Flock of griffon vultures./ iStock

Flying can be less energetically costly than swimming, an activity traditionally considered the most energy-efficient mode of locomotion among vertebrates. This is the finding of an international study published in Science, with contributions from the Spanish National Research Council (CSIC). For the first time, the study explicitly incorporates the role of the atmosphere as an energy source for flying animals.

Textbooks typically rank animal modes of locomotion according to their energetic costs: swimming is considered the least energy-intensive, followed by flying, while running requires the most energy. However, this conventional ranking is based on laboratory measurements of animals swimming in tanks, flying in wind tunnels or running on treadmills. The new international study reveals that this ranking changes under natural conditions.

“Our study shows that soaring birds can travel at an energetic cost even lower than that of swimming, which has traditionally been considered the least energy-intensive form of locomotion in vertebrates,” explains Ainara Cortés Avizanda, a CSIC researcher at the Doñana Biological Station (EBD).

Led by the Max Planck Institute of Animal Behavior (MPI-AB) and Swansea University, the study involved 68 researchers from around the world. It is one of the most comprehensive analyses to date of the energetic costs of vertebrate flight under natural conditions. In addition to the EBD, other Spanish institutions contributed to the research, including Miguel Hernández University and the University of Granada..

17 years of tracking to measure the energetic cost of flight

The research combines 17 years of GPS and accelerometer tracking data from 850 individuals belonging to 38 bird and bat species across different regions of the world. In total, the team analysed 1.4 million GPS locations and nearly 195,000 flight segments. These data were complemented by four years of radar observations, providing information on small migratory passerines. For this study, the Doñana Biological Station (CSIC) contributed valuable data collected through GPS tracking projects involving hundreds of individuals from several bird species over recent decades.

The team integrated these records with atmospheric data, including wind conditions and updrafts, to estimate the energetic costs of movement under real-world conditions. This analysis made it possible to calculate how much time animals spent flapping their wings versus soaring and to compare the energy expended per unit of distance travelled.

The results distinguish between two main strategies: relying primarily on flapping flight or exploiting atmospheric currents to soar. Among soaring birds, body size plays an important role: in general, the larger the bird, the less it needs to flap its wings and the lower its cost of transport.

“Soaring birds such as the Andean condor take advantage of rising air currents and flap their wings very little. As a result, their cost of transport can be up to 79% lower than that of birds of similar body mass that rely on continuous flapping, such as the trumpeter swan,” explains José Antonio Donázar, José Antonio Donázar, a CSIC researcher at the EBD. 

Vultures travel more than four million kilometres

Vultures, including the Andean condor, are among the clearest examples of the importance of atmospheric currents. The seven vulture species analysed in the study accumulated 128,000 flight-days and travelled more than 4.3 million kilometres — a distance equivalent to more than 100 trips around the Earth.

During these journeys, the vultures flapped their wings for just 0.1% to 7% of their total flight time. Rather than maintaining continuous flapping, these birds exploit rising columns of air to gain altitude and substantially reduce their energy expenditure.

This strategy, however, depends on favourable atmospheric conditions. The study also reveals that larger soaring birds have less flexibility in deciding when and where to fly because they depend more heavily on the availability of suitable updrafts.

Wind can help or hinder movement

Birds that rely on flapping flight follow a different strategy. Although flying requires greater energy expenditure, it allows them to operate under a wider range of environmental conditions.

Wind has a particularly important influence on this group. Tailwinds facilitate movement, while headwinds increase aerodynamic drag and can raise the energetic cost of flight.

Taken together, the findings show that the atmosphere can act both as a source of energy and as a factor that hinders movement. Understanding atmospheric conditions is therefore essential to explaining the movement strategies of different species.

Implications for conservation in a changing climate

These findings provide an important tool for conservation. They offer a new framework for predicting the energetic costs of flight and determining how climate change could affect where and when animals travel in the future.

“Changes in wind patterns and thermal updrafts associated with climate change could alter migration routes, geographic distributions and how many species use the landscape,” explains a Julio Blas, a CSIC researcher at the EBD and co-author of the paper.

The study provides a framework for estimating the energetic costs of flight under natural conditions and exploring how changes in the atmosphere may influence animal movements. This information can help anticipate changes in movement patterns and improve tools for biodiversity conservation and management.

“This study highlights a particularly important idea: the atmosphere is not simply the medium through which birds and bats fly, but an essential energy resource that can determine when, where and how they move,” says Cortés-Avizanda. “This perspective changes how we understand animal mobility and provides tools to anticipate how flying species will respond to a world with increasingly variable climate patterns.”