Feeding en route: Prey availability and traits influence prey selection by an avian predator on migration.

During animal migration, ephemeral communities of taxa at all trophic levels co-occur over space and time. The interactions between predators and prey along migration corridors are ecologically and evolutionarily significant. However, these interactions remain understudied in terrestrial systems and warrant further investigations using novel approaches. We investigated the predator-prey interactions between a migrating avivorous predator and ephemeral avian prey community in the fall migration season. We tested for associations between avian traits and prey selection and hypothesized that prey traits (i.e. relative size, flocking behaviour, habitat, migration tendency and availability) would influence prey selection by a sexually dimorphic raptor on migration. To document prey consumption, we sampled trace prey DNA from beaks and talons of migrating sharp-shinned hawks Accipiter striatus (n = 588). We determined prey availability in the ephemeral avian community by extracting weekly abundance indices from eBird Status and Trends data. We used discrete choice models to assess prey selection and visualized the frequency of prey in diet and availability on the landscape over the fall migration season. Using eDNA metabarcoding, we detected prey species on 94.1% of the hawks sampled (n = 525/588) comprising 1396 prey species detections from 65 prey species. Prey frequency in diet and eBird relative abundance of prey species were correlated over the migration season for top-selected prey species, suggesting prey availability is an important component of raptor-songbird interactions during fall. Prey size, flocking behaviour and non-breeding habitat association were prey traits that significantly influenced predator choice. We found differences between female and male hawk prey selection, suggesting that sexual size dimorphism has led to distinct foraging strategies on migration. This research integrated field data collected by a volunteer-powered raptor migration monitoring station and public-generated data from eBird to reveal elusive predator-prey dynamics occurring in an ephemeral raptor-songbird community during fall migration. Understanding dynamic raptor-songbird interactions along migration routes remains a relatively unexplored frontier in animal ecology and is necessary for the conservation and management efforts of migratory and resident communities.

Durante la migración animal, las comunidades efímeras de taxones de todos los niveles tróficos coexisten en el espacio y el tiempo. Las interacciones entre depredadores y presas a lo largo de los corredores migratorios son significativas desde el punto de vista ecológica y evolutivo. Sin embargo, estas interacciones siguen siendo poco estudiadas en los sistemas terrestres y justifican más investigaciones utilizando enfoques novedosos. Investigamos las interacciones depredador­presa entre un depredador avívoro migratorio y una comunidad de presas aviares efímeras en la temporada migratoria otoñal. Probamos las asociaciones entre los rasgos de las aves y la selección de presas y planteamos la hipótesis de que los rasgos de las presas (tamaño relativo, comportamiento de bandada, hábitat, tendencia migratoria y disponibilidad) influirían en la selección de presas por parte de una rapaz sexualmente dimórfica durante la migración. Para documentar el consumo de presas, recogimos rastros de ADN de presas de picos y garras de Gavilán Americano Accipiter striatus (n = 588) migratorios. Determinamos la disponibilidad de presas en la comunidad de aves efímeras extrayendo índices de abundancia semanales de los datos de eBird Estado y Tendencias. Utilizamos modelos de elección discreta para evaluar la selección de presas y visualizamos la frecuencia de las presas en la dieta y la disponibilidad en el paisaje durante la temporada migratoria otoñal. Utilizando el metacódigo de barras del ADN ambiental, detectamos especies de presas en el 94,1% de los halcones muestreados (n = 525/588), comprendiendo 1396 detecciones de 65 especies de presas. La frecuencia de presas en la dieta y la abundancia relativa de especies de presas en eBird se correlacionaron a lo largo de la temporada de migración para las principales especies de presas seleccionadas, lo que sugiere que la disponibilidad de presas es un componente importante de las interacciones entre aves rapaces y aves canoras durante el otoño. El tamaño de las presas, el comportamiento de las bandadas y la asociación con el hábitat no reproductivo fueron rasgos de presa que influyeron significativamente en la elección de los depredadores. Encontramos diferencias entre la selección de presas de gavilán hembra y macho, lo que sugiere que el dimorfismo sexual de tamaño ha conducido a distintas estrategias de alimentación durante la migración. Esta investigación integró datos de campo recopilados por una estación de monitoreo de migración de rapaces impulsada por voluntarios y datos generados públicamente por eBird para revelar la esquiva dinámica depredador­presa que ocurre en una comunidad efímera de rapaces y aves canoras durante la migración otoñal. Comprender las interacciones dinámicas entre rapaces y aves canoras a lo largo de las rutas migratorias sigue siendo una frontera relativamente inexplorada en la ecología animal y es necesaria para los esfuerzos de conservación y gestión de las comunidades migratorias y residentes.

Eco-evolutionary drivers of avian migratory connectivity.

Migratory connectivity, reflecting the extent by which migrants tend to maintain their reciprocal positions in seasonal ranges, can assist in the conservation and management of mobile species, yet relevant drivers remain unclear. Taking advantage of an exceptionally large (~150,000 individuals, 83 species) and more-than-a-century-long dataset of bird ringing encounters, we investigated eco-evolutionary drivers of migratory connectivity in both short- and long-distance Afro-Palearctic migratory birds. Connectivity was strongly associated with geographical proxies of migration costs and was weakly influenced by biological traits and phylogeny, suggesting the evolutionary lability of migratory behaviour. The large intraspecific variability in avian migration strategies, through which most species geographically split into distinct migratory populations, explained why most of them were significantly connected. By unravelling key determinants of migratory connectivity, our study improves knowledge about the resilience of avian migrants to ecological perturbations, providing a critical tool to inform transboundary conservation and management strategies at the population level.

High pathogenicity avian influenza: targeted active surveillance of wild birds to enable early detection of emerging disease threats.

Avian influenza (AI) is an important disease that has significant implications for animal and human health. High pathogenicity AI (HPAI) has emerged in consecutive seasons within the UK to cause the largest outbreaks recorded. Statutory measures to control outbreaks of AI virus (AIV) at poultry farms involve disposal of all birds on infected premises. Understanding of the timing of incursions into the UK could facilitate decisions on improved responses. During the autumnal migration and wintering period (autumn 2019- spring 2020), three active sampling approaches were trialled for wild bird species considered likely to be involved in captive AI outbreaks with retrospective laboratory testing undertaken to define the presence of AIV.Faecal sampling of birds (n = 594) caught during routine and responsive mist net sampling failed to detect AIV. Cloacal sampling of hunter-harvested waterfowl (n = 146) detected seven positive samples from three species with the earliest detection on the 17 October 2020. Statutory sampling first detected AIV in wild and captive birds on 3 November 2020. We conclude that hunter sourced sampling of waterfowl presents an opportunity to detect AI within the UK in advance of outbreaks on poultry farms and allow for early intervention measures to protect the national poultry flock.

Predator swamping reduces predation risk during nocturnal migration of juvenile salmon in a high-mortality landscape.

Animal migrations are costly and are often characterized by high predation risk for individuals. Three of the most oft-assumed mechanisms for reducing risk for migrants are swamping predators with high densities, specific timing of migrations and increased body size. Assessing the relative importance of these mechanisms in reducing predation risk particularly for migrants is generally lacking due to the difficulties in tracking the fate of individuals and population-level characteristics simultaneously. We used acoustic telemetry to track migration behaviour and survival of juvenile sockeye salmon (Oncorhynchus nerka) smolts released over a wide range of conspecific outmigration densities in a river associated with poor survival. The landscape was indeed high risk; smolt survival was poor (˜68%) over 13·5 km of river examined even though migration was rapid (generally <48 h). Our results demonstrate that smolts largely employ swamping of predators to reduce predation risk. Increased densities of co-migrant conspecifics dramatically improved survival of smolts. The strong propensity for nocturnal migration resulted in smolts pausing downstream movements until the next nightfall, greatly increasing relative migration durations for smolts that could not traverse the study area in a single night. Smolt size did not appear to impact predation risk, potentially due to unique characteristics of the system or our inability to tag the entire size range of outmigrants. Movement behaviours were important in traversing this high-risk landscape and provide rare evidence for swamping to effectively reduce individual predation risk.

A conceptual framework on the role of magnetic cues in songbird migration ecology.

Migrating animals perform astonishing seasonal movements by orienting and navigating over thousands of kilometres with great precision. Many migratory species use cues from the sun, stars, landmarks, olfaction and the Earth's magnetic field for this task. Among vertebrates, songbirds are the most studied taxon in magnetic-cue-related research. Despite multiple studies, we still lack a clear understanding of when, where and how magnetic cues affect the decision-making process of birds and hence, their realised migratory behaviour in the wild. This understanding is especially important to interpret the results of laboratory experiments in an ecologically appropriate way. In this review, we summarise the current findings about the role of magnetic cues for migratory decisions in songbirds. First, we review the methodological principles for orientation and navigation research, specifically by comparing experiments on caged birds with experiments on free-flying birds. While cage experiments can show the sensory abilities of birds, studies with free-flying birds can characterise the ecological roles of magnetic cues. Second, we review the migratory stages, from stopover to endurance flight, in which songbirds use magnetic cues for their migratory decisions and incorporate this into a novel conceptual framework. While we lack studies examining whether and when magnetic cues affect orientation or navigation decisions during flight, the role of magnetic cues during stopover is relatively well studied, but mostly in the laboratory. Notably, many such studies have produced contradictory results so that understanding the biological importance of magnetic cues for decisions in free-flying songbirds is not straightforward. One potential explanation is that reproducibility of magnetic-cue experiments is low, probably because variability in the behavioural responses of birds among experiments is high. We are convinced that parts of this variability can be explained by species-specific and context-dependent reactions of birds to the study conditions and by the bird's high flexibility in whether they include magnetic cues in a decision or not. Ultimately, this review should help researchers in the challenging field of magnetoreception to design experiments meticulously and interpret results of such studies carefully by considering the migration ecology of their focal species.

Genetic Divergence and Connectivity among Gene Pools of Polyprion americanus.

Three regional gene pools of Polyprion americanus have been described so far, i.e., the North Atlantic, the Southwest Atlantic, and the Indo-Pacific Ocean. However, there is taxonomic uncertainty about the Southeast Atlantic population and there is suspicion on the existence of a third species of Polyprion in that area. Additionally, prior studies have shown a lack of genetic structuring in the Atlantic North. Nonetheless, a more conspicuous characterization of intensity, periodicity, and direction of migration are needed to properly understand the wreckfish connectivity pattern in the North Atlantic population. This study addresses the interspecific concerns highlighted above as well as the intrapopulation structure of P. americanus from the Atlantic North, using the mitochondrial DNA Cytochrome Oxidase I gene and nuclear DNA microsatellite markers on a comprehensive sampling effort. The highly divergent gene pool from South Africa was characterized by the specific Mitochondrial DNA PamCOI.Saf haplotype. Its molecular composition and phylogenetic status were conspicuously intermediate between P. americanus and P. oxygeneios, which suggests its putative hybrid origin between those species. Microsatellite variation exhibited a high differentiation (24%) among four putative Polyprion spp. gene pools which contrasts with the large genetic homogeneity within the Atlantic North stock (FSC = 0.002). The significant migration rates inferred upon Bayesian algorithms suggest a longitudinal bi-directional connectivity pattern which strengthens the migratory hypothesis previously suggested on demographic data in the Atlantic North gene pool.

Quantifying year-round nocturnal bird migration with a fluid dynamics model.

To understand the influence of biomass flows on ecosystems, we need to characterize and quantify migrations at various spatial and temporal scales. Representing the movements of migrating birds as a fluid, we applied a flow model to bird density and velocity maps retrieved from the European weather radar network, covering almost a year. We quantified how many birds take-off, fly, and land across Western Europe to (1) track bird migration waves between nights, (2) cumulate the number of birds on the ground and (3) quantify the seasonal flow into and out of the study area through several regional transects. Our results identified several migration waves that crossed the study area in 4 days only and included up to 188 million (M) birds that took-off in a single night. In spring, we estimated that 494 M birds entered the study area, 251 M left it, and 243 M birds remained within the study area. In autumn, 314 M birds entered the study area while 858 M left it. In addition to identifying fundamental quantities, our study highlights the potential of combining interdisciplinary data and methods to elucidate the dynamics of avian migration from nightly to yearly time scales and from regional to continental spatial scales.

Infected juvenile salmon can experience increased predation during freshwater migration.

Predation risk for animal migrants can be impacted by physical condition. Although size- or condition-based selection is often observed, observing infection-based predation is rare due to the difficulties in assessing infectious agents in predated samples. We examined predation of outmigrating sockeye salmon (Oncorhynchus nerka) smolts by bull trout (Salvelinus confluentus) in south-central British Columbia, Canada. We used a high-throughput quantitative polymerase chain reaction (qPCR) platform to screen for the presence of 17 infectious agents found in salmon and assess 14 host genes associated with viral responses. In one (2014) of the two years assessed (2014 and 2015), the presence of infectious haematopoietic necrosis virus (IHNv) resulted in 15-26 times greater chance of predation; in 2015 IHNv was absent among all samples, predated or not. Thus, we provide further evidence that infection can impact predation risk in migrants. Some smolts with high IHNv loads also exhibited gene expression profiles consistent with a virus-induced disease state. Nine other infectious agents were observed between the two years, none of which were associated with increased selection by bull trout. In 2014, richness of infectious agents was also associated with greater predation risk. This is a rare demonstration of predator consumption resulting in selection for prey that carry infectious agents. The mechanism by which this selection occurs is not yet determined. By culling infectious agents from migrant populations, fish predators could provide an ecological benefit to prey.

Birds of three worlds: moult migration to high Arctic expands a boreal-temperate flyway to a third biome.

BACKGROUND: Knowledge on migration patterns and flyways is a key for understanding the dynamics of migratory populations and evolution of migratory behaviour. Bird migration is usually considered to be movements between breeding and wintering areas, while less attention has been paid to other long-distance movements such as moult migration. METHODS: We use high-resolution satellite-tracking data from 58 taiga bean geese Anser fabalis fabalis from the years 2019-2020, to study their moult migration during breeding season. We show the moulting sites, estimate the migratory connectivity between the breeding and the moulting sites, and estimate the utilization distributions during moult. We reveal migration routes and compare the length and timing of migration between moult migrants and successful breeders. RESULTS: All satellite-tracked non-breeding and unsuccessfully breeding taiga bean geese migrated annually to the island of Novaya Zemlya in the high Arctic for wing moult, meaning that a large part of the population gathers at the moulting sites outside the breeding range annually for approximately three months. Migratory connectivity between breeding and moulting sites was very low (rm = - 0.001, 95% CI - 0.1562-0.2897), indicating that individuals from different breeding grounds mix with each other on the moulting sites. Moult migrants began fall migration later in autumn than successful breeders, and their overall annual migration distance was over twofold compared to the successful breeders. CONCLUSIONS: Regular moult migration makes the Arctic an equally relevant habitat for the taiga bean goose population as their boreal breeding and temperate wintering grounds, and links ecological communities in these biomes. Moult migration plays an important role in the movement patterns and spatio-temporal distribution of the population. Low migratory connectivity between breeding and moulting sites can potentially contribute to the gene flow within the population. Moult migration to the high Arctic exposes the population to the rapid impacts of global warming to Arctic ecosystems. Additionally, Novaya Zemlya holds radioactive contaminants from various sources, which might still pose a threat to moult migrants. Generally, these results show that moult migration may essentially contribute to the way we should consider bird migration and migratory flyways.

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.

Messy eaters: Swabbing prey DNA from the exterior of inconspicuous predators when foraging cannot be observed.

Complex coevolutionary relationships among competitors, predators, and prey have shaped taxa diversity, life history strategies, and even the avian migratory patterns we see today. Consequently, accurate documentation of prey selection is often critical for understanding these ecological and evolutionary processes. Conventional diet study methods lack the ability to document the diet of inconspicuous or difficult-to-study predators, such as those with large home ranges and those that move vast distances over short amounts of time, leaving gaps in our knowledge of trophic interactions in many systems. Migratory raptors represent one such group of predators where detailed diet studies have been logistically challenging. To address knowledge gaps in the foraging ecology of migrant raptors and provide a broadly applicable tool for the study of enigmatic predators, we developed a minimally invasive method to collect dietary information by swabbing beaks and talons of raptors to collect trace prey DNA. Using previously published COI primers, we were able to isolate and reference gene sequences in an open-access barcode database to identify prey to species. This method creates a novel avenue to use trace molecular evidence to study prey selection of migrating raptors and will ultimately lead to a better understanding of raptor migration ecology. In addition, this technique has broad applicability and can be used with any wildlife species where even trace amounts of prey debris remain on the exterior of the predator after feeding.

Count trends for migratory Bald Eagles reveal differences between two populations at a spring site along the Lake Ontario shoreline.

The recovery of Bald Eagles (Haliaeetus leucophalus), after DDT and other organochlorine insecticides were banned in the United States, can be regarded as one of the most iconic success stories resulting from the Endangered Species Act. Interest remains high in the recovery and growth of the Bald Eagle population. Common to evaluating growth and recovery rates are counts at nesting sites and analyses of individuals fledged per season. But this is merely one snapshot that ignores survival rates as eagles grow to maturity. By analyzing indices from migration counts, we get a different snapshot better reflecting the survival of young birds. Different populations of Bald Eagles breed at different sites at different times of the year. Typical migration count analyses do not separate the populations. A separation of two distinct populations can be achieved at spring count sites by taking advantage of the tendency for northern summer breeding birds to migrate north in spring earlier than southern winter breeding birds who disperse north later in spring. In this paper I analyze migratory indices at a spring site along Lake Ontario. The analysis shows that eagles considered to be primarily of the northern summer breeding population showed an estimated growth rate of 5.3 ± 0.85% (SE) per year with 49% of eagles tallied in adult plumage, whereas the migrants considered to be primarily of the southern breeding population had an estimated growth rate of 14.0 ± 1.79% with only 22% in adult plumage. Together these results argue that the populations of southern breeding Bald Eagles are growing at a substantially higher rate than northern breeding eagles. These findings suggest that aggregate population indices for a species at migration counting sites can sometimes obscure important differences among separate populations at any given site and that separating counts by time period can be a useful way to check for differences among sub-populations.