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1. 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)

   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 arrivalmore »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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2. CHANGES IN SPRING ARRIVAL DATES OF RUFOUS HUMMINGBIRDS (SELASPHORUS RUFUS) IN WESTERN NORTH AMERICA IN THE PAST CENTURY

   Courter, Jason R. ( March 2017 , Wilson Ornithological Society Annual Meeting)

   Warming temperatures have been linked to advancing spring migration dates of birds, although most studies have been conducted at individual sites. Problems may arise if birds arrive or depart before or after associated food resources reach critical lifecycle stages. I compared mean first arrival dates of Rufous Hummingbird (Selaphorus rufus), a prolific pollinator and long-distance migrant, between 1895-1969 and 2006-2015 at eight locations in Oregon, Washington, and British Columbia. Historical arrivals were reported through the North American Bird Phenology Program and recent arrivals were estimated from temporal occupancy patterns using eBird checklists. Results indicated that hummingbirds arrived 8 and 11more »days later in the recent time period in two coastal cities in Oregon and 7-17 days earlier in northern, more inland cities in Washington and British Columbia. Spring temperatures have increased in the past century in much of this region and birds arrived earlier in years with warmer spring temperatures, suggesting that migratory advancements were climate-related. Later arrivals reported in coastal regions of Oregon in the recent time period may suggest that Rufous Hummingbirds are bypassing coastal areas to take advantage of more predictable conditions along inland migratory routes, or are shifting their breeding ranges northward, notions both supported by declining population trends observed in Breeding Bird Survey data. My results provide justification for the investigation of the ecological impacts of climate change on birds in coastal vs. inland environments and a framework for comparing information from two extensive and emerging datasets to better understand the impacts of climate change on bird migration.« less
3. Perceived wintering latitude determines timing of song output in a migratory bird

   https://doi.org/10.1002/ece3.5922  

   Brewer, Dustin E ; McGill, Clint A ; Fudickar, Adam M. ( December 2019 , Ecology and evolution)

   Migratory bird populations frequently consist of individuals that overwinter variable distances from the breeding site. Seasonal changes in photoperiod, which varies with latitude, underlie seasonal changes in singing frequency in birds. Therefore, migratory populations that consist of individuals that overwinter at different latitudes with large overwintering ranges could experience within-population variation in seasonal production of song. To test the influence of overwintering latitude on intrapopulation variance in song production in the spring, we subjected two groups of Eastern Song Sparrows (Melospiza melodia melodia) from the same partially migratory breeding population to different photoperiodic schedules associated with a 1,300-km difference inmore »overwintering location. One group remained on the natural photoperiodic schedule of the breeding site (resident group) while the other group experienced a nonbreeding photoperiod that mimicked a southern migration in the fall followed by a northern migration back to the breeding site in the spring (migratory group). We compared song output between the two groups in three different stages (nonbreeding, prebreeding, and breeding). Little singing occurred during nonbreeding stage sample dates (20 November, 6 December) for the resident group, and no singing occurred for the migrant group. During the prebreeding stage (27 January, 7 February), significantly more singing occurred in the resident group than in the migrant group. During the breeding stage (21 March, 4 April), after a simulated migration for the migrants, song output was similar in both groups. These results suggest that within-population variation in wintering latitude may contribute to variation in seasonal changes in singing behavior, which may covary with readiness to breed. Studies utilizing confirmed migrants and residents, rather than merely simulated migrants and residents, are also needed to better understand these processes.« less
4. Migratory strategy drives species-level variation in bird sensitivity to vegetation green-up

   https://doi.org/https://doi.org/10.1038/s41559-021-01442-y  

   Youngflesh, Casey ; Socolar, Jacob ; Amaral, Bruna R ; Arab, Ali ; Guralnick, Robert P. ; Hurlbert, Allen H. ; LaFrance, Raphael ; Mayor, Stephen J. ; Miller, David A. ; Tingley, Morgan W. ( January 2021 , Nature ecology evolution)

   Animals and plants are shifting the timing of key life events in response to climate change, yet despite recent documentation of escalating phenological change, scientists lack a full understanding of how and why phenological responses vary across space and among species. Here, we used over 7 million community-contributed bird observations to derive species-specific, spatially explicit estimates of annual spring migration phenology for 56 bird species across eastern North America. We show that changes in the spring arrival of migratory birds are coarsely synchronized with fluctuations in vegetation green-up and that the sensitivity of birds to plant phenology varied extensively. Bird arrivalmore »responded more synchronously with vegetation green-up at higher latitudes, where phenological shifts over time are also greater. Critically, species’ migratory traits explained variation in sensitivity to green-up, with species that migrate more slowly, arrive earlier and overwinter further north showing greater responsiveness to earlier springs. Identifying how and why species vary in their ability to shift phenological events is fundamental to predicting species’ vulnerability to climate change. Such variation in sensitivity across taxa, with long-distance neotropical migrants exhibiting reduced synchrony, may help to explain substantial declines in these species over the last several decades.« less
5. Exposure to Non-Native Tropical Milkweed Promotes Reproductive Development in Migratory Monarch Butterflies

   https://doi.org/10.3390/insects10080253  

   Majewska, Ania A. ; Altizer, Sonia ( August 2019 , Insects)

   Background: North American monarchs (Danaus plexippus) are well-known for their long-distance migrations; however, some monarchs within the migratory range have adopted a resident lifestyle and breed year-round at sites where tropical milkweed (Asclepias curassavica) is planted in the southern coastal United States. An important question is whether exposure to exotic milkweed alters monarch migratory physiology, particularly the ability to enter and remain in the hormonally-induced state of reproductive diapause, whereby adults delay reproductive maturity. Cued by cooler temperatures and shorter photoperiods, diapause is a component of the monarch’s migratory syndrome that includes directional flight behavior, lipid accumulation, and the exceptionalmore »longevity of the migratory generation. Methods: Here, we experimentally test how exposure to tropical milkweed during the larval and adult stages influences monarch reproductive status during fall migration. Caterpillars reared under fall-like conditions were fed tropical versus native milkweed diets, and wild adult migrants were placed in outdoor flight cages with tropical milkweed, native milkweed, or no milkweed. Results: We found that monarchs exposed to tropical milkweed as larvae were more likely to be reproductively active (exhibit mating behavior in males and develop mature eggs in females) compared to monarchs exposed to native milkweed. Among wild-caught fall migrants, females exposed to tropical milkweed showed greater egg development than females exposed to native or no milkweed, although a similar response was not observed for males. Conclusions: Our study provides evidence that exposure to tropical milkweed can increase monarch reproductive activity, which could promote continued residency at year-round breeding sites and decrease monarch migratory propensity.« less