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1. Aminopeptidease-N modulation assists lean mass anabolism during refueling in the white-throated sparrow

   https://doi.org/https://doi.org/10.1098/rspb.2020.2348  

   Griego, M. (January 2021, Proceedings of the Royal Society of London Series B Containing papers of a biological character)

   null (Ed.)

   Songbirds meet the extreme metabolic demands of migration by burning both stored fat and protein. However, catabolizing these endogenous tissues for energy leads to organ atrophy, and reductions in gastrointestinal tissue can be as great as 50% of the pre-flight mass. Remarkably, during stopover refuelling birds quickly regain digestive mass and performance. Aminopep- tidase-N (APN) is a brush-border enzyme responsible for late-stage protein digestion and may critically assist tissue reconstruction during the stopover, thus compensating for reduced gut size. We hypothesized that birds recover- ing from a fast would differentially upregulate APN activity relative to disaccharidases to rapidly process and assimilate dietary protein into lean mass. We fasted 23 wild-caught migratory white-throated sparrows (Zonotrichia albicollis) for 48h to mimic mass reductions experienced during migratory flight and measured intestinal APN activity before the fast, immediately after the fast, and during recovery at 24 h and 48 h post- fast. Total fat mass, lean mass and basal metabolic rate were measured daily. We show that fasted birds maintain APN activity through the fast, despite a 30% reduction in intestine mass, but during refuelling, APN activity increases nearly twofold over pre-fasted individuals. This suggests that dynamically regulating APN may be necessary for rapid protein reconstruction during the stopover. 

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2. Artificial light at night is a top predictor of bird migration stopover density

   https://doi.org/10.1038/s41467-023-43046-z  

   Horton, Kyle G.; Buler, Jeffrey J.; Anderson, Sharolyn J.; Burt, Carolyn S.; Collins, Amy C.; Dokter, Adriaan M.; Guo, Fengyi; Sheldon, Daniel; Tomaszewska, Monika Anna; Henebry, Geoffrey M. (December 2023, Nature Communications)

   Abstract As billions of nocturnal avian migrants traverse North America, twice a year they must contend with landscape changes driven by natural and anthropogenic forces, including the rapid growth of the artificial glow of the night sky. While airspaces facilitate migrant passage, terrestrial landscapes serve as essential areas to restore energy reserves and often act as refugia—making it critical to holistically identify stopover locations and understand drivers of use. Here, we leverage over 10 million remote sensing observations to develop seasonal contiguous United States layers of bird migrant stopover density. In over 70% of our models, we identify skyglow as a highly influential and consistently positive predictor of bird migration stopover density across the United States. This finding points to the potential of an expanding threat to avian migrants: peri-urban illuminated areas may act as ecological traps at macroscales that increase the mortality of birds during migration. 

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3. Metabolic reduction after long-duration flight is not related to fat-free mass loss or flight duration in a migratory passerine

   https://doi.org/10.1242/jeb.215384  

   Gerson, Alexander R.; DeSimone, Joely G.; Black, Elizabeth C.; Dick, Morag F.; Groom, Derrick J. (October 2020, The Journal of Experimental Biology)

   null (Ed.)

   ABSTRACT Migratory birds catabolize large quantities of protein during long flights, resulting in dramatic reductions in organ and muscle mass. One of the many hypotheses to explain this phenomenon is that decrease in lean mass is associated with reduced resting metabolism, saving energy after flight during refueling. However, the relationship between lean body mass and resting metabolic rate remains unclear. Furthermore, the coupling of lean mass with resting metabolic rate and with peak metabolic rate before and after long-duration flight have not previously been explored. We flew migratory yellow-rumped warblers ( Setophaga coronata ) in a wind tunnel under one of two humidity regimes to manipulate the rate of lean mass loss in flight, decoupling flight duration from total lean mass loss. Before and after long-duration flights, we measured resting and peak metabolism, and also measured fat mass and lean body mass using quantitative magnetic resonance. Flight duration ranged from 28 min to 600 min, and birds flying under dehydrating conditions lost more fat-free mass than those flying under humid conditions. After flight, there was a 14% reduction in resting metabolism but no change in peak metabolism. Interestingly, the reduction in resting metabolism was unrelated to flight duration or to change in fat-free body mass, indicating that protein metabolism in flight is unlikely to have evolved as an energy-saving measure to aid stopover refueling, but metabolic reduction itself is likely to be beneficial to migratory birds arriving in novel habitats. 

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4. 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)

   Synopsis 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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5. Broad-Scale Weather Patterns Encountered during Flight Influence Landbird Stopover Distributions

   https://doi.org/10.3390/rs12030565  

   Clipp, Hannah L.; Cohen, Emily B.; Smolinsky, Jaclyn A.; Horton, Kyle G.; Farnsworth, Andrew; Buler, Jeffrey J. (February 2020, Remote Sensing)

   The dynamic weather conditions that migrating birds experience during flight likely influence where they stop to rest and refuel, particularly after navigating inhospitable terrain or large water bodies, but effects of weather on stopover patterns remain poorly studied. We examined the influence of broad-scale weather conditions encountered by nocturnally migrating Nearctic-Neotropical birds during northward flight over the Gulf of Mexico (GOM) on subsequent coastal stopover distributions. We categorized nightly weather patterns using historic maps and quantified region-wide densities of birds in stopover habitat with data collected by 10 weather surveillance radars from 2008 to 2015. We found spring weather patterns over the GOM were most often favorable for migrating birds, with winds assisting northward flight, and document regional stopover patterns in response to specific unfavorable weather conditions. For example, Midwest Continental High is characterized by strong northerly winds over the western GOM, resulting in high-density concentrations of migrants along the immediate coastlines of Texas and Louisiana. We show, for the first time, that broad-scale weather experienced during flight influences when and where birds stop to rest and refuel. Linking synoptic weather patterns encountered during flight with stopover distributions contributes to the emerging macro-ecological understanding of bird migration, which is critical to consider in systems undergoing rapid human-induced changes. 

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