Researchers studied 302 Himalayan species over the span of 10 years to analyse migrational patterns
The majority of birds in the Himalayas migrate downslope during the winter, while a small proportion of species in low elevations migrate upslope, a new study examining birds from eastern and western Himalayas found.
Diet and habitat availability also drives the seasonal elevational shifts in the birds, said Himalayan birds that show the greatest elevational shifts remain within the narrowest thermal regimes, which was recently published in the journal Global Ecology and Biogeography.
The study examined seasonal elevational shifts among 302 Himalayan bird species using eBird, a community science dataset, for the years 2011 through 2022.
The scientists examined diet, dispersal capability and thermal regime to see if they could influence elevational shifts. Thermal regime is a term used to highlight the temporal and spatial distribution of temperature.
It found that 65 per cent of the species in the eastern and western Himalayas displayed seasonal elevational shifts within their species range compared with other bird communities reported in high seasonality or the peak season.
However, the researchers think that climate change might have an impact on their behaviour.
Species that breed at high altitudes in the Himalayas do not spend the winter there, the paper said. Urocissa flavirostris, Dendrocopos darjellensis, Garrulax ocellatus, and Alcippe vinipectus are four of 37 species known to have a summer median elevational limit of over 2,500 metres. The researchers did not, however, observe any short or small upslope shifts for these birds.
Temperate mountains, especially at high altitudes with cold winters and warm summers, experience a significant shift in seasonality, the researchers wrote.
The data of 37,944 birds from the western Himalayas and 9,992 from the eastern regions, totalling 47,936 were examined. In the eastern Himalayas, the elevational distribution among 198 species showed that 55 per cent of the birds had significant lower shifts while 59 per cent reflected median shifts and 56 per cent recorded upper elevations limits, the study found.
About 10 per cent of the species showed upslope shifts while 35 per cent of them did not depict any shift in elevation. Species shifting on higher elevations were lower elevation birds, the study noted.
Out of 221 species In the western Himalayas, 57 per cent showed downslope shift, five per cent showed upslope shifts and about 38 per cent showed no elevational shift changes. Birds in the east and west shows significant negative association with regards to thermal regime across lower, median and upper elevational limits, the research paper found.
However, the range of the thermal regime varied between 10 and 30 degrees Celsus and was never found to be zero. The availability of food was a deciding factor for the birds to determine the elevational shift.
As for dispersal ability or the ability of the species to establish itself in that area, fruit-eating species (frugivores) shifted shorter distances compared to invertivores, that feed on invertebrates, the paper said.
The shifting of frugivores was “statistically significant at the median limit in the western Himalayas and all three limits in the eastern Himalayas,” the paper said.
“In the eastern Himalayas, granivores (species eating eating grain and seeds) shifted further downslope which was significant at their lower limit while omnivores shifted shorter distances downslope which was significant at their upper limit. Invertivores and granivores on average shifted farther downslope than omnivores and frugivores,” the researchers wrote.
About 117 species common to the eastern and western Himalayas had greater elevational shifts for populations residing in the west compared to the east, which was found to be at the median elevational limit, the analysis found.
Birds were found to be in significantly narrower thermal regimes in the eastern Himalayas than the west across all elevational limits.
The research noted that diet and habitat preference restricted the migration of these birds.
Citing an example, it said, “Buff-barred warbler or Phylloscopus pulcher migrates nearly 1,900 metres downslope from 3,700 m to 1,800 m. While theoretically it could move further down to better match its breeding range temperatures, diet and habitat preferences may prevent it from doing so.”
“Elevational movements are relatively short distance, elevational shifts correspond to the wider result of birds moving short distances to track thermal regimes,” it explained further.
However, the researchers could not find the reasons why such pattern of elevational shifts repeats across distantly related groups of populations of birds or taxa.
The scientists assumed survival trade-offs between the seasonal movement into lower elevation communities, which are more species rich and increased interspecific competition against coping with colder temperatures and lower food availability in the winter.
Change in land use and climate change can have a combined effect elevationally migration migrating birds in the future, said Tarun Menon, PhD student at Indian Institute of Science, Bengaluru and lead author of the research.
“Birds that breed at high elevations may have to move higher up the mountain to find breeding habitats. At the same time, when these species migrate downslope in winter, the continuing loss of low elevation habitat may affect the populations of these species,” he said.
However, there would be a limit to which these birds could move, Menon said. Despite their ability to breed at higher elevations, the lack of food and habitat may limit the ability of birds to move upward.
Resident birds are also likely to shift their ranges to higher altitudes, he added. “But they may not be able to move as much as migratory birds, as they generally don't move much and may not be able to adapt to conditions at higher elevations. They will only be able to achieve upslope shifts up to a certain extent,” Menon said.
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