Background: Every year, billions of migratory animals cross the planet in pursuit of increased foraging opportunities, improved safety, and higher reproductive output. In so doing, these migrants transport nutrients, energy, and other organisms (including seeds, mollusks, parasites, and pathogens) between disparate locations. Migrants also forage and are preyed upon throughout their journeys, thereby establishing transport and trophic interactions with resident communities. Migratory animals thus couple ecological communities across the globe and may mediate their diversity and stability. However, as yet, the influence of migrants and their services on these communities is often overlooked, and as a consequence of global changes, migrations are threatened worldwide. Advances: We review several examples in which interactions between migratory animals and resident communities have been quantified, illustrating the processes by which migrants may uniquely alter energy flow, food-web topology and stability, trophic cascades, and the structure and dynamics of (meta-)communities. For example, the inputs of nutrients and energy originating from distant localities by migrants can dramatically increase resource availability, with rippling consequences for productivity at various trophic levels and the potential to drive the transition between alternative stable states. Migrant-mediated transport of propagules of other organisms can lead to the establishment of new or lost species, as well as influencing gene flow and genetic mixing among resident populations. Similarly, migrants can alter parasite transmission, diversity, and evolution by harboring a broader range of parasites than residents and by either facilitating or hindering the long-distance dispersal of parasites. Foraging by migrants can also have profound effects on community processes and ecosystem functions. For instance, grazing by migratory animals can alter nutrient cycling, primary productivity, biomass of edible plants, competitive interactions between plant species, and ultimately, the composition and long-term persistence of the entire plant community. The most striking difference between migrant and resident consumers is, however, the pulsed nature of migrant utilization and the timing of their interactions. Together, these fundamentally define the relationship between migrant abundance and primary production (in the case of migrant herbivores) or the stability of food webs (in the case of migratory predators). Outlook: Our Review demonstrates that the highly predictable, seasonally pulsed nature of animal migration, together with the spatial scales at which it operates and the immense number of individuals involved, not only set migration apart from other types of movement, but render it a uniquely potent, yet underappreciated, dimension of biodiversity that is intimately embedded within resident communities. Given the potential for migration to influence ecological networks worldwide, we suggest integrative network approaches, through which studies of community dynamics and ecosystem functioning may explicitly consider animal migrations, understand the ramifications of their declines, and assist in developing effective conservation measures.

Summary: Similar to other infectious diseases, the prevalence of low pathogenic avian influenza viruses (LPAIV) has been seen to exhibit marked seasonal variation. However, mechanisms driving this variation in wild birds have yet to be tested. We investigated the validity of three previously suggested drivers for the seasonal dynamics inLPAIVinfections in wild birds: (i) host density, (ii) immunologically naïve young and (iii) increased susceptibility in migrants. To address these questions, we sampled a keyLPAIVhost species, the mallardAnas platyrhynchos, on a small spatial scale, comprehensively throughout a complete annual cycle, measuring both current and past infection (i.e. viral and seroprevalence, respectively). We demonstrate a minor peak inLPAIVprevalence in summer, a dominant peak in autumn, during which half of the sampled population was infected, and no infections in spring. Seroprevalence of antibodies to a conserved gene segment of avian influenza virus (AIV) peaked in winter and again in spring. The summer peak ofLPAIVprevalence coincided with the entrance of unfledged naïve young in the population. Moreover, juveniles were more likely to be infected, shed higher quantities of virus and were less likely to have detectable antibodies toAIVthan adult birds. The arrival of migratory birds, as identified by stable hydrogen isotope analysis, appeared to drive the autumn peak inLPAIVinfection, with both temporal coincidence and higher infection prevalence in migrants. Remarkably, seroprevalence in migrants was substantially lower than viral prevalence throughout autumn migration, further indicating that each wave of migrants amplified localAIVcirculation. Finally, while host abundance increased throughout autumn, it peaked in winter, showing no direct correspondence with either of theLPAIVinfection peaks. At an epidemiologically relevant spatial scale, we provide strong evidence for the role of migratory birds as key drivers for seasonal epizootics ofLPAIV, regardless of their role as vectors of these viruses. This study exemplifies the importance of understanding host demography and migratory behaviour when examining seasonal drivers of infection in wildlife populations.

Recent demand for increased understanding of avian influenza virus in its natural hosts, together with the development of high-throughput diagnostics, has heralded a new era in wildlife disease surveillance. However, survey design, sampling, and interpretation in the context of host populations still present major challenges. We critically reviewed current surveillance to distill a series of considerations pertinent to avian influenza virus surveillance in wild birds, including consideration of what, when, where, and how many to sample in the context of survey objectives. Recognizing that wildlife disease surveillance is logistically and financially constrained, we discuss pragmatic alternatives for achieving probability-based sampling schemes that capture this host–pathogen system. We recommend hypothesis-driven surveillance through standardized, local surveys that are, in turn, strategically compiled over broad geographic areas. Rethinking the use of existing surveillance infrastructure can thereby greatly enhance our global understanding of avian influenza and other zoonotic diseases.

Long-distance migratory birds are often considered extreme athletes, possessing a range of traits that approach the physiological limits of vertebrate design. In addition, their movements must be carefully timed to ensure that they obtain resources of sufficient quantity and quality to satisfy their high-energy needs. Migratory birds may therefore be particularly vulnerable to global change processes that are projected to alter the quality and quantity of resource availability. Because long-distance flight requires high and sustained aerobic capacity, even minor decreases in vitality can have large negative consequences for migrants. In the light of this, we assess how current global change processes may affect the ability of birds to meet the physiological demands of migration, and suggest areas where avian physiologists may help to identify potential hazards. Predicting the consequences of global change scenarios on migrant species requires (i) reconciliation of empirical and theoretical studies of avian flight physiology; (ii) an understanding of the effects of food quality, toxicants and disease on migrant performance; and (iii) mechanistic models that integrate abiotic and biotic factors to predict migratory behaviour. Critically, a multi-dimensional concept of vitality would greatly facilitate evaluation of the impact of various global change processes on the population dynamics of migratory birds.

Gut microbes are increasingly recognised for their role in regulating an animal's metabolism and immunity. However, identifying repeatable associations between host physiological processes and their gut microbiota has proved challenging, in part because microbial communities often respond stochastically to host physiological stress (e.g. fasting, forced exercise or infection). Migratory birds provide a valuable system in which to test host–microbe interactions under physiological extremes because these hosts are adapted to predictable metabolic and immunological challenges as they undergo seasonal migrations, including temporary gut atrophy during long‐distance flights. These physiological challenges may either temporarily disrupt gut microbial ecosystems, or, alternatively, promote predictable host–microbe associations during migration. To determine the relationship between migration and gut microbiota, we compared gut microbiota composition between migrating and non‐migrating (“resident”) conspecific shorebirds sharing a flock. We performed this across two sandpiper species, Calidris ferruginea and Calidris ruficollis, in north‐western Australia, and an additional C. ruficollis population 3,000 km away in southern Australia. We found that migrants consistently had higher abundances of the bacterial genus Corynebacterium (average 28% abundance) compared to conspecific residents (average <1% abundance), with this effect holding across both species and sites. However, other than this specific association, community structure and diversity was almost identical between migrants and residents, with migration status accounting for only 1% of gut community variation when excluding Corynebacterium. Our findings suggest a consistent relationship between Corynebacterium and Calidris shorebirds during migration, with further research required to identify causal mechanisms behind the association, and to elucidate functionality to the host. However, outside this specific association, migrating shorebirds broadly maintained gut community structure, which may allow them to quickly recover gut function after a migratory flight. This study provides a rare example of a repeatable and specific response of the gut microbiota to a major physiological challenge across two species and two distant populations.