How Mountain Birds Use Altitudinal Migration to Save Energy

Have you ever noticed that birds in the mountains appear at different elevations as the seasons change?

At one point in the year, birds may be abundant on the lower slopes. Several months afterwards, their calls can be heard higher up. Although this movement may seem straightforward, research indicates that each flight is part of an intelligent survival strategy.

A worldwide study led by the University of East Anglia has examined how mountain birds react to seasonal shifts and climate.

Rather than responding to temperature alone, their movements reflect a finely balanced trade-off between energy expenditure, food availability and competition.

Birds shift elevation to save energy

Bird species richness generally declines with increasing elevation. The lower parts of mountains accommodate numerous species, whereas higher areas support fewer.

One long-established explanation connects this distribution to evolution. Over millions of years, species became adapted to particular climate zones and remained within limited temperature ranges.

A second explanation centres on energy. Birds require fuel to fly, stay warm and breed, while food supplies vary with both season and elevation. From an energy perspective, birds travel towards locations where the rewards outweigh the effort required.

These shifts are therefore shaped by present-day conditions, rather than ancient evolutionary history alone.

Researching birds across the world

The scientists examined seasonal movements in 34 mountain regions around the world. Their dataset included 10,998 bird populations representing 2,684 species.

The observations were supplied through participatory science programmes, in which birdwatchers contributed records over many years.

Seasonal elevation records enabled the team to measure how far birds travelled uphill or downhill. They then used computer models to forecast movement patterns according to energy efficiency.

The model results closely reflected observations from the real world, providing strong evidence for explanations based on energy.

Mountain birds migrate vertically

Many birds living in mountains undertake altitudinal migration, travelling up and down slopes instead of migrating between continents. They alter their elevation between breeding and non-breeding periods, commonly by only a few hundred metres.

“A lot of mountain birds perform altitudinal migration moving up and down the mountain with the seasons. This behavior is common but not well studied,” said Dr. Marius Somveille from UEA’s School of Environmental Sciences.

The study found that approximately 31 percent of bird populations resident on mountain slopes use altitudinal migration.

Just a small number travelled more than 1,000 metres, much as only a limited number of bird species migrate exceptionally long distances across latitudes.

Energy directs mountain bird movement

Were temperature the only factor controlling these movements, birds would closely track warm areas throughout the year. The evidence instead reveals another pattern. During colder periods, many populations moved uphill, widening their exposure to temperatures rather than escaping the cold.

This pattern supports the theory of energy efficiency. Birds weigh the costs of keeping warm against food access and competition.

Because they are endothermic, birds can tolerate cooler conditions when sufficient food is available. The energy gained through improved feeding opportunities can exceed the additional cost of producing heat.

The computer simulations applied a cost-benefit method. They assessed energy obtained from vegetation productivity against energy costs associated with thermoregulation and breeding.

Their outcomes corresponded closely with the recorded distribution of birds across seasons and elevations.

Latitude and regional patterns

Altitudinal migration was also recorded close to the equator, where seasonal shifts in temperature are limited.

Around 18 percent of tropical bird populations displayed vertical movements. In these regions, food supplies and competition are likely to be the main drivers.

Latitude nevertheless influenced the broader trend. Approximately 20 percent of bird species in India’s Southern Ghats undertook altitudinal migration, compared with roughly 43 percent in eastern Taiwan.

In the Swiss Alps, the proportion rose to nearly 57 percent. Stronger seasonality at greater distances from the equator made vertical movement more beneficial.

Climate change and future bird movements

The research identified a notable relationship: climbing 115 metres up a mountain had effects comparable to travelling one degree of latitude north or south. Mountains, in other words, function as compressed forms of global migration routes.

“We found that energy efficiency appears to drive both the seasonal distribution of birds across latitudes and along mountain slopes,” said Dr. Somveille.

Altitudinal migration performs a similar function to long distance migration. Each approach allows birds to manage seasonal change while maintaining balanced energy budgets.

Model forecasts indicated that warming by itself would lead to only modest changes in distribution, including small uphill movements caused by reduced heat costs.

Overall, the results indicate that future changes in energy availability at different elevations could alter mountain bird communities more than rising temperatures themselves.