take this bold step by relating the survival of two swal- low species between breeding seasons in Europe to satellite- determined data on the condition of rainfall-dependent veg- etation across the African continent. Clearly, understanding both within- and between-season population connectivity will provide a more accurate picture of what a ?population? truly is for a migratory species. Expanding Research in Time and Space Ornithologists have come a long way in the past two decades in recognizing the importance of long-term cli- matic cycles and rare disturbance events in shaping the life histories of birds. This increasing emphasis on long-term events has been fueled by the inadequacy of equilibrium- based models for explaining real populations in natural ecosystems, as well as by the need to place human-caused change in its proper context. We have made real progress in demonstrating the role of known climatic events (such as the El Ni?o Southern Oscillation) in shaping the life history of migratory and nonmigratory birds (Sillett et al. 2000). And we now understand that disturbance factors, such as se- vere weather and fire, have probably affected migratory birds as much as periods of stasis have (Rotenberry et al. 1995). Taking an even broader view of environmental insta- bility, Winkler (Chap. 30) makes a tantalizing suggestion that dispersal strategies of temperate zone birds may have been shaped by the historical loss and gain of large areas of ap- propriate habitat during the Pleistocene.The unit of study has been changing as well. Avian ecologists now recognize that ?communities? the size of study plots do not necessar- ily represent the dynamics of populations that interact across vast areas of natural and human-influenced landscapes (Pul- liam 1988). We have a long way to go before we can under- stand the dynamics of populations over the relevant land- scape mosaic during both the breeding and nonbreeding seasons, but this should remain a research priority. A Focus on Individuals Recent advances in behavioral ecology have brought us a major shift in emphasis over the past two decades. The in- dividual bird is now taking center stage in our efforts to understand the ecology and evolution of migratory birds. Accompanying this greater interest in the performance of individuals is an increase in our ability to track their fates. With radiotelemetry we are better able to examine the de- cision-making strategies of individuals at the local scale, and thanks to advances in technology in satellite telemetry capabilities for larger species, we can now track birds through entire migrations. Cochran and Wikelski (Chap. 22) elaborate on these advances and provide an exciting vi- sion for our ability to track individuals from space over even greater distances. Ornithologists often focus on measuring the abundance of individuals over space. However, aware that interpreting presence/absence data is fraught with problems, ecologists and conservation biologists are increasingly concerned with the actual fitness consequences of different ecological deci- sions. Although fitness itself is the measure of ultimate im- portance, for most organisms?particularly highly mobile The Renaissance of Migratory Bird Biology 441 ones?fitness is almost impossible to measure. It is even more difficult to assess how any one aspect of the life his- tory of a bird affects its fitness. So ornithologists are em- ploying, and in some cases developing, new techniques to measure the condition of individuals that in all likelihood correlate with fitness itself. Fitness indices, such as hormone levels, mass changes, and immune responses, do indeed rep- resent an advance from the past but must still be compared with more direct measures of fitness so that we can be cer- tain of their utility. The traditional view is that most migratory birds form pair bonds that facilitate cooperation between the sexes to raise young. Recently, the nature of the pair bond has been challenged in several different ways (Stutchbury et al., Chap. 24). First, genetic analyses have shown that in migratory songbirds, many pairs are not truly monogamous. The male that participates in the cooperative activities associated with the pair bond does not necessarily sire all of the young. This discovery has led to a rich line of research on what factors favor the occurrence of extra-pair fertilizations and which pair member is in control of the reproductive decisions. Second, several interesting facets of male-female dynamics during the nonbreeding season have been discovered. For example, the number of taxa where it is known that pair bonds are developed and maintained during the nonbreed- ing season has expanded. Classically, extra-seasonal pair bonds were thought to be restricted to a few groups of long- lived migratory species?such as waterfowl. Now we know that a scattering of songbirds also develop such bonds. However, the possibility that pair bonds are maintained be- tween seasons needs further exploration. Also interesting is the recent evidence that males and females compete for re- sources during the nonbreeding season. Evidence thus far suggests that males are behaviorally dominant over females and that this has consequences for male survival. Not un- derstood, however, is how this can act as an evolutionary stable strategy. MISSING PERSPECTIVES As comprehensive as we believe this book is, we cannot help thinking about the important topics that did not receive coverage. Some of these areas may prove to be critical in any future synthesis about the ecology and evolution of bird migration. We would therefore like to touch briefly upon some of the holes in the tapestry of this volume with the hope that mentioning these gaps will still inspire future research. Plasticity in Migratory Programs The role of genetic control versus developmental plasticity and environmental influence of the migratory program, and their interaction, is a subject that will loom large in any future discussion of the evolution of migration patterns. Research in the past two decades has established for some species the importance of genetically controlled endoge- nous rhythms in migratory behavior that play a key role in establishing the timing and at least general features of the geography of migration. However, the facultative response of Zugunruhe and other components of the migration pro- gram have been confirmed only in short-distance migrants and one or two Palearctic migratory species. How individ- uals make their own unique migration decisions and re- spond to variation in the environment is an area for physi- ologist and cognitive ethologists to explore, but it has profound implications for how migration can be coupled with or decoupled from environmentally based selection. Furthermore, the responsiveness of hard-wired behaviors to selection is an area that is just beginning to be explored. Such plasticity in avian migratory programs is evident at both the individual and population levels. It is likely that even the most spectacular long-distance journeys evolved through gradual stages from residency, to partial migration where only some individuals in a population migrate, to complete short-distance movements, and finally to long- distance migrations. Detailed study of populations display- ing partial migration systems (Cristol et al. 1998) may pro- vide insights into the trade-offs underlying all migration systems. Partial migration systems are not a focus of any of the contributions to this volume, but their study represents a continuing research opportunity. The Role of Pioneers and Vagrants When it comes to normal dispersal, we are just beginning to frame the right questions and develop the methodologies to approach such questions for migratory species (Winkler, Chap. 30). However, the role of individuals that disperse well outside of the ?normal? range of a species?so-called vagrant birds?has not been investigated from an evolu- tionary or ecological perspective (but see Bildstein and Zalles, Chap. 13). While these individuals provide much of the sport for birders, the numbers in which they occur sug- gest that they may also play an important role in the long- term success of lineages of migratory birds. In the highly unpredictable and rapidly changing Quaternary and Holo- cene climates and biomes, birds that produce pioneering phenotypes may be ecological winners. The existence of ex- treme vagrants may be a highly visible endpoint to a gradi- ent of hyperdispersal in Temperate Zone migratory birds. The ecological and taxonomic correlates of avian vagrancy may be an area of high-risk research with a large potential pay-off, but the possible role of vagrants in speciation (migration-dosing [Bildstein and Zalles, Chap. 13]) remains an important area of research and discovery. From Orientation to Habitat Selection Orientation mechanisms have inspired some of the most elegant experimental work in animal behavior and physiol- ogy. After decades of this research, however, we are still far from understanding how birds locate and choose particular 442 ? ? ? ? ?? ? ? ? ? sites at either end of their migratory route and how they are able to retrace their trips so precisely. The prospect of track- ing individuals opens up the possibility of developing greater understanding for the mechanisms by which partic- ular habitat patches are selected. FUTURE DIRECTIONS The hallmark of success of any scientific field is the degree to which it attracts the best and brightest of the next gener- ation of investigators. We believe that the understanding of how migratory behavior shapes the adaptations of birds is inherently fascinating. More generally, migration systems provide an excellent opportunity for examining how envi- ronmental challenges at different stages of a life cycle shape avian adaptations. New technologies, better natural history information, and new perspectives have reinvigorated the field of ornithology and, more specifically, migratory bird biology. Thus we believe that this volume, in each of its chapters, provides any young investigator a wealth of im- portant and interesting questions to pursue. We offer the following additional questions as our take on what some of the most interesting challenges of the coming decades might be. Our hope is that when another symposium is or- ganized in 10 years, we have a few answers and a new list of even more compelling questions. 1. How can we integrate phylogenetic hypotheses for the evolution of different clades with our increasing understanding of paleoenvironments in different re- gions? How much were the current migration systems shaped by deep (Tertiary) versus recent (Quaternary) events? 2. What are the likely ecological attributes of taxa that gave rise to migratory populations? 3. To what degree do density-dependent versus density- independent events during migration contribute to the population dynamics of migratory birds? 4. What factors determine seasonal population limita- tion and at what time of year are populations of mi- gratory birds most limited? 5. What are the primary factors that regulate popula- tions of migratory birds? 6. What are the trade-offs in foraging ecology and trophic adaptations in species that migrate between temperate and tropical habitats? 7. How do social interactions and dominance relation- ships in one season affect events in subsequent seasons? 8. What cognitive abilities distinguish migratory and non- migratory species? 9. How do breeding populations map onto winter distri- butions across migratory species and vice versa? What are the social and ecological factors that underlie these patterns? 10. To what degree do competitive interactions between migrant species and between migrant and resident species shape the distribution and habitat use of mi- grant species year round? 11. What ecological and climatic factors shape patterns of migration and determine the travel and stopover strategies? 12. How does migration contribute to evolutionary processes within species and to the formation of new species? 13. To what degree are the annual time activity budget and migration strategies genetically constrained and what is the importance of behavioral plasticity? L I T E R AT U R E C I T E D Chesser, T., and D. J. Levey. 1998. Austral migrants and the evolu- tion of migration in New World birds: diet, habitat, and mi- gration revisited. American Naturalist 152:311?319. Cristol, D. A., M. B. Baker, and C. Carbone. 1999. Differential mi- gration: latitudinal segregation by age and sex class. Current Ornithology 15:33?88. Hayes, F. E. 1995. Definitions for migrant birds: what is a Neotropical migrant? Auk 112:521?523. Loiselle, B. A., and J. G. Blake. 1992. Population variation in a tropical bird community: implications for conservation. Bio- Science 42:838?845. MacArthur, R. H. 1959. On the breeding distribution patterns of North American migrant birds. Auk 76:318?325. Newton, I., and L. Dale. 1996. Relationship between migration and latitude among west European birds. Journal of Animal Ecology 65:137?146. Outlaw, D. C., G. Voelker, B. Mil?, and D. Girman. 2003. The evolution of long-distance migration and historical biogeog- raphy of the Catharus thrushes: a molecular phylogenetic ap- proach. Auk 120(2):299?310. Pulliam, H. R. 1988. Sources, sinks, and population regulation. American Naturalist 132:652?661. Rotenberry, J. T., R. J. Cooper, J. M. Wunderle, and K. G. Smith. 1995. When and how are populations limited? The roles of insect outbreaks, fire, and other natural perturbations. Pages 55?84 in Ecology and Management of Neotropical Migra- tory Birds (T. E. Martin and D. M. Finch, eds.). Oxford Uni- versity Press, Oxford. Salomenson, F. 1955. The evolutionary significance of bird mi- gration. Biologiske Meddelelser 22:1?62. Sillett T. S., R. T. Holmes, and T. W. Sherry. 2000. Impacts of a global climate cycle on population dynamics of a migratory songbird. Science 288:2040?2042. The Renaissance of Migratory Bird Biology 443