Diverse migration strategy between freshwater and seawater habitats in the freshwater eel genus Anguilla

The freshwater eels of the genus Anguilla, which are catadromous, migrate between freshwater growth habitats and offshore spawning areas. A number of recent studies, however, found examples of the temperate species Anguilla anguilla, Anguilla rostrata, Anguilla japonica, Anguilla australis and Anguilla dieffenbachii that have never migrated into fresh water, spending their entire life history in the ocean. Furthermore, those studies found an intermediate type between marine and freshwater residents, which appear to frequently move between different environments during their growth phase. The discovery of marine and brackish-water residents Anguilla spp. suggests that they do not all have to be catadromous, and it calls into question the generalized classification of diadromous fishes. There has been little available information, however, concerning migration in tropical Anguilla spp. Anguilla marmorata, shows three fluctuation patterns: (1) continuous residence in fresh water, (2) continuous residence in brackish water and (3) residence in fresh water after recruitment, while returning to brackish water. Such migratory patterns were found in other tropical species, Anguilla bicolor bicolor and Anguilla bicolor pacifica. In A. b. bicolor collected in a coastal lagoon of Indonesia, two further patterns of habitat use were found: (1) constantly living in either brackish water or sea water with no freshwater life and (2) habitat shift from fresh water to brackish water or sea water. The wide range of environmental habitat use indicates that migratory behaviour of tropical Anguilla spp. is facultative among fresh, brackish and marine waters during their growth phases after recruitment to the coastal areas. Further, the migratory behaviours of tropical Anguilla spp. appear to differ in each habitat in response to inter and intra-specific competition. The results suggest that tropical Anguilla spp. have a flexible pattern of migration, with an ability to adapt to various habitats and salinities. The ability of anguillids to reside in environments of various salinities would be a common feature between tropical and temperate species without a latitudinal cline. Thus, the migration of Anguilla spp. into fresh water is clearly not an obligatory behaviour. This evidence of geographical variability among Anguilla spp. suggests that habitat use is determined by environmental conditions in each site.     

Social transmission of migratory knowledge: quantifying the risk of losing migratory behavior

When migration is a learned behavior, small populations have a significant problem of maintaining migratory knowledge across generations. These populations risk losing migratory behavior entirely, which may exacerbate existing stressors on population size. Here we investigated the success of various behavioral, demographic, and social factors towards maintaining migration within small populations. Using a discrete-time probabilistic model to simulate repeated migrations, we found that migratory group size plays an important role in maintaining migratory knowledge within the population. Rare, large groups allow for migratory knowledge to be spread to many individuals at once. When a population learns migration information incrementally, the presence of individuals that can learn quickly, therefore transitioning rapidly into leaders, has a profound impact on migrational persistence. Furthermore, small populations are better able to maintain migratory behavior when groups rely on informed leaders as compared to using collective group knowledge, even when that collective knowledge is heavily weighted towards knowledgeable individuals. Finally, we found that both species with short lifespans and species that migrate with fixed group compositions are at especially high risk of losing their migration behavior at small population sizes.     

Investigating connectivity and seasonal differences in wind assistance in the migration of Common Sandpipers

Many migratory bird species have undergone recent population declines, but there is considerable variation in trends between species and between populations employing different migratory routes. Understanding species-specific migratory behaviours is therefore of critical importance for their conservation. The Common Sandpiper Actitis hypoleucos is an Afro-Palaearctic migratory bird species whose European populations are in decline. We fitted geolocators to individuals breeding in England or wintering in Senegal to determine their migration routes and breeding or non-breeding locations. We used these geolocator data in combination with previously published data from Scottish breeding birds to determine the distributions and migratory connectivity of breeding (English and Scottish) and wintering (Senegalese) populations of the Common Sandpiper, and used simulated random migrations to investigate wind assistance during autumn and spring migration. We revealed that the Common Sandpipers tagged in England spent the winter in West Africa, and that at least some birds wintering in Senegal bred in Scandinavia; this provides insights into the links between European breeding populations and their wintering grounds. Furthermore, birds tagged in England, Scotland and Senegal overlapped considerably in their migration routes and wintering locations, meaning that local breeding populations could be buffered against habitat change, but susceptible to large-scale environmental changes. These findings also suggest that contrasting population trends in England and Scotland are unlikely to be the result of population-specific migration routes and wintering regions. Finally, we found that birds used wind to facilitate their migration in autumn, but less so in spring, when the wind costs associated with their migrations were higher than expected at random. This was despite the wind costs of simulated migrations being significantly lower in spring than in autumn. Indeed, theory suggests that individuals are under greater time pressures in spring than in autumn because of the time constraints associated with reproduction.     

Recruitment of Anguilla spp. glass eels in the Waikato River,New Zealand. Evidence of declining migrations?

The timing of Anguilla spp. glass eel recruitment into the Waikato River, North Island, New Zealand, was studied over a 2 year period (2004–2005). While glass eels of both the shortfin eel Anguilla australis and the endemic longfin eel Anguilla dieffenbachii were caught, the former comprised >97% of the species composition. There was a positive correlation of glass eel migrations with spring tides, with peak migration periods typically occurring within a few hours of the peak of high tide, and between 2 and 4 days after the day of spring tide. Both water temperature and discharge had significant inverse relationships with glass eel catches, with temperature explaining >30% of the variance in catch periodicity. Comparison of catch data 30 years apart showed that main migration periods appear to occur several weeks earlier today than previously. Reduced catch per unit effort and duration of runs from recent years' sampling (compared with the 1970s) indicate that a reduction in recruitment may also have occurred during this period, something recorded in other temperate species of Anguilla .     

Flying with diverse passengers: greater richness of parasitic nematodes in migratory birds

Environmental changes are simultaneously affecting parasitic diseases and animal migrations, making it important to understand the disease dynamics of migratory species, including their range of infections and investment into defences. There is an urgent need for such knowledge because migratory animals, especially birds, are important for pathogen transmission and also particularly sensitive to environmental changes. Here we compare the nematode species richness and relative immune investment (via relative spleen size) of almost 200 migratory and non‐migratory species within three diverse groups of birds (Anseriformes, Accipitriformes and Turdidae) with worldwide distributions and varied ecology. Our results provide the first large‐scale demonstration that migratory birds face greater challenge from macroparasites as they have significantly dissimilar nematode fauna and higher nematode species richness compared to non‐migratory species. Even though birds with relatively large spleens had more nematode species, there was no difference in relative spleen size between migratory and non‐migratory bird species. The physiological stress of migration can be exacerbated by the potential range of pathologies induced by their richer nematode communities, particularly in combination with environmental perturbations. Altered migration stemming from global changes can also have important consequences for nematode transmission. Synthesis Most studies on parasites of migratory birds versus non‐migratory birds focus upon blood parasites; here we compared the diversity of another important parasite group – nematodes (roundworms) in three orders of birds. We found for any given order, migratory species and species with proportionally larger spleens generally have a wider range of nematodes. It is unclear why migratory species harbour more nematode species. Global climate change is expected to influence both bird migration patterns and infectious diseases, which may increase host susceptibility to parasitism and also introduce diverse nematodes to new areas and potential hosts.     

The need for integrative approaches to understand and conserve migratory ungulates

Over the last two centuries overhunting, anthropogenic barriers and habitat loss have disrupted many ungulate migrations. We review the literature on ungulate migration disruptions and find that for many species the disruption of migratory routes causes a rapid population collapse. Previous research has focused on the proximal ecological factors that might favour migration, particularly spatiotemporal variation in resources and predation. However, this does not provide an adequate basis for understanding and mitigating anthropogenic effects on migratory populations. Migration is a complex behaviour and we advocate an integrative approach that incorporates population dynamics, evolution, genetics, behaviour and physiology, and that borrows insights and approaches from research on other taxa. We draw upon research on avian migration to illustrate research approaches that might also be fruitful in ungulates. In particular, we suggest that the migratory cycle should be evaluated in the context of seasonal population limitation, an approach we highlight with a preliminary demographic perturbation analysis of the Serengeti wildebeest ( Connochaetes taurinus ) population. We provide suggestions for avenues of future research and highlight areas where we believe rapid progress can be made by applying recent advances in theory, technology and analytical approaches.     

Lamellipodia-based migrations of larval epithelial cells are required for normal closure of the adult epidermis of Drosophila

Cell migrations are an important feature of animal development. They are, furthermore, essential to wound healing and tumour progression. Despite recent progress, it is still mysterious how cell migration is spatially and temporally regulated during morphogenesis and how cell migration is coordinated with other cellular behaviours to shape tissues and organs. The formation of the abdominal epithelium of Drosophila during metamorphosis provides an attractive system to study morphogenesis. Here, the diploid adult histoblasts replace the polyploid larval epithelial cells (LECs). Using in vivo 4D microscopy, I show that, besides apical constriction and apoptosis, the LECs undergo extensive coordinated migrations. The migrations follow a transition from a stationary (epithelial) to a migratory mode. The migratory behaviour is stimulated by autocrine Dpp signalling. Directed apical lamellipodia-like protrusions propel the cells. Initially, planar cell polarity determines the orientation of LEC migration. While LECs are migrating they also constrict apically, and changes in activity of the small GTPase Rho1 can favour one behaviour over the other. This study shows that the LECs play a more active role in morphogenesis than previously thought, with their migrations contributing to abdominal closure. It furthermore provides insights into how the migratory behaviour of cells is regulated during morphogenesis.     

Leadership, social learning, and the maintenance (or collapse) of migratory populations

Long-distance animal migrations are complex, population-level phenomena that emerge in seasonal landscapes as a result of the interplay between environmental influences (e.g., resources, predators) and social interactions among conspecifics. When landscapes change with respect to phenology or connectivity, the dynamics of migratory species can abruptly shift, in many cases leading to a cessation of migration and dramatic decreases in population size. We develop a difference equation modeling framework to explore how the social transfer of knowledge from informed “leader” individuals enhances the performance of seasonally migratory versus resident populations. The model permits a wide range of population-level behaviors including alternative stable states, partial migration equilibria, and complex dynamics, but we focus our efforts on investigations of migration collapse mediated by a lack of informed leaders that can arise from changes in landscape structure, survivorship, reproduction, and/or social learning. Migration collapse is a hysteretic phenomenon in this model and results either in extinction of the population or purely resident behavior. The hysteretic nature of migration failure, which hinges on cultural transmission of knowledge, highlights a potentially critical role for behavior and social learning in aspects of spatial ecology and conservation biology.     

Potential interactions between diadromous fishes of U.K. conservation importance and the electromagnetic fields and subsea noise from marine renewable energy developments

The considerable extent of construction and operation of marine renewable energy developments (MRED) within U.K. and adjacent waters will lead, among other things, to the emission of electromagnetic fields (EMF) and subsea sounds into the marine environment. Migratory fishes that respond to natural environmental cues, such as the Earth's geomagnetic field or underwater sounds, move through the same waters that the MRED occupy, thereby raising the question of whether there are any effects of MRED on migratory fishes. Diadromous species, such as the Salmonidae and Anguillidae, which undertake large-scale migrations through coastal and offshore waters, are already significantly affected by other human activities leading to national and international conservation efforts to manage any existing threats and to minimize future concerns, including the potential effect of MRED. Here, the current state of knowledge with regard to the potential for diadromous fishes of U.K. conservation importance to be affected by MRED is reviewed. The information on which to base the review was found to be limited with respect to all aspects of these fishes' migratory behaviour and activity, especially with regards to MRED deployment, making it difficult to establish cause and effect relationships. The main findings, however, were that diadromous species can use the Earth's magnetic field for orientation and direction finding during migrations. Juveniles of anadromous brown trout (sea trout) Salmo trutta and close relatives of S. trutta respond to both the Earth's magnetic field and artificial magnetic fields. Current knowledge suggests that EMFs from subsea cables may interact with migrating Anguilla sp. (and possibly other diadromous fishes) if their movement routes take them over the cables, particularly in shallow water (<20 m). The only known effect is a temporary change in swimming direction. Whether this will represent a biologically significant effect, for example delayed migration, cannot yet be determined. Diadromous fishes are likely to encounter EMFs from subsea cables either during the adult movement phases of life or their early life stages during migration within shallow, coastal waters adjacent to natal rivers. The underwater sound from MRED devices has not been fully characterized to determine its acoustic properties and propagation through the coastal waters. MRED that require pile driving during construction appear to be the most relevant to consider. In the absence of a clear understanding of their response to underwater sound, the specific effects on migratory species of conservation concern remain very difficult to determine in relation to MRED. Based on the studies reviewed, it is suggested that fishes that receive high intensity sound in close proximity to construction may be physiologically affected to some degree, whereas those at farther distances, potentially up to several km, may exhibit behaviour responses; the effect of which is unknown and will be dependent on the properties of the received sound and receptor characteristics and condition. Whether there are behavioural effects on the fishes during operation is unknown but any change to the environment and subsequent response by the fishes would need to be considered over the lifetime of the MRED. It is not yet possible to determine if effects relating to sound exposure are biologically significant. The current assumptions of limited effects are built on an incomplete understanding of how the species move around their environment and interact with natural and anthropogenic EMFs and subsea sound. A number of important knowledge gaps exist, principally whether migratory fish species on the whole respond to the EMF and the sound associated with MRED. Future research should address the principal gaps before assuming that any effect on diadromous species results in a biological effect.     

‘Chancing on a spectacle:’ co-occurring animal migrations and interspecific interactions

Migrations of diverse wildlife species often converge in space and time, with their journeys shaped by similar forces (i.e. geographic barriers and seasonal resources and conditions); we term this ‘co-migration’. Recent studies have illuminated multi-speciesmigrations by land and sea including the simultaneous movements of numerous insects, birds, bats and of fish invertebrates marine predators. Beyond their significance as natural wonders, species with overlapping migrations may interact ecologically, with potential effects on population and community dynamics. Direct and indirect ecological interactions (including predation and competition) between migrant species remain poorly understood, in part because migration is the least-studied phase of animals’ annual cycles. To address this gap, we conducted a literature review to examine whether animal migration studies incorporate multiple species and to what extent they investigate interspecific interactions between co-migrants. Following a key word search, we read all migration research papers in 23 relevant peer-reviewed journals during 2008–2017. Thirty percent of animal migration papers reported two or more species with coinciding migrations, suggesting that co-migrations are common, although few studies investigated or discussed these mixed-species migrations further. Synthesizing these, we present examples and describe five types of ecological interactions between migrating species, including predator–prey, host–parasite and commensal relationships. Considering migratory animals as interacting with migrant communities will enhance understanding of the drivers of migration and could improve predictions about wildlife responses to global change. Further research focused on multi-species migrations could also inform conservation efforts for migratory animal populations, many of which are declining or shifting, with unexplored consequences for other co-migratory species.     

Exploring intraspecific variation in migratory destinations to investigate the drivers of migration

Various benefits (e.g. tracking of resources and of climate niche) and costs (e.g. distance travelled) are hypothesized to drive seasonal animal migrations. Until now, these potential factors have been investigated together at the species level, but migratory movements are made at the individual level, leading to intraspecific variability. Here, we use ringing/recovery data from 1308 individuals belonging to thirteen North American bird species to analyse patterns in intraspecific variability of migratory destinations in order to investigate which factors underpin bird migration and how individuals trade-off among multiple factors. Our results suggest that migratory destinations have been shaped by access to resources (most important during the breeding season) and climatic niche tracking (during winter, mostly). However, the benefits of resource surpluses and climate niche tracking appear to be traded off against the cost of migratory distance, which seems to strongly constrain where individuals migrate.     

When Does this Fish Spawn? Fishermen’s Local Knowledge of Migration and Reproduction of Brazilian Coastal Fishes

Fishermen’s local knowledge of fishing resources may be an important source of information to improve artisanal tropical fisheries management, such as those found in Brazil, where most data on fish biology is lacking. We aim to study the local ecological knowledge that Brazilian coastal fishers have about reproductive aspects (season, places and migration) of 13 coastal fish species of commercial importance. We selected fishermen with more than 30 years of fishing practice and we interviewed a total of 67 fishermen: 29 from the southeastern coast, from the communities of Puruba, Almada, Picinguaba and Bertioga, and 38 from the northeastern coast, from the communities of Valença, Arembepe and Porto Sauípe. In the interviews, we used standardized questionnaires and showed photos of fish species. Our results indicate some general patterns in fishes’ reproduction according to fishermen knowledge: fish species spawn in open ocean, near reefs or in coastal rivers (estuaries); some fishes reproduce during the summer and others in winter, while some have more defined spawning months. The main fish migratory patterns mentioned by interviewees were: long migrations along the coast, usually in the South to North direction, short migrations among reefs, fishes that do not migrate, migrations between the shore and open ocean and migrations between the sea and coastal rivers. Fishermen’s knowledge differed among fish species: most fishermen did not know spawning places or seasons of large pelagic fishes, which raised concerns of their possible depletion. We compared such ethnoichthyological information with available scientific data, indicating promising insights about reproduction and migration of Brazilian coastal fishes. Data gathered from local fishermen may provide inexpensive and prompt information, potentially applicable to fisheries management. Our approach might be useful to several other small-scale fisheries, especially the tropical ones, where there is a high diversity of target species and a low biological and ecological knowledge about these species.     

The economics of bird migration

Piersma, T. 2000. The economics of bird migration. Ostrich 71(1): 362.

Through their migrations birds spin a worldwide web of migratory routes. Even at the level of individuals and populations, the migrations can cover a large part of the globe. Many factors are involved in the evolution and maintenance of all these migration systems: the contingencies of earth history, the sensory and metabolic evolutionary potential of flying organisms, the distribution of predators and parasites, and energetic considerations. In this presentation I aim to briefly outline the use of energetics in studies on the maintenance of today's flyways. My overview will focus on the shorebird species seasonally migrating between wintering grounds in coastal African and the breeding grounds in the Canadian, Greenlandic, European and Siberian Arctic.

Every individual animal has to balance its energy budget but depending on size, it can balance the discrepancies between energy expenditure and energy intake through the use or storage of nutrients at various time scales. Energy thus provides a common currency to relate ecological phenomena that are separated in space and time. I will describe methods to estimate energy expenditure, energy intake and the use of energy stores, and how these estimates can be translated as an annual energy budget of a coastal shorebird species. This will then be used to evaluate the repercussions of wintering at tropical latitudes and at temperate latitudes, and how such knowledge might help us to understand the evolution of a migration system. Population-specific energy budgets provide only one of the possible uses of energetics approach. Studies at the individual level, at which it is rarely possible to quantify such budgets completely, nevertheless benefit from energetic considerations when the functional significance of variations in migratory timing and seasonal body mass and plumage changes are the focus of attention.     

Drivers of animal migration and implications in changing environments

Migratory species are widespread in terrestrial, aquatic and aerial environments, and are important both ecologically and economically. Since migration is an adaptive response to particular conditions, environmental changes (climate or otherwise) will potentially alter the selective pressures on movement behavior. Such changes may also interfere with, or disrupt, a species’ ability to migrate. In either case, environmental changes could lead to the reduction or total loss of a migration, yet we have little understanding of when to expect these outcomes to occur. Here, I argue that an understanding of both the proximate and ultimate drivers of migration is needed if we are to predict the fate of migrations under changing environmental conditions. I review what is currently known about the drivers of animal migration, but show that we also need a more complete synthesis of migratory patterns across diverse ecosystems and taxonomic groups. The current understanding of migration indicates that (1) drivers of migration vary across species and ecosystems, and (2) a species’ ability to adapt to environmental change successfully depends in part on its migration drivers. Together, these findings suggest a way forward for studying and generating predictions of how changing environmental conditions will differentially impact species by taxonomic group and geographic region of the world.     

Changes in the strategy of migration of gulls (Laridae) along the western coast of the Caspian Sea as a result of environmental changes in space and time

This paper generalizes the data obtained in 1995–2014 in the regions of the Sulak and Turali lagoons of Dagestan (the western coast of the Middle Caspian). The lagoons are located in a “bottleneck” that is a narrow migration corridor traversed by one of the largest migration routes of trans-palearctic species in Russia. This route is a part of the West Siberian–East African migration range. The migration traffic and territorial localization of the Laridae populations participating in the total migratory flow along the western coast of the Caspian Sea have been determined. The present-day migratory range of Laridae that covers the space from West Europe to the Baikal Lake and West India is specified. It is determined that Dagestan is crossed not by one but two independent and stable migration flows of Laridae, which fly across the transit region in different migration routes, but at the same periods of time. Three types of migration intensity of Laridae across the study area are defined: weak, average, and mass migration. Both spring and autumn migrations include five peaks of migration activity (migratory waves). Migratory timing, taxonomical composition, and abundance of different Laridae species vary for each degree type of migratory waves. The key determinants of migratory wave intensities are the abundance of migratory populations and weather conditions of a year. Over the last 5–7 years there has been a steady decrease in the abundance of some Laridae species on the western coast of the Middle Caspian that takes place under the impact of a set of regulating factors, which act across the whole migration range. The decrease in the abundance of migratory Laridae leads to a “blurring” of clear boundaries between migratory waves and migration intensity, changing migration routes of some Laridae populations, which now have shifted from traditional wintering routes along the western coast of the Caspian Sea and countries of the Middle East and northeastern Africa to India.     

Mapping Global Diversity Patterns for Migratory Birds

Nearly one in five bird species has separate breeding and overwintering distributions, and the regular migrations of these species cause a substantial seasonal redistribution of avian diversity across the world. However, despite its ecological importance, bird migration has been largely ignored in studies of global avian biodiversity, with few studies having addressed it from a macroecological perspective. Here, we analyse a dataset on the global distribution of the world’s birds in order to examine global spatial patterns in the diversity of migratory species, including: the seasonal variation in overall species diversity due to migration; the contribution of migratory birds to local bird diversity; and the distribution of narrow-range and threatened migratory birds. Our analyses reveal a striking asymmetry between the Northern and Southern hemispheres, evident in all of the patterns investigated. The highest migratory bird diversity was found in the Northern Hemisphere, with high inter-continental turnover in species composition between breeding and non-breeding seasons, and extensive regions (at high latitudes) where migratory birds constitute the majority of the local avifauna. Threatened migratory birds are concentrated mainly in Central and Southern Asia, whereas narrow-range migratory species are mainly found in Central America, the Himalayas and Patagonia. Overall, global patterns in the diversity of migratory birds indicate that bird migration is mainly a Northern Hemisphere phenomenon. The asymmetry between the Northern and Southern hemispheres could not have easily been predicted from the combined results of regional scale studies, highlighting the importance of a global perspective.     

Linking the continental migratory cycle of the monarch butterfly to understand its population decline

Threats to several of the world's great animal migrations necessitate a research agenda focused on identifying drivers of their population dynamics. The monarch butterfly is an iconic species whose continental migratory population in eastern North America has been declining precipitously. Recent analyses have linked the monarch decline to reduced abundance of milkweed host plants in the USA caused by increased use of genetically modified herbicide‐resistant crops. To identify the most sensitive stages in the monarch's annual multi‐generational migration, and to test the milkweed limitation hypothesis, we analyzed 22 years of citizen science records from four monitoring programs across North America. We analyzed the relationships between butterfly population indices at successive stages of the annual migratory cycle to assess demographic connections and to address the roles of migrant population size versus temporal trends that reflect changes in habitat or resource quality. We find a sharp annual population decline in the first breeding generation in the southern USA, driven by the progressively smaller numbers of spring migrants from the overwintering grounds in Mexico. Monarch populations then build regionally during the summer generations. Contrary to the milkweed limitation hypothesis, we did not find statistically significant temporal trends in stage‐to‐stage population relationships in the mid‐western or northeastern USA. In contrast, there are statistically significant negative temporal trends at the overwintering grounds in Mexico, suggesting that monarch success during the fall migration and re‐establishment strongly contributes to the butterfly decline. Lack of milkweed, the only host plant for monarch butterfly caterpillars, is unlikely to be driving the monarch's population decline. Conservation efforts therefore require additional focus on the later phases in the monarch's annual migratory cycle. We hypothesize that lack of nectar sources, habitat fragmentation, continued degradation at the overwintering sites, or other threats to successful fall migration are critical limiting factors for declining monarchs.     

Accipiter hawks of the Laurentian Upland and the Interior Plains undertake the longest migrations: insights from birds banded or recovered in Veracruz,Mexico

Bird banding has allowed us to understand diverse aspects of the life histories of migratory raptors. However, most banding stations are located at northern latitudes so what we know about the movements of these raptors is biased toward higher latitudes, primarily from Canada and the United States, leaving important gaps in our knowledge of their movements at lower latitudes. Our objective was to describe the migratory movements of Sharp-shinned (Accipiter striatus) and Cooper’s (A. cooperii) hawks based on banding and recapture records of birds that migrate through Veracruz, Mexico. More specifically, we sought to determine their breeding, migration, and non-breeding locations, estimate their migration distances, and contribute to a better understanding of their migration patterns. With a total of 80 records, we calculated migration distances and used Kernel Density Estimation analyses to identify where these hawks were recaptured or recovered by season. The distribution of recaptures and recoveries largely coincided with breeding locations in the Laurentian Upland and the Interior Plains physiographic regions. All migration records follow a trajectory that extends from the midwestern United States to the Gulf coastal plain of Mexico. The mean breeding season migration distance to Veracruz was 3374 km (a difference of 27 degrees of latitude) for Sharp-shinned Hawks and 2926 km (a difference of 25 degrees of latitude) for Cooper’s Hawks. Our non-breeding records indicate that populations of Accipiter hawks from these North American populations migrate the longest distances to reach Central America, the southernmost distribution of their migratory populations. Distances covered by both species represent round-trip migrations that may be as long as 10,000 km. Our results support those of previous studies and illustrate how continental physiography influences the migration routes, migratory behavior, and migratory connectivity of these hawks.     

Initiating and guiding migration: lessons from border cells

Cell migration occurs in many different contexts. Amoebae and other isolated cells migrate in culture. In animals, 'professional' migratory cells of the immune system constantly survey the body for intruders, whereas other cell types perform specific developmentally regulated migrations. One simple model for the latter type of event is migration of border cells during Drosophila oogenesis. Recent findings have shed light on how border cell fate is induced and on how the migration is guided. This article discusses the implications of these studies and compares (invasive) migration through a tissue with what is known about cells crawling on a flat substratum.