Have you ever wondered how it is that birds can fly for a long time without having to take a break and recover their energy? The amount of energy birds expend when moving can be a determining factor for when they finish their migrations. During flight, birds may choose to flap their wings as fast as they can or over as wide an arc as they need to maintain steady flight to make it to a suitable spot at the end of migration without running out of fuel. Researchers are trying to figure out how important these two flying characteristics are for energy savings and how they may change because of the demands of the environment.
While there is existing evidence that birds beat their wings slower as their energy demands decrease, previous work has shown that this might not be entirely true. Knowing this, Krishnamoorthy Krishnan from Swansea University, UK, and a group of more than 20 researchers from around the world were motivated to use the information of wing movement from 14 different species in the wild to determine how different birds behave when they are flying. But first, Krishnan and the team developed a new technique to estimate how wide birds sweep their wings up and down during each wingbeat, in addition to how fast they move, based on the movements detected by a motion sensor mounted on migrating birds. In the lab, they attached a motion sensor and a magnetometer to the back of two pigeons (Columbia livia) and a dunlin (Calidris alpha) as well as a magnet on one wing of each bird, which the magnetometer detected each time the wing swept up and down, to measure how wide and how fast the wings were sweeping. The team then set the birds the challenge of flying at different speeds in a wind tunnel to compare how well the motion sensor recorded each wingbeat picked up by the magnetometer.
Having confirmed that they could detect wingbeat signatures in the movements recorded by the motion sensors as the birds flew in the wind tunnel, the team then investigated the maneuvers of 12 wild species, including the red-tailed tropicbird (Phaethon rubricauda), the western barn owl (Tyto alba) and feral pigeons in Germany to learn more about how these birds use their wings during flight across different airspeeds and climbing rates in their natural environment. The tropicbird and barn owl swept their wings with a wider arc as well as faster when the birds were climbing. Airspeed had no effect on any of these three birds in the wild. In contrast, the data from the wind tunnel suggested that pigeons needed wider wing movements when the airspeed increased, indicating that the birds adopt a different flying strategy whenever the wind changes, which could be associated with increased energy demands for a more challenging flight. Lastly, Krishnan and colleagues were curious to know whether the size and shape of the bird can play a role in how fast the birds move their wings. However, after looking at all 14 species, they concluded that all have very similar wing movement patterns, regardless of their different sizes or shapes.
Krishnan and colleagues’ lab-based study has revealed how researchers can extract previously unnoticed detail about bird wing movements in the wild from motion sensors mounted on their bodies. Their observations show that birds beat their wings with a wider arc when they need more propulsion under challenging conditions, such as when climbing, or when they encounter winds during long migrations.
