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1. Inclement weather forces stopovers and prevents migratory progress for obligate soaring migrants

   https://doi.org/10.1186/s40462-021-00274-6  

   Mallon, Julie M. ; Bildstein, Keith L. ; Fagan, William F. ( December 2021 , Movement Ecology)

   null (Ed.)

   Abstract Background Migrating birds experience weather conditions that change with time, which affect their decision to stop or resume migration. Soaring migrants are especially sensitive to changing weather conditions because they rely on the availability of environmental updrafts to subsidize flight. The timescale that local weather conditions change over is on the order of hours, while stopovers are studied at the daily scale, creating a temporal mismatch. Methods We used GPS satellite tracking data from four migratory Turkey Vulture ( Cathartes aura ) populations, paired with local weather data, to determine if the decision to stopover by migrating Turkey Vultures was in response to changing local weather conditions. We analyzed 174 migrations of 34 individuals from 2006 to 2019 and identified 589 stopovers based on variance of first passage times. We also investigated if the extent of movement activity correlated with average weather conditions experienced during a stopover, and report general patterns of stopover use by Turkey Vultures between seasons and across populations. Results Stopover duration ranged from 2 h to more than 11 days, with 51 % of stopovers lasting < 24 h. Turkey Vultures began stopovers immediately in response to changes in weather variables that did not favor thermal soaring (e.g., increasing precipitation fraction and decreasing thermal updraft velocity) and their departure from stopovers was associated with improvements in weather that favored thermal development. During stopovers, proportion of activity was negatively associated with precipitation but was positively associated with temperature and thermal updraft velocity. Conclusions The rapid response of migrating Turkey Vultures to changing weather conditions indicates weather-avoidance is one of the major functions of their stopover use. During stopovers, however, the positive relationship between proportion of movement activity and conditions that promote thermal development suggests not all stopovers are used for weather-avoidance. Our results show that birds are capable of responding rapidly to their environment; therefore, for studies interested in external drivers of weather-related stopovers, it is essential that stopovers be identified at fine temporal scales. 

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2. Heterothermic Migration Strategies in Flying Vertebrates

   https://doi.org/10.1093/icb/icad053  

   McGuire, Liam P ; Leys, Ryan ; Webber, Quinn M ; Clerc, Jeff ( June 2023 , Integrative And Comparative Biology)

   Migration is a widespread and highly variable trait among animals. Population-level patterns arise from individual-level decisions, including physiological and energetic constraints. Many aspects of migration are influenced by behaviors and strategies employed during periods of stopover, where migrants may encounter variable or unpredictable conditions. Thermoregulation can be a major cost for homeotherms which largely encounter ambient temperatures below the lower critical temperature during migration, especially during the rest phase of the daily cycle. In this review we describe the empirical evidence, theoretical models, and potential implications of bats and birds that use heterothermy to reduce thermoregulatory costs during migration. Torpor-assisted migration is a strategy described for migrating temperate insectivorous bats, whereby torpor can be used during periods of inactivity to drastically reduce thermoregulatory costs and increase net refueling rate, leading to shorter stopover duration, reduced fuel load requirement, and potential consequences for broad-scale movement patterns and survival. Hummingbirds can adopt a similar strategy, but most birds are not capable of torpor. However, there is an increasing recognition of the use of more shallow heterothermic strategies by diverse bird species during migration, with similarly important implications for migration energetics. A growing body of published literature and preliminary data from ongoing research indicate that heterothermic migration strategies in birds may be more common than traditionally appreciated. We further take a broad evolutionary perspective to consider heterothermy as an alternative to migration in some species, or as a conceptual link to consider alternatives to seasonal resource limitations. There is a growing body of evidence related to heterothermic migration strategies in bats and birds, but many important questions related to the broader implications of this strategy remain.

    

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3. Biomass burning in the Neotropics is exposing migrating birds to elevated fine particulate matter concentrations

   https://doi.org/10.1111/geb.13767  

   La Sorte, Frank A. ; Zuckerberg, Benjamin ; Lepczyk, Christopher A. ; Aronson, Myla F. J. ; Horton, Kyle G. ( October 2023 , Global Ecology and Biogeography)

   A unique risk faced by nocturnally migrating birds is the disorienting influence of artificial light at night (ALAN). ALAN originates from anthropogenic activities that can generate other forms of environmental pollution, including the emission of fine particulate matter (PM_2.5). PM_2.5concentrations can display strong seasonal variation whose origin can be natural or anthropogenic. How this variation affects seasonal associations with ALAN and PM_2.5for nocturnally migrating bird populations has not been explored.

   Western Hemisphere.

   2021

   Nocturnally migrating passerine (NMP) bird species.

   We combined monthly estimates of PM_2.5and ALAN with weekly estimates of relative abundance for 164 NMP species derived using observations from eBird. We identified groups of species with similar associations with monthly PM_2.5. We summarized their shared environmental, geographical, and ecological attributes.

   PM_2.5was lowest in North America, especially at higher latitudes during the boreal winter. PM_2.5was highest in the Amazon Basin, especially during the dry season (August–October). ALAN was highest within eastern North America, especially during the boreal winter. For NMP species, PM_2.5associations reached their lowest levels during the breeding season (<10 μg/m³) and highest levels during the nonbreeding season, especially for long‐distance migrants that winter in Central and South America (~20 μg/m³). Species that migrate through Central America in the spring encountered similarly high PM_2.5concentrations. ALAN associations reached their highest levels for species that migrate (~12 nW/cm²/sr) or spend the nonbreeding season (~15 nW/cm²/sr) in eastern North America.

   We did not find evidence that the disorienting influence of ALAN enhances PM_2.5exposure during stopover in the spring and autumn for NMP species. Rather, our findings suggest biomass burning in the Neotropics is exposing NMP species to consistently elevated PM_2.5concentrations for an extended period of their annual life cycles.

    

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4. Migrating songbirds on stopover prepare for, and recover from, oxidative challenges posed by long‐distance flight

   https://doi.org/10.1002/ece3.1601  

   Skrip, Megan M. ; Bauchinger, Ulf ; Goymann, Wolfgang ; Fusani, Leonida ; Cardinale, Massimiliano ; Alan, Rebecca R. ; McWilliams, Scott R. ( July 2015 , Ecology and Evolution)

   Managing oxidative stress is an important physiological function for all aerobic organisms, particularly during periods of prolonged high metabolic activity, such as long‐distance migration across ecological barriers. However, no previous study has investigated the oxidative status of birds at different stages of migration and whether that oxidative status depends on the condition of the birds. In this study, we compared (1) energy stores and circulating oxidative status measures in (a) two species of Neotropical migrants with differing migration strategies that were sampled at an autumn stopover site before an ecological barrier; and (b) a species of trans‐Saharan migrant sampled at a spring stopover site after crossing an ecological barrier; and (2) circulating oxidative measures and indicators of fat metabolism in a trans‐Saharan migrant after stopovers of varying duration (0–8 nights), based on recapture records. We found fat stores to be positively correlated with circulating antioxidant capacity in Blackpoll Warblers and Red‐eyed Vireos preparing for fall migration on Block Island,USA, but uncorrelated in Garden Warblers on the island of Ponza, Italy, after a spring crossing of the Sahara Desert and Mediterranean Sea. In all circumstances, fat stores were positively correlated with circulating lipid oxidation levels. Among Garden Warblers on the island of Ponza, fat anabolism increased with stopover duration while oxidative damage levels decreased. Our study provides evidence that birds build antioxidant capacity as they build fat stores at stopover sites before long flights, but does not support the idea that antioxidant stores remain elevated in birds with high fuel levels after an ecological barrier. Our results further suggest that lipid oxidation may be an inescapable hazard of using fats as the primary fuel for flight. Yet, we also show that birds on stopover are capable of recovering from the oxidative damage they have accrued during migration, as lipid oxidation levels decrease with time on stopover. Thus, the physiological strategy of migrating songbirds may be to build prophylactic antioxidant capacity in concert with fuel stores at stopover sites before a long‐distance flight, and then repair oxidative damage while refueling at stopover sites after long‐distance flight.

    

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5. Inbound arrivals: using weather surveillance radar to quantify the diurnal timing of spring trans‐Gulf bird migration

   https://doi.org/10.1111/ecog.06644  

   Abbott, Annika L. ; Deng, Yuting ; Badwey, Katie ; Farnsworth, Andrew ; Horton, Kyle G. ( August 2023 , Ecography)

   More than two billion birds migrate through the Gulf of Mexico each spring en route to breeding grounds in the USA and Canada. This region has a long history of complex natural and anthropogenic environments as the northern Gulf coast provides the first possible stopover habitats for migrants making nonstop trans‐Gulf crossings during spring migration. However, intense anthropogenic activity in the region, which is expanding rapidly at present, makes migrants vulnerable to a multitude of obstacles and increasingly fragments and alters these habitats. Understanding the timing of migrants' overwater arrivals has biological value for expanding our understanding of migration ecology relative to decision‐making for nonstop flights, and is imperative for advancing conservation of this critical region through the identification of key times in which to direct conservation actions (e.g. temporary halting of wind turbines, reduction of light pollution). We explored 10 years of weather surveillance radar data from five sites along the northern Gulf of Mexico coast to quantify the daily timing and intensity of arriving trans‐Gulf migrants. On a daily scale, we found that migrant intensity peaked an average of nine hours after local sunrise, occurring earliest at easternmost sites. On a seasonal level, the greatest number of arrivals occurred between late April and early May, with peak intensity occurring latest at westernmost sites. Overall intensity of migration across all 10 years of data was greatest at the westernmost sites and decreased moving farther to the east. These findings emphasize the differential spatial and temporal patterns of use of the Gulf of Mexico region by migrating birds, information that is essential for improving our understanding of the ecology of trans‐Gulf migration and for supporting data‐driven approaches to conservation actions for the migratory birds passing through this critical region. 

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