The research, published in the journal Science, reviews laboratory estimates of flight in 50 species of birds and bats, concluding that size, flapping flight, and air currents determine energy expenditure in the wild. Thanks to the atmosphere, flying can be the 'cheapest' mode of travel for animals.
“Wild birds rewrite the rules of travel,” comments ecology professor José Antonio Sánchez Zapata, a signatory of the study. He explains that while an Andean condor and a swan weigh roughly the same (around 11 kilograms), the condor expends approximately one-fifth of the energy the swan does to cover each kilometer of flight. The difference lies in the air: the condor utilizes rising thermal currents and flaps its wings only 0.1% of its flight time, whereas the swan flaps 99%.
Researcher Martina Scacco from the Max Planck Institute notes that the sky is an essential resource that can drastically change the cost of a journey. Emily Shepard from Swansea University adds that laboratory rules suggested condors and swans should have the same energy expenditure due to similar body mass, but observing the condors' soaring and swans' flapping hinted at a difference. The study quantified this difference, finding the energy cost of transport for a condor was 79% lower than for a swan, with gliding flight proving cheaper than swimming.
This is the largest study to examine the energy cost of long-distance travel in wild animals, combining 17 years of tracking data from 850 birds and bats across 38 species. Spanish science contributed data on vultures and storks, thanks to the UMH, the Doñana Biological Station (EBD-CSIC), and the University of Granada (UGR).
The study also considers the role of the atmosphere in the global displacement of numerous wild species, offering a key insight for the conservation and management of aerial species. Changes in atmospheric conditions affect migratory routes and foraging areas, and climate change could alter where and when animals travel in the future.
The team used accelerometers to distinguish between high-energy-cost flapping and low-cost gliding. GPS data was associated with wind and updrafts encountered during the flight, allowing calculation of the energy expended per meter traveled. The 50 species were divided into gliders and flappers, showing that heavier gliding birds had lower transport costs.
The Spanish team provided data from tracking griffon vultures and white storks. The analyzed gliding vultures flew a combined total of over 4.3 million kilometers, flapping only between 0.1% and 7% of the time. The heaviest vulture, the Andean condor, flapped the least, effectively using atmospheric energy like a 'free elevator'.
Birds relying on flapping use an energy-costly movement for variable conditions, where tailwinds assist and headwinds create resistance. The work received funding from European, national, and international sources.
