Partial migration in birds: tests of three hypotheses in a tropical lekking frugivore

1. Partially migratory species provide opportunities to understand which ecological factors cause some animals to migrate when others remain resident year round. Partial migration in birds has been explained by the dominance, arrival-time, and body-size hypotheses. 2. Testing these hypotheses has proven difficult due to the similarities of the predictions they make in temperate-breeding long-distance migrants. In tropical altitudinal migrants, however, these hypotheses make different predictions regarding the sex, age, and condition of migrants and residents. 3. Among white-ruffed manakins in Costa Rica, young birds were not more likely to migrate (as predicted by the dominance hypothesis), nor were females more likely to migrate (as predicted by the arrival-time hypothesis). All condition-related variables interacted with sex, together explaining much of the variation in migratory behaviour. 4. I re-articulate the body-size hypothesis in the context of tropical altitudinal bird migration, focusing explicitly on how limited foraging opportunities and differences in individual condition affect fasting ability during torrential rains. Despite ample food, the smallest birds or those stressed by parasites or moult may risk starvation at breeding elevations due to a reduction in foraging time. These results highlight how intrinsic and extrinsic factors may interact to produce observed patterns of within- and among-species variation in migratory behaviour.     

Being En Route

Through an ethnography of unauthorized migration from El Salvador to the United States, I explore "clandestinity" as a hidden, yet known, dimension of social reality. Unauthorized migrants who are en route to the United States have to make themselves absent from the spaces they occupy. When they become clandestine, such migrants embody illegality; in some cases, they literally "go underground" should they die and be buried en route. Because their presence is prohibited, unauthorized migrants do not fully arrive even when they reach their destinations. There are parallels between the ways that migrants are present in yet absent from nations, and the ways that ethnographers are present in yet absent from the field. This ethnography of migrants en route therefore suggests how anthropological knowledge practices also produce realities that are hidden, yet known.     

“Grateful” subjects: class and capital at the border in Philippine–Canada migration

This paper considers how Philippine migrants prepare themselves and are prepared by others for emigration to Canada. In particular, it emphasizes how class-differentiated migrants are rendered socially homogenous as they are encouraged to be “grateful” transnational citizen subjects throughout their migration trajectories, commencing with initial decisions to migrate. Preparations for migration include individual decisions to increase marketability by acquiring particular kinds of skill sets matched to one of a variety of immigration streams. Despite such individual projects, the inequalities associated with gendered and racialized characteristics of Philippine migration trajectories and class dynamics are enduring for many migrants, though not all. Historical structural processes shaping the contours of global migration in the example of Philippine–Canada migration are compounded by the contradictory practices and outcomes associated with various preparations for migration. Current reforms to Canada’s immigration system to a “just-in-time” model promise to cause major disruptions to long-held migrant plans. Meanwhile, migrants are preconditioned to accept uncritically the multiple forms of subordination encouraged through the policies of multiple states and to accommodate themselves to new immigrant/citizen social identities which are devalued in a multiplicity of ways. This paper shows how, through the collusion of agents that migrants encounter in multiple sites, the disciplining of mobile citizens becomes more formalized and the contradictions between migrant ambition and neoliberal imperatives more visible.     

Social ties at work: Roma migrants and the community dynamics

This research examines how the internal social dynamics of Roma communities at home shape their propensity to migrate. It is theoretically grounded in the literature on social capital and focuses on two core concepts: ‘migration-rich’ and ‘migration-poor’ communities. The research is based on in-depth interviews and informal discussions with Roma from six (mainly rural) communities of Transylvania (Romania) and includes qualitative data gathered from migrants as well as from people who did not migrate. The findings challenge existing conceptualizations of Roma migration as either explained by poverty alone or by cultural arguments (such as nomadism). This paper indicates that even in the context of severe poverty, social networks are actually decisive for migration. It demonstrates that the patterns of migration tend to be community-specific and shaped by a locally shared culture (ethos) on migration. The research suggests policy choices according to the community profile and its internal dynamics.     

Dendritic cell migration and lymphocyte homing imprinting

For an effective adaptive immune response to occur, dendritic cells (DC), which are the most efficient antigen-presenting cells, must be able to sample the peripheral microenvironment and migrate towards secondary lymphoid organs (SLO) where they activate naive lymphocytes. Upon activation, lymphocytes proliferate and acquire the capacity to migrate to extralymphoid compartments. Although the molecular mechanisms controlling lymphocyte homing to lymphoid and to some extralymphoid tissues have been described in significant detail, it is much less clear how DC migration is controlled. Do DC obey similar adhesion cues that lymphocytes do, or do they have their own "zip codes"? This is relevant from a therapeutic standpoint because effective DC-based vaccines should be able to reach the appropriate tissues in order to generate protective immune responses. Here, we discuss some of the mechanisms used by DC to reach their target tissues. Once DC arrive at their destination, they are exposed to the tissue microenvironment, which likely modulates their functional properties in a tissue-specific fashion. This local DC "education" is probably responsible among other things; for the acquisition of tissue-specific homing imprinting capacity by which DC instruct lymphocytes to migrate to specific tissues. Finally, we discuss how dysregulation of these signals may play a key role in disease.     

Moving ethnography online: researching Brazilian migrants' online togetherness

Abstract

This article contributes to recent scholarship on the changing nature of fieldwork practices within migration research, focusing on the practice of online ethnography. It makes a case for the significance of the internet and, more specifically, social network sites, in the experience of many migrants. I state that online togetherness is an integral part of the lives of many migrants which also interrelates with ‘offline’ aspects of their social lives. Therefore, I argue that current research on migration would benefit from a more balanced combination of offline and online ethnography, taking into account how online connectivity affects the nature of migration and the conditions of being a migrant. Methodologically, I suggest that ethnography is well suited for generating understandings of the significance of the internet in the experience of migrants, but that a number of adjustments in methods of data collection and analysis must be made.     

Life History Consequences of Bioenergetic and Biomechanical Constraints on Migration

In this paper I test the hypothesis that bioenergetic and biomechanicalconstraints to migration play a pivotal role in shaping thelife history characteristics of migrants. Firstly, I examinebioenergetic constraints on the ability to migrate by activetransport and how they shape the life histories of insects andfish, and, secondly, the consequences of biomechanical constraintsto the migration by passive transport in insects and spiders. In both insects and fish the mass-specific energetic costs ofactive transport (flight and swimming, respectively) decreasewith body size, and hence selection should favor large sizein migrant species. Because their habitats are ephemeral, migrantinsects must grow rapidly. In fish, migrant species are ableto exploit resources unavailable to more sedentary species andhence should also show an enhanced rate of growth. These predictionsare supported by comparisons within populations, between populations,and among species in both groups. In contrast to the above, biomechanical factors limit the uppersize at which insects and spiders can migrate by passive transport.Theory predicts that ballooning will be most likely in spidersconsiderably less than 6 mm in length. Therefore, species thatmigrate as adults are predicted to be smaller than those thatdo not. This prediction is supported by a comparison of migratoryand non-migratory spider species from the United Kingdom. Theaverage length of species that migrate as adults, and of migratingyoung of spiders too large to balloon as adults, is about 2mm. Further, within this geographic species assemblage, thesize distribution of adult spiders is markedly peaked in the2 mm region, suggesting that biomechanical constraints on ballooningmay have a major influence on the evolution of body size inspiders.     

Predicting conditions for migration: effects of density dependence and habitat quality

Migration is widespread among animals, but the factors that influence the decision to migrate are poorly understood. Within a single species, populations may be completely migratory, completely sedentary or partially migratory. We use a population model to derive conditions for migration and demonstrate how migratory survival, habitat quality and density dependence on both the breeding and non-breeding grounds influence conditions for migration and the proportion of migrants within a population. Density dependence during the season in which migratory and sedentary individuals use separate sites is necessary for partial migration. High levels of density dependence at the non-shared sites widen the range of survival values within which we predict partial migration, whereas increasing the strength of density dependence at the shared sites narrows the range of survival values within which we predict partial migration. Our results have important implications for predicting how contemporary populations with variable migration strategies may respond to changes in the quality or quantity of habitat.     

Control of neural crest cell behavior and migration: Insights from live imaging

Neural crest cells (NCCs) are a remarkable, dynamic group of cells that travel long distances in the embryo to reach their target sites. They are responsible for the formation of craniofacial bones and cartilage, neurons and glia in the peripheral nervous system and pigment cells. Live imaging of NCCs as they traverse the embryo has been critical to increasing our knowledge of their biology. NCCs exhibit multiple behaviors and communicate with each other and their environment along each step of their journey. Imaging combined with molecular manipulations has led to insights into the mechanisms controlling these behaviors. In this Review, we highlight studies that have used live imaging to provide novel insight into NCC migration and discuss how continued use of such techniques can advance our understanding of NCC biology.Key words: live imaging, neural crest, EMT, Rho GTPase, ephrin, PCP signaling, cadherin, VEGFNeural crest cells (NCCs) are a pluripotent population of cells that migrate from the dorsal neuroepithelium and give rise to multiple cell types including neurons and glia of the peripheral nervous system, pigment cells and craniofacial bone and cartilage.¹ An important hallmark of NCCs is their remarkable ability to migrate over long distances and along specific pathways through the embryo. NCC migration begins with an epithelial to mesenchymal transition (EMT), in which NCCs lose adhesions with their neighbors and segregate from the neuroepithelium.^2,3 Following EMT, NCCs acquire a polarized morphology and initiate directed migration away from the neural tube. While migrating along their pathways to their target tissues, NCCs are guided by extensive communication with one another and by other cues from the extracellular environment. Each of these aspects of NCC migration requires precise regulation of cell motile behaviors, although the mechanisms controlling them are still not well understood. A critical step toward understanding the molecular control of NCC motility is characterization of NCC behaviors as they migrate in their native environment. In the past 15 years, multiple studies have analyzed specific behaviors associated with NCCs along the various stages of their journey and have begun to identify molecules controlling these behaviors. In this review we will focus specifically on these studies that employ live imaging and will highlight the strength of live imaging to reveal mechanisms regulating NCC motility and migration pathways.     

El Ni?o-Southern Oscillation Is Linked to Decreased Energetic Condition in Long-Distance Migrants

Predicting how migratory animals respond to changing climatic conditions requires knowledge of how climatic events affect each phase of the annual cycle and how those effects carry-over to subsequent phases. We utilized a 17-year migration dataset to examine how El Niño-Southern Oscillation climatic events in geographically different regions of the Western hemisphere carry-over to impact the stopover biology of several intercontinental migratory bird species. We found that migratory birds that over-wintered in South America experienced significantly drier environments during El Niño years, as reflected by reduced Normalized Difference Vegetation Index (NDVI) values, and arrived at stopover sites in reduced energetic condition during spring migration. During El Niño years migrants were also more likely to stopover immediately along the northern Gulf coast of the southeastern U.S. after crossing the Gulf of Mexico in small suboptimal forest patches where food resources are lower and migrant density often greater than larger more contiguous forests further inland. In contrast, NDVI values did not differ between El Niño and La Niña years in Caribbean-Central America, and we found no difference in energetic condition or use of coastal habitats for migrants en route from Caribbean-Central America wintering areas. Birds over-wintering in both regions had consistent median arrival dates along the northern Gulf coast, suggesting that there is a strong drive for birds to maintain their time program regardless of their overall condition. We provide strong evidence that not only is the stopover biology of migratory landbirds influenced by events during the previous phase of their life-cycle, but where migratory birds over-winter determines how vulnerable they are to global climatic cycles. Increased frequency and intensity of ENSO events over the coming decades, as predicted by climatic models, may disproportionately influence long-distance migrants over-wintering in South America.     

How do migratory songbirds cross the Sahara?

Many European songbirds winter in Africa south of the Sahara.Along their migratory routes they must fly over the huge desert belt of the Sahara twice a year. For decades ornithologists have assumed that most migrants cross this 'ecological barrier' in one long non-stop flight of thousands of kilometres. Results of recent research, however, suggest that many of the songbirds that migrate across the Sahara follow an intermittent migratory strategy with regular stopovers in the desert.     

Breeding latitude predicts timing but not rate of spring migration in a widespread migratory bird in South America

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Evidence for Repeated Independent Evolution of Migration in the Largest Family of Bats

Background

How migration evolved represents one of the most poignant questions in evolutionary biology. While studies on the evolution of migration in birds are well represented in the literature, migration in bats has received relatively little attention. Yet, more than 30 species of bats are known to migrate annually from breeding to non-breeding locations. Our study is the first to test hypotheses on the evolutionary history of migration in bats using a phylogenetic framework.

Methods and Principal Findings

In addition to providing a review of bat migration in relation to existing hypotheses on the evolution of migration in birds, we use a previously published supertree to formulate and test hypotheses on the evolutionary history of migration in bats. Our results suggest that migration in bats has evolved independently in several lineages potentially as the need arises to track resources (food, roosting site) but not through a series of steps from short- to long-distance migrants, as has been suggested for birds. Moreover, our analyses do not indicate that migration is an ancestral state but has relatively recently evolved in bats. Our results also show that migration is significantly less likely to evolve in cave roosting bats than in tree roosting species.

Conclusions and Significance

This is the first study to provide evidence that migration has evolved independently in bat lineages that are not closely related. If migration evolved as a need to track seasonal resources or seek adequate roosting sites, climate change may have a pivotal impact on bat migratory habits. Our study provides a strong framework for future research on the evolution of migration in chiropterans.     

Directions in modelling partial migration: how adaptation can cause a population decline and why the rules of territory acquisition matter

Modelling of partial migration in birds has progressed from simple graphical representations to sophisticated analyses that use evolutionary invasion analysis to determine how the success of the two strategies (stay year round on the breeding grounds, or migrate) can become frequency dependent. Here I build two models to relax two assumptions commonly made in models and often violated in nature: that individuals do not vary in any trait other than their migratory propensity, and that the prior residence effect (which grants priority access of good habitats to non‐migrants) operates at maximum strength. The same framework can incorporate and merge aspects of various hypotheses proposed to explain partial migration (dominance, body size, arrival timing, and limited foraging opportunities), and shows that either small (subdominant) or large (dominant) individuals may emerge as the more likely migrants; the latter case occurs when it is easy for socially dominant migrants to win back prime breeding locations upon their arrival. The dynamics of territory acquisition is shown to be an important and understudied topic, as variations in the relative importance of prior residency versus resource holding power can shift a population from complete migration to complete year‐round residency. These models also highlight exceptions to a tacit assumption in discussions of evolution of migration under climate change, which is that populations can decline if genetic adaptation or phenotypic plasticity do not occur fast enough. Competition can also yield the opposite pattern where adaptation itself leads to a population decline.     

Why fly the extra mile? Latitudinal trend in migratory fuel deposition rate as driver of trans‐equatorial long‐distance migration

Trans‐equatorial long‐distance migrations of high‐latitude breeding animals have been attributed to narrow ecological niche widths. We suggest an alternative hypothesis postulating that trans‐equatorial migrations result from a possible increase in the rate at which body stores to fuel migration are deposited with absolute latitude; that is, longer, migrations away from the breeding grounds surpassing the equator may actually enhance fueling rates on the nonbreeding grounds and therewith the chance of a successful, speedy and timely migration back to the breeding grounds. To this end, we first sought to confirm the existence of a latitudinal trend in fuel deposition rate in a global data set of free‐living migratory shorebirds and investigated the potential factors causing this trend. We next tested two predictions on how this trend is expected to impact the migratory itineraries on northward migration under the time‐minimization hypothesis, using 56 tracks of high‐latitude breeding shorebirds migrating along the East Asian‐Australasian Flyway. We found a strong positive effect of latitude on fuel deposition rate, which most likely relates to latitudinal variations in primary productivity and available daily foraging time. We next confirmed the resulting predictions that (1) when flying from a stopover site toward the equator, migrants use long jumps that will take them to an equivalent or higher latitude at the opposite hemisphere; and (2) that from here onward, migrants will use small steps, basically fueling only enough to make it to the next suitable staging site. These findings may explain why migrants migrate “the extra mile” across the equator during the nonbreeding season in search of better fueling conditions, ultimately providing secure and fast return migrations to the breeding grounds in the opposite hemisphere.     

Reverse migration as a cause of vagrancy

Capsule Reverse migration in autumn does not occur to the same degree in all species of migrants, but is related to migratory direction.

Aims To identify factors determining degree of reverse migration and specifically to test whether it occurs in long-distance migrant species irrespective of their standard (normal) migration direction. Methods Multiple regression analysis on the number of individuals occurring as reverse migrants observed in northwest European countries.

Methods Multiple regression analysis on the number of individuals occurring as reverse migrants observed in northwest European countries.

Results The number of reverse migrants observed in northwest European countries is strongly correlated with standard migratory direction, estimated population size and detectability, but an effect of the distance travelled from the breeding areas is not supported. The pattern holds true for subsamples of the data set, including British and Irish records or Scandinavian records only, and when controlling for phylogeny.

Conclusion Birds that migrate eastward in autumn from their breeding grounds, mostly in eastern Europe, are more likely to consistently reverse migrate than those species migrating southward mostly from southern Europe.     

Regulation of migration in Mythimna separata (Walker) in China: a review integrating environmental, physiological, hormonal, genetic, and molecular factors

Each year the Mythimna separate (Walker), undertakes a seasonal, long-distance, multigeneration roundtrip migration between southern and northern China. Despite its regularity, the decision to migrate is facultative, and is controlled by environmental, physiological, hormonal, genetic, and molecular factors. Migrants take off on days 1 or 2 after eclosion, although the preoviposition period lasts ≈7 d. The trade-offs among the competing physiological demands of migration and reproduction are coordinated in M. separata by the "oogenesis-flight syndrome." Larvae that experience temperatures above or below certain thresholds accompanied by appropriate humidity, short photoperiod, poor nutrition, and moderate density tend to develop into migrants. However, there is a short window of sensitivity within 24 h after adult eclosion when migrants can be induced to switch to reproductive residents if they encounter extreme environmental factors including starvation, low temperature and long photoperiod. Juvenile hormone (JH) titer is low before migration but high titers are associated with termination of migratory behavior and the switch to reproduction. Early release of JH by the corpora allata in environmentally stressed 1-d old adults, otherwise destined by larval conditions to be migrants, switches them to residents. Offspring inherit parental additive genetic effects governing migratory behavior. However, they also retain flexibility in expression of both flight and reproductive life history traits. The insect neuropeptide, allatotropin, which activates corpora allata to synthesize JH, controls adult flight and reproduction. Future research directions to better understand regulation of migration in this species are discussed.     

Genetic control of migratory behaviour in birds

Although it has long been suspected that biannual migration in birds has a direct genetic basis, only in the last decade have details of the inheritance of behavioural traits such as migratory activity and directional preferences been demonstrated. A model has now been developed to estimate how inexperienced first-time migrants manage to reach their unknown winter quarters on the basis of inherited spatio-temporal programs. Furthermore, in obligate partial migrants the decision to migrate or not has been shown to have a strong genetic base. Migratoriness and sedentariness in partial migrants have been shown to have a high potential for rapid evolution. A recent set of results has suggested that novel migratory habits can evolve in less than 25 years. A possible consequence is that environmental changes, including 'greenhouse' effects, might considerably alter avian migration systems by acting on genetic variation for migratory tendencies.     

Migration in butterflies: a global overview

Insect populations including butterflies are declining worldwide, and they are becoming an urgent conservation priority in many regions. Understanding which butterfly species migrate is critical to planning for their conservation, because management actions for migrants need to be coordinated across time and space. Yet, while migration appears to be widespread among butterflies, its prevalence, as well as its taxonomic and geographic distribution are poorly understood. The study of insect migration is hampered by their small size and the difficulty of tracking individuals over long distances. Here we review the literature on migration in butterflies, one of the best-known insect groups. We find that nearly 600 butterfly species show evidence of migratory movements. Indeed, the rate of ‘discovery’ of migratory movements in butterflies suggests that many more species might in fact be migratory. Butterfly migration occurs across all families, in tropical as well as temperate taxa; Nymphalidae has more migratory species than any other family (275 species), and Pieridae has the highest proportion of migrants (13%; 133 species). Some 13 lines of evidence have been used to ascribe migration status in the literature, but only a single line of evidence is available for 92% of the migratory species identified, with four or more lines of evidence available for only 10 species – all from the Pieridae and Nymphalidae. Migratory butterflies occur worldwide, although the geographic distribution of migration in butterflies is poorly resolved, with most data so far coming from Europe, USA, and Australia. Migration is much more widespread in butterflies than previously realised – extending far beyond the well-known examples of the monarch Danaus plexippus and the painted lady Vanessa cardui – and actions to conserve butterflies and insects in general must account for the spatial dependencies introduced by migratory movements.     

Ecology and behavior of Gecarcoidea natalis, the Christmas Island red crab, during the annual breeding migration.

The terrestrial crab Gecarcoidea natalis is endemic to the forests of Christmas Island but must migrate each year to the coast to breed. During 1993 and 1995, radio-tracking, mark and recapture, and counting methods were used to establish the routes, walking speeds, direction of travel, and destinations of migrating crabs, as well as crab numbers and distribution. The density of crabs ranged from 0.09 to 0.57 crabs per square meter, which gave a population estimate of 43.7 million adult crabs on the island. During the dry season the crabs were relatively inactive but on arrival of the wet season immediately began their migration. The crabs generally walked in straight lines, and most crabs from around the Island traveled toward the northwest shore instead of simply walking toward the nearest shore. The maximum recorded distance walked by a red crab in one day was 1460 m, but the mean was 680 m per day in 1993 and 330 m in 1995. Comparing the 1993 and 1995 study seasons, there was a 3-week difference in the timing of the start of the migration, but the spawning date was fixed by the lunar phase and took place 17 to 18 days after mating. In 1993, late rain prompted a "rushed" migration and crabs walked directly to their shore destinations; in contrast, in 1995 most crabs made stops of 1 to 7 days during the downward migration. By giving the crabs a chance to feed along the way and minimizing the time that the population was concentrated near the shore, these stops may be important in ensuring that the animals have enough food after the long dry season. Furthermore, this behavior implies that the crabs are able to judge how far away they are from the shore during the migration.