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1. Intra‐specific variation in migration phenology of American Kestrels ( Falco sparverius ) in response to spring temperatures

   https://doi.org/10.1111/ibi.12953  

   Powers, Breanna F.; Winiarski, Jason M.; Requena‐Mullor, Juan M.; Heath, Julie A. (March 2021, Ibis)

   null (Ed.)

   Abstract In migratory birds, among- and within-species heterogeneity in response to climate change may be attributed to differences in migration distance and environmental cues that affect timing of arrival at breeding grounds. We used eBird observations and a within-species comparative approach to examine whether migration distance (with latitude as a proxy) and weather predictors can explain spring arrival dates at the breeding site in a raptor species with a widespread distribution and diverse migration strategies, the American Kestrel Falco sparverius. We found an interactive effect between latitude and spring minimum temperatures on arrival dates, whereby at lower latitudes (short-distance migrants) American Kestrels arrived earlier in warmer springs and later in colder springs, but American Kestrels at higher latitudes (long-distance migrants) showed no association between arrival time and spring temperatures. Increased snow cover delayed arrival at all latitudes. Our results support the hypothesis that short-distance migrants are better able to respond to conditions on the breeding ground than are long-distance migrants, suggesting that long-distance migrants may be more vulnerable to shifts in spring conditions that could lead to phenological mismatch between peak resources and nesting. 

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2. Behavioral responses to spring snow conditions contribute to long-term shift in migration phenology in American robins

   https://doi.org/10.1088/1748-9326/ab71a0  

   Oliver, Ruth Y.; Mahoney, Peter J.; Gurarie, Eliezer; Krikun, Nicole; Weeks, Brian C.; Hebblewhite, Mark; Liston, Glen; Boelman, Natalie (April 2020, Environmental Research Letters)

   Abstract Migratory birds have the capacity to shift their migration phenology in response to climatic change. Yet the mechanistic underpinning of changes in migratory timing remain poorly understood. We employed newly developed global positioning system (GPS) tracking devices and long-term dataset of migration passage timing to investigate how behavioral responses to environmental conditions relate to phenological shifts in American robins (Turdus migratorius) during spring migration to Arctic-boreal breeding grounds. We found that over the past quarter-century (1994–2018), robins have migrated ca. 5 d/decade earlier. Based on GPS data collected for 55 robins over three springs (2016–2018), we found the arrival timing and likelihood of stopovers, and timing of arrival to breeding grounds, were strongly influenced by dynamics in snow conditions along migratory paths. These findings suggest plasticity in migratory behavior may be an important mechanism for how long-distance migrants adjust their breeding phenology to keep pace with advancement of spring on breeding grounds. 

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3. Holding steady: Little change in intensity or timing of bird migration over the Gulf of Mexico

   https://doi.org/10.1111/gcb.14540  

   Horton, Kyle G.; Van Doren, Benjamin M.; La Sorte, Frank A.; Cohen, Emily B.; Clipp, Hannah L.; Buler, Jeffrey J.; Fink, Daniel; Kelly, Jeffrey F.; Farnsworth, Andrew (January 2019, Global Change Biology)

   Abstract Quantifying the timing and intensity of migratory movements is imperative for understanding impacts of changing landscapes and climates on migratory bird populations. Billions of birds migrate in the Western Hemisphere, but accurately estimating the population size of one migratory species, let alone hundreds, presents numerous obstacles. Here, we quantify the timing, intensity, and distribution of bird migration through one of the largest migration corridors in the Western Hemisphere, the Gulf of Mexico (the Gulf). We further assess whether there have been changes in migration timing or intensity through the Gulf. To achieve this, we integrate citizen science (eBird) observations with 21 years of weather surveillance radar data (1995–2015). We predicted no change in migration timing and a decline in migration intensity across the time series. We estimate that an average of 2.1 billion birds pass through this region each spring en route to Nearctic breeding grounds. Annually, half of these individuals pass through the region in just 18 days, between April 19 and May 7. The western region of the Gulf showed a mean rate of passage 5.4 times higher than the central and eastern regions. We did not detect an overall change in the annual numbers of migrants (2007–2015) or the annual timing of peak migration (1995–2015). However, we found that the earliest seasonal movements through the region occurred significantly earlier over time (1.6 days decade⁻¹). Additionally, body mass and migration distance explained the magnitude of phenological changes, with the most rapid advances occurring with an assemblage of larger‐bodied shorter‐distance migrants. Our results provide baseline information that can be used to advance our understanding of the developing implications of climate change, urbanization, and energy development for migratory bird populations in North America. 

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4. Seasonal Patterns of Fat Deposits in Relation to Migratory Strategy in Facultative Migrants

   https://doi.org/10.3389/fevo.2021.691808  

   Cornelius, Jamie M.; Hahn, Thomas P.; Robart, Ashley R.; Vernasco, Ben J.; Zahor, Dorothy L.; Glynn, Kenneth J.; Navis, Corrie J.; Watts, Heather E. (June 2021, Frontiers in Ecology and Evolution)

   null (Ed.)

   Physiological preparations for migration generally reflect migratory strategy. Migrant birds fuel long-distance flight primarily with lipids, but carrying excess fuel is costly; thus, the amount of fat deposited prior to departure often reflects the anticipated flight duration or distance between refueling bouts. Seasonal pre-migratory deposition of fat is well documented in regular seasonal migrants, but is less described for more facultative species. We analyze fat deposits of free-living birds across several taxa of facultative migrants in the songbird subfamily Carduelinae, including house finches ( Haemorhous mexicanus ), American goldfinches ( Spinus tristis ), pine siskins ( Spinus pinus ) and four different North American ecotypes of red crossbills ( Loxia curvirostra ), to evaluate seasonal fat deposition during facultative migratory periods. Our data suggest that the extent of seasonal fat deposits corresponds with migratory tendency in these facultative taxa. Specifically, nomadic red crossbills with a seasonally predictable annual movement demonstrated relatively large seasonal fat deposits coincident with the migratory periods. In contrast, pine siskins, thought to be more variable in timing and initiation of nomadic movements, had smaller peaks in fat deposits during the migratory season, and the partial migrant American goldfinch and the resident house finch showed no peaks coincident with migratory periods. Within the red crossbills, those ecotypes that are closely associated with pine habitats showed larger peaks in fat deposits coincident with autumn migratory periods and had higher wing loading, whereas those ecotypes associated with spruces, Douglas-fir and hemlocks showed larger peaks coincident with spring migratory periods and lower wing loading. We conclude that population averages of fat deposits do reflect facultative migration strategies in these species, as well as the winter thermogenic challenges at the study locations. A difference in seasonal fattening and wing loading among red crossbill ecotypes is consistent with the possibility that they differ in their migratory biology, and we discuss these differences in light of crossbill reproductive schedules and phenologies of different conifer species. 

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5. Genetic analysis of museum samples suggests temporal stability in the Mexican nonbreeding distribution of a neotropical migrant

   https://doi.org/10.1111/jav.03369  

   Turbek, Sheela P; Polich, Alexandria; Bossu, Christen M; Rayne, Christine; Carpenter, Amanda; Rodríguez_Otero, Genaro; Gómez_Villaverde, Sergio; Rodríguez_Vásquez, Fabiola; Hernández-Baños, Blanca E; McCormack, John; et al (January 2025, Journal of Avian Biology)

   Seasonal migration is highly labile from an evolutionary perspective and known to rapidly evolve in response to selective pressures. However, long‐distance migratory birds rely partially on innate genetic programs and may be constrained in their ability to alter their migratory behavior. We take advantage of recent advances in our ability to genotype historical DNA samples to examine the temporal stability of migratory connections between breeding and nonbreeding populations (i.e. migratory connectivity) and population‐level nonbreeding distributions in the Wilson's warblerCardellina pusilla, a long‐distance migratory songbird. By assigning historical and contemporary samples collected across the nonbreeding range to genetically distinct breeding clusters, we suggest that broad‐scale population‐level nonbreeding distributions within this species have remained largely consistent within Mexico from the mid‐1900s to the present day. These findings support the idea that the nonbreeding distributions of long‐distance migrants may remain stable over long time scales, even in the face of rapid environmental change. 

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