- A study of eBird checklists from 2011 to 2022 found that Himalayan bird species that made the greatest elevational shifts had the narrowest thermal regimes or the range of temperatures a species needs for development.
- About 65% of 302 species observed in eastern and western Himalayas moved downslope for the winter, while 5-10% moved upslope.
- This study implies that the conservation of lower elevation habitats may benefit high elevation birds that move downslope for the winter.
While there is a multitude of global research on the latitudinal migration of wild species, studies on elevational movements, particularly on montane avifauna, are few and far between. Seasonal elevational movement is a widespread behaviour, which when studied closely can shed light on why animals move up or down their montane ecosystem, and how the conservation strategy of these species can be improved.
A study published in September 2023 looked at 302 species of birds in the Himalayas and found that 65% of the species moved downslope for the winter, while 5-10% moved upslope. It found that species that displayed the greatest elevational shifts – which moved up the highest within the geographic area – possessed the narrowest thermal regime, which is the range of temperatures that a species experiences year-round. In other words, birds that made the most movement were those that were adapted to the lowest range of temperatures. “In addition to tracking thermal regimes, diet and potentially habitat availability/ preferences may drive seasonal elevational shifts,” the study mentioned.
“Even the species that moved the most and stayed within the narrowest thermal regimes did not manage to match their exact breeding temperatures,” said Tarun Menon, a Ph.D. scholar at the Indian Institute of Science (IISc) in Bengaluru and the lead author of the study. “This means that if the ideal breeding temperature (summer season) of a bird is, say 20 degrees Celsius, it shifts during the winter to a lower elevation that offers a similar temperature, say 15 degrees (as opposed to sub-zero temperatures in its breeding range). The bird could move further down the mountain to find an elevation where it is 20 degrees, but it doesn’t. There may be several reasons for this – like habitat or food availability.”
Why and how do Himalayan birds move across elevations?
Starkly different environmental characteristics are observed across elevations, despite the physical distance being quite short compared to latitudinal distances that display a similar extent of differences. This causes animals to move across elevations seasonally, a behaviour that has evolved independently across taxa. Such migrations have been observed in at least 12% of all avian species. In this study, the species that showed the greatest shifts tended to be ones that bred at the highest elevations.
Extreme seasonal weather conditions (cold winters, warm summers) that lead to limitations in food availability and physiological adaptations drive birds to move downslope in the winter. During the summers, they move upslope to explore nesting opportunities and forage for insects, fruits and other food. Such movements are termed as ‘elevational shifts’ in this study rather than ‘migrations’ as they may not be displayed by the entire population.
Published in Global Ecology and Biogeography journal, the study used community science data from 47,936 eBird checklists (2011 to 2022) from Sikkim, northern West Bengal, Bhutan, Arunachal Pradesh, Jammu and Kashmir, Ladakh, Uttarakhand and Himachal Pradesh. These ecosystems proved to be ideal for this study, given their high diversity of birds and large elevational gradient, allowing for the study authors to understand what drives movement.
Data cleaning procedures and randomisation tests were used to estimate seasonal movements. “Using these data, we ran phylogenetic least squares regressions (PGLS) to test if the extent of elevational shift is driven by thermal regime, dispersal ability and diet. Diet influenced elevational shift in both eastern and western Himalayas, while dispersal ability did not drive elevational shifts,” stated the study. PGLS allows scientists to look for correlations between various species’ traits while accounting for the relatedness between the species of interest. This is because closely related species are more likely to have similar traits which may lead to spurious correlations.
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Differences in findings between eastern and western Himalayas
“We quantified the difference between breeding and non-breeding elevations as a measure of the elevational shifts in a species,” the study authors explained. As for calculating the thermal regimes, “For each species, we calculated the thermal regime as the difference between the mean maximum June temperature at the species’ breeding elevation and the mean minimum January temperature at the species’ wintering elevation.”
Despite 117 species of birds being common between the eastern and western ranges of the Himalayas, the two regions yielded slightly different findings. Birds in the former displayed narrower thermal regimes than their counterparts in the west. This was attributed to higher species diversity and thus higher interspecific competition in the east, along with lower seasonality.
In the eastern Himalayas, of the 198 bird species studied, 55%, 59% and 56% of species showed a significant downslope shift at their lower, median and upper elevational limits, while 10% showed upslope shifts. Meanwhile, 35%, 31%, and 34% species showed no shifts across seasons. In the western Himalayas, of the 221 species studied, 57%, 65% and 64% of species showed downslope shifts at their lower, median, and upper limits, while 5%, 4% and 5% showed upslope shifts. Meanwhile, 38%, 31% and 31% of species showed no elevational shifts.
“We consider lower, median and upper limits to consider the full range occupied by a shifting population,” said Menon. “All individuals of a species do not shift uniformly to the same elevation.”
Some species that existed in both regions displayed different behaviours. For example, the Whistler’s warbler (Phylloscopus whistleri) shifts 553 metres more in the west than in the east, while the buff-barred warbler (Phylloscopus pulcher) shifts 935 metres more in the east than in the west.
Elevational shifts in conservation and climate change
Understanding elevational shifts may help understand the life history of the Himalayas, notes the study. Invertivores (birds that feed on invertebrates) shifted longer distances than frugivores (that feed on fruit or fruit-like produce). Invertivores and granivores (that feed on seeds or grains) shifted farther downslope than frugivores and omnivores. These statistics suggest the availability and variety of food at various elevations – “insect resources and seeds might be limiting at higher elevations in the winter and not fruit resources,” explained the study.
“Lower elevation habitats cannot be ignored,” emphasised Menon. “They are most at risk to land use changes and urbanisation. These areas are not only important for conserving low elevation bird species but also high elevation species that look to feed during the winters at the lower elevations.”
Menon also highlighted the potential impacts of climate change in the context of elevational shifts, pointing to birds’ adaptability to unpredictable changes in seasonality. “Birds time their elevation movements and breeding to when they want their chicks to hatch, when resources may be most available. This means chicks hatch during summer, when there is light rain and abundance of food. However, with erratic weather patterns induced by climate change, birds are not able to change their cycles as drastically. This could be leading to failure in breeding or a lower rate of success.”
All species are embedded in a network with other species of their ecosystem, and not all species shift or migrate at the same rate, according to Umesh Srinivasan, who is an Assistant Professor at the Centre for Ecological Sciences at IISC and is currently Menon’s advisor. “When some species are compelled to migrate due to rising temperatures, symbiotic relationships may be broken, causing local extinctions.”
“Moreover, when species begin shifting their ranges to areas they’ve never lived in before, they interact in unpredictable ways with the new environment and its species. We don’t know how these interactions may play out,” adds Srinivasan. According to him, many species are able to track temperatures by 93% accuracy, and are sensitive to slight changes in temperature.”
Birds that do not migrate or shift are even more at risk, according to Menon. Rising temperatures are causing birds to keep moving higher upslope – however, this behaviour is likely to be restricted to species that are able to track temperatures and migrate elevationally. Species that don’t migrate at all, constrained by their ability to track temperatures, are highly vulnerable to climate change impacts.
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Banner image: Goulds shortwing. Extreme seasonal weather conditions (cold winters or warm summers) that lead to limitations in food availability and physiological adaptations, drive Himalayan birds to move downslope in the winter. Photo by Tarun Menon.