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.

Hemolytic parasites affect survival in migrating red-tailed hawks.

Migrating birds face a myriad of hazards, including higher exposure to parasites and numerous competing energy demands. It follows that migration may act as a selective filter and limit population growth. Understanding how individual-level physiological condition and disease status scale up to population dynamics through differential survival of individuals is necessary to identify threats and management interventions for migratory populations, many of which face increasing conservation challenges. However, linking individual physiological condition, parasite infection status and survival can be difficult. We examined the relationship among two measures of physiological condition [scaled-mass index and heterophil/leukocyte (H/L) ratio], hematozoa (i.e. hemoparasites) presence and abundance, and constitutive immunity in 353 autumn migrating red-tailed hawks (Buteo jamaicensis calurus) from 2004 to 2018. Hematazoa (i.e. Haemoproteus and Leucocytozoon) were in the blood smears from 139 red-tailed hawks (39.4%). H/L ratio decreased with scaled-mass index. Adults had a significantly higher H/L ratio than juveniles. Our two measures of immune defences, hemolytic-complement activity and bacteria-killing ability, were highly positively correlated. Our most notable finding was a negative relationship between Haemoproteus parasitemia and survival (i.e. documented individual mortality), indicating that haemosporidian parasites influence survival during a challenging life stage. The effect of haemosporidian parasites on individuals is often debated, and we provide evidence that parasitemia can affect individual survival. In contrast, we did not find evidence of trade-offs between survival and immune defences.

Snap-freezing in the Field: Effect of Sample Holding Time on Performance of Bactericidal Assays.

Comparative analyses in biology rely on the quality of available data. Methodological differences among studies may introduce variation in results that obscure patterns. In the field of eco-immunology, functional immune assays such as antimicrobial capacity assays are widely used for among-species applications. Sample storage time and animal handling time can influence assay results in some species, but how sample holding time prior to freezing influences assay results is unknown. Sample holding time can vary widely in field studies on wild animals, prompting the need to understand the implications of such variation on assay results. We investigated the hypothesis that sample holding time prior to freezing influences assay results in six species (Leiocephalus carinatus, Iguana iguana, Loxodonta africana, Ceratotherium simum, Columba livia, and Buteo swainsoni) by comparing antibacterial capacity of serum with varying processing times prior to snap-freezing. Blood was collected once from each individual and aliquots were placed on ice and assigned different holding times (0, 30, 60, 180, and 240 min), after which each sample was centrifuged, then serum was separated and snap-frozen on dry ice and stored at -80ºC for 60 days prior to assaying. For each aliquot, we conducted antibacterial capacity assays with serial dilutions of serum inoculated with E. coli and extracted the dilution at 50% antibacterial capacity for analysis. We found a decrease in antibacterial capacity with increased holding time in one of the six species tested (B. swainsoni), driven in part by complete loss of antibacterial capacity in some individuals at the 240-min time point. While the majority of species' antibacterial capacity were not affected, our results demonstrate the need to conduct pilot assays spanning the anticipated variation in sample holding times to develop appropriate field protocols.

Seasonal muscle ultrastructure plasticity and resistance of muscle structural changes during temperature increases in resident black-capped chickadees and rock pigeons.

Resident birds in temperate zones respond to seasonally fluctuating temperatures by adjusting their physiology, such as changes in basal metabolic rate or peak metabolic rate during cold exposure, or altering their organ sizes, so as to match the thermogenic requirements of their current environment. Climate change is predicted to cause increases in the frequency of heat and cold wave events, which could increase the likelihood that birds will face an environmental mismatch. Here, we examined seasonality and the effects of acute and chronic heat shock to 33°C and subsequent recovery from heat shock on the ultrastructure of the superficial pectoralis muscle fiber diameter, myonuclear domain (MND) and capillary density in two temperate bird species of differing body mass, the black-capped chickadee (Poecile atricapillus) and the rock pigeon (Columba livia). We found that muscle fiber ultrastructure did not change with heat treatment. However, in black-capped chickadees, there was a significant increase in fiber diameter in spring phenotype birds compared with summer phenotype birds. In rock pigeons, we saw no differences in fiber diameter across seasons. Capillary density did not change as a function of fiber diameter in black-capped chickadees, but did change seasonally, as did MND. Across seasons, as fiber diameter decreased, capillary density increased in the pectoralis muscle of rock pigeons. For both species in this study, we found that as fiber diameter increased, so did MND. Our findings imply that these two temperate birds employ different muscular growth strategies that may be metabolically beneficial to each.