The largest living soaring birds are reluctant flappers. Wing flapping costs calories, and if the next meal is hundreds of kilometres away, saving energy is most important. The Andean condor (Vultur gryphus), a scavenger weighing up to 16 kg, is a soaring specialist. These birds are known to avoid flapping their wings until it's absolutely necessary and rely instead on external air currents to subsidise their flight costs. Given this reliance on air currents, learning how condors interact with their surroundings might reveal the limits to their aerial athleticism and may even offer a glimpse into the past, when impossibly giant birds and pterosaurs roamed the sky.
Hannah Williams, Emily Shepherd and colleagues from Swansea University, UK, together with Sergio Lambertucci and colleagues from the Universidad Nacional del Comahue, Argentina, decided to take a closer look at condors flying in the sky. This goes beyond casual birdwatching – how do you observe an animal cruising 1 km high for hours on end? Williams and Shepherd used remote data logging devices. They caught eight juvenile condors in a mountainous region of Argentina and tagged them with the devices before quickly releasing the animals. The loggers measured the birds’ location and motion, as well as the ambient pressure and temperature around them, and were pre-programmed to drop off the birds while they were roosting. Williams and Shepherd specifically chose to study juvenile birds as they are more likely to roost in accessible locations – handy for retrieving the loggers later.
Incredibly, the condors covered a lot of ground with their wearable tech, with one bird travelling over 300 km in a single day. In addition, they flapped for only 1% of their total flight time of 235 h, which is a record low for powered flight in any bird. One of the condors even went for 5 h in the air, covering 170 km without flapping once. This is a very cheap way to get around, as riding on air currents requires 15 times less energy than powered flapping for the equivalent time. On average, the birds resorted to flapping for only 4 mins per day, with three of these minutes spent taking off from the ground. This behaviour didn't even change much with weather conditions, telling us that the birds’ athletic abilities aren't strictly restricted by the environment around them. Instead, the amount of time they spent flapping was linked to the effort required to take off. The energy used during 3 min of take-off is equivalent to 50 min of soaring, so it's critical for the condors’ calorific budget that they don't land unnecessarily.
The other incidences when the condors resorted to flapping usually occurred as the birds moved between thermal updraughts, particularly when the thermals were weak, or when the birds were at low altitude. Given the risks of grounding, when they are most vulnerable to predation by opportunistic mammals, the condors invest precious energy flapping their wings to get to the next energy-saving thermal.
The researchers conclude that for Andean condors, landing is risky and take-off is expensive, but flight is cheap. These heavy birds exploit their environment to their full potential, allowing them to travel colossal distances on a tight budget.
