seasonal mountain bird migration

How energy eFFIciency drives seasonal mountain bird migration Birds move up and down mountains primarily for food, to escape competition from other species and not to track temperature as previously thought, according to a new study; patterns of this migration can guide habitat and species management, and help to understand the impact of agricultural or land-use change on birds Nikhil Sreekandan Migration is the movement of birds across long distances, from temperate to tropical regions during winters, and vice versa. The birds move to areas with more or increasing resources and to stay within the climate niche to which they are adapted. 574c065c-6f6d-4320-820a-b3bea6fef4c7 They also migrate short distances up and down mountains around the world. And a long-standing hypothesis as to why they do this was that they were moving away from warmer areas to cooler ones. However, a new paper in Science Advances has now challenged this predominant theory to paint a more complex picture. Against the gradient In the study, researchers from the U.K., the U.S., and Taiwan analysed citizen science data across 34 mountain regions worldwide and found that elevational migration in birds is widespread even in the equatorial tropics, where the temperature on mountains changes very little across seasons. The data reportedly showed that 36.5% of all migrant mountain bird populations would be in the temperature sweet spot if they stayed put instead of moving around. “A lot of birds actually move upslope during winter, which means they go against the temperature gradient,” Marius Somveille, lead author of the paper and ecology and conservation lecturer at the University of East Anglia in the U.K., said. Instead, the researchers have argued that the seasonal movement of birds in mountains is driven by a need to optimise their energy budgets. “Using citizen science data, we were able to show that birds in mountains across the world move for ease of access to food, to escape competition with other species, and to minimise thermoregulation costs — each contributing towards optimising their energy budget,” Dr. Somveille said. The energy budget is the nite amount of energy a bird has to spend on various activities, including staying warm, nding food, protecting itself, rearing its young, etc. The researchers examined seasonal bird distribution data of 10,998 populations belonging to 2,684 species in the 34 mountain regions. Then they compared real-world distribution patterns recorded in the eBird database with a simulation that related the energy available to the birds in their environment to their energy demands. The simulation was run on the ‘Seasonally Explicit Distributions Simulator’ (SEDS) model — which scientists had originally developed to project the seasonal distribution of birds. BIG SHOT X NASA astronaut and Artemis II commander Reid Wiseman peers out of one of the Orion spacecraft’s main cabin windows, looking back at the earth, as the crew travelled towards the moon on April 2. REUTERS CM THE GIST Birds migrate to areas with more or increasing resources and to stay within the climate niche to which they are adapted. They also migrate short distances up and down mountains around the world In a new study, researchers from the U.K., the U.S., and Taiwan analysed citizen science data across 34 mountain regions worldwide and found that elevational migration in birds is widespread even in the equatorial tropics Ayellow-billed chough in North Sikkim. Resources a ect the way birds are distributed in mountains much more than temperature. DIBYENDU ASH (CC BY-SA) “Here, we adapted it to the case of elevational gradients in mountains,” Dr. Somveille said. “Essentially, it simulates how birds should be distributed if they were optimising for energy e ciency.” The simulation assumed that the energy available on a mountain is proportional to how green the plants nearby were. “The idea is that the greener the environment, the more productive it is and therefore the more energy is owing across its food webs,” Dr. Somveille explained. ‘Revealed a global pattern’ “We begin with a mountain empty of birds, and then we start simulating bird populations one by one. Initially, they position themselves in an area with the most resources and favourable temperature,” he added. “But once more bird populations are added, leading to competition for food, populations start dispersing and positioning themselves di erently, and then at one point, they start to migrate because that’s the best strategy.” The simulation stopped once all the resources in the mountain had been depleted. When the team compared the seasonal distribution from the simulation with the real world, 28 of 34 mountain slopes matched. “The model creating distribution patterns similar to the real world gives us some condence that it is capturing something that potentially explains what the birds are doing,” Dr. Somveille said. Be it human activity or climate change, a better understanding of what a ects resource distribution is key to making predictions for how it will impact birds MARIUS SOMVEILLE Lecturer at the University of East Anglia Tarun Menon, who recently completed his Ph.D. on seasonal elevational migration in the Himalaya at the Indian Institute of Science, Bengaluru, commended the unique SEDS model: “There has been previous research from tropical mountains where birds are tracking fruit availability, which need not be correlated with temperature. But this study looks at mountain ranges worldwide and has revealed a global pattern in terms of energy e ciency.” ‘A better understanding’ When the simulations ran with the worst-case climate scenarios by 2100, it found that the pattern of bird distribution along mountain ranges was largely similar. “The direct e ect on birds will be small, but through a ecting energy e ciency, it would still be enough to generate some movement towards higher elevation,” Dr. Somveille said. The model predicted an average upslope shift of around 129 m in this time for mountain birds. While the study highlighted broad patterns that could guide habitat and species management, Dr. Somveille expressed hope to adapt the model to study speci c species guilds: “Now, with the availability of numerous remote sensing products, can we maybe estimate the density of fruits or insects across a whole mountain? Then we could see which group of bird species could be more a ected by, say, agricultural change or land use change,” he said. “We found that in the winter, there are relatively more terrestrial and ying insects at lower elevations, while in the summer, they are relatively more abundant at higher elevations,” Dr. Menon said, referring to his Ph.D. work on insectivorous birds. “The study

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