Research led by the University of East Anglia (UEA) sheds new light on how mountain birds adapt to changes in climate.
Scientists know that species diversity changes as you go up a mountain, but it is not clearly understood why this is the case.
One theory is that it is mostly because of long-term evolution, and the climate niches species have adapted to over millions of years. Another - the 'energy efficiency' hypothesis - suggests it is about how species today manage their energy budgets and compete for available resources that vary in space and time.
To test this, researchers looked at seasonal changes in the elevational distributions of birds - how high in the mountain birds go at different times of year - for nearly 11,000 avian populations across 34 mountain regions worldwide.
Publishing their findings today in the journal Science Advances, they found that many birds do not strictly follow the temperatures they are supposedly adapted to.
Instead, their movements match what would be expected if they were trying to use and acquire energy in the most efficient way based on today's environments, with computer models simulating what birds should do to save energy corresponding with what they do in real life.
Lead author Dr Marius Somveille, of UEA's School of Environmental Sciences, said: "A lot of mountain birds perform altitudinal migration - moving up and down the mountain with the seasons. This behaviour is common but not well-studied, and it has long been debated whether environmental conditions and biodiversity change with elevation in the same way they do with increasing latitude.
"We found that energy efficiency appears to drive both the seasonal distribution of birds across latitudes and along mountain slopes. This suggests that elevational gradients in avian distributions might be a condensed version of corresponding latitudinal gradients, and that altitudinal migration serves the same ecological purpose as long-distance migration, like flying towards the tropics for winter, saving energy and surviving in changing conditions.
"Understanding this is important as it helps us better predict how mountain birds will cope with global change."
Dr Somveille added: "Human activity is affecting where energy and resources are available in mountain environments. Lower elevations are losing habitat due to human activity, while higher elevations stay more protected because they're harder to reach. These shifts are likely to significantly change where birds can live and how they spread out across mountains."
The team used publicly available participatory science data for 10,998 populations, belonging to 2684 species, to provide the most extensive quantification of seasonal distribution patterns of mountain birds to date.
Focusing on elevational gradients as natural replicates across mountain regions worldwide, and using seasonality as a natural experiment, the observed seasonal distribution of mountain birds was compared with computer model predictions based on energy efficiency.
The researchers found more than 30 per cent of the avian populations studied that live year-round on the mountain slopes are altitudinal migrants - defined as having average seasonal altitudes separated by more than 200 metres - confirming altitudinal migration to be a "notable phenomenon globally".
Among these, only a few populations radically shift their distribution along the elevational gradient, for example by more than 1000 metres on average, mirroring the pattern observed for latitudinal migration, where few avian species migrate very long distances.
Altitudinal migration was found to be widespread - in 339 of 1852 populations - within the equatorial tropics despite minimal seasonal temperature changes in these regions.
However, the proportion of altitudinal migrants in a mountain region nonetheless increases with latitude, for example the further north of the equator that populations are, with the tropical Southern Ghats of India having approximately 20 per cent of bird species as altitudinal migrants, while the temperate Swiss Alps have about 57 per cent.
Located in subtropical mid-latitude, eastern Taiwan has approximately 43 per cent of bird species that are altitudinal migrants, such as the Taiwan Yuhina. Overall, this latitudinal pattern supports the idea that altitudinal migration is an adaptation to seasonality.
"Overall, our results suggest that the seasonal distribution of birds in mountains is largely shaped by a complex interplay between species minimising energy costs while maximizing energy acquisition and considering what competitors are doing," said Dr Somveille.
"Also, that altitudinal migration is a behavioural mechanism allowing birds to optimise their energy budgets in the face of seasonality and competition. This explains the many populations that appear to engage in upslope migration during the colder season, as a distributional strategy that is energetically efficient for some species given the dynamics of competition for access to food."
Dr Somveille, who started the research while at University College London, collaborated with scientists from the Georgia Institute of Technology and Yale University in the US, and Academia Sinica in Taiwan. It was supported by funding from the Wolfson Foundation and Royal Society.
'Climate, ecological dynamics, and the seasonal distribution of birds in mountains', Marius Somveille, Benjamin G Freeman, Frank A La Sorte and Mao-Ning Tuanmu, is published in Science Advances on February 6.
