Gulls alter their flight paths to get the benefit of updraughts from buildings, allowing them to conserve energy, and helping explain why they are drawn to urban areas, research conducted by biologists and aerospace engineers has shown. By improving our knowledge of airflow in urban environments, the work could also help with the design of unmanned aerial vehicles.
This is the first evidence that human activities, by altering the pattern of air flow, can change how birds use space. It has long been understood that urban airflows are particularly complex, but there has been no research into the implications of this for birds.
The study, published by the Royal Society, was conducted by Dr Emily Shepard of Swansea University’s College of Science, in collaboration with colleagues from the department of Aerospace Engineering at the University of Bristol. It featured in a special issue of Philosophical Transactions B, co-edited by Dr Shepard and devoted entirely to flight.
The team studied gulls at two sites in Swansea on the seafront, where buildings look out onto the Bay. They used equipment such as anemometers and helium balloons to measure wind speed and direction, and an ornithodolite – like binoculars with a rangefinder and compass – which allows birds to be tracked and their co-ordinates established.
- The number of birds gliding along the line of buildings was strongly correlated with wind direction, with most birds following this path when the wind was blowing at right angles to the buildings, producing updraughts that birds can use to fly at very low cost.
- The energy savings for gulls from judicious use of airspace and flight path selection can be substantial.
- Birds did not always exploit the strongest updraughts, as that would have involved flying very close to the buildings at high speed; instead they opted to fly at quite a conserved range of speeds, and at positions relative to the buildings that could provide them greater stability in the face of cross-wind gusts. This shows that the best strategy is not necessarily to fly in regions with the strongest updraughts - flight safety seems to affect their choice of flight path as well.
The researchers suggest that understanding the strategies used by birds could help with the design of unmanned aerial vehicles which also have to negotiate complex airflows
Dr Emily Shepard of the College of Science at Swansea University said:
Our paper provides a new take on energy-saving strategies amongst gulls. It shows that they alter their flight paths to take advantage of updraughts associated with buildings, in the same way as they would do with cliffs.
Populations of herring gulls and lesser black-backed gulls are declining in many rural areas but expanding in urban locations. We suggest that urban areas are associated with cheap flight costs, as buildings provide ample updraughts under a range of conditions, which might be one of the factors making urban areas profitable for soaring birds.
However, we also show that urban airflows are very complex and that gulls appear to modify their fine-scale trajectories in a manner that affords them greater flight control in the face of gusts. So exploiting airflows in urban areas is not without its risks.
Dr Shepard compared what birds have to face in flight to the challenges faced by humans when swimming:
As terrestrial animals, it can be difficult for humans to imagine what it must be like to travel in a medium that is also moving.
If you swim in a pool when no-one else is swimming, it feels easier to slip through because the water hasn’t been churned up by anyone else. This is just small-scale turbulence. But add on top of this how it feels to swim in the sea, where the tide can pull you back as you try to swim back to the shore.
Now imagine that you have to swim through the sea to get to the office every day. Sometimes the currents would be with you, sometimes they would be against you, this and the choppiness of the water will have a huge effect on how hard you have to work.
If you had to do this every day, you would get pretty good at predicting the sea state and current direction.
This is analogous to the scenario that flying animals face all the time.
The air is hardly ever still and this has a profound effect on their flight behaviour.
Read the Fine-scale flight strategies of gulls in urban airflows indicate risk and reward in city living research paper here.
- Monday 15 August 2016 13.13 BST
- Monday 15 August 2016 13.21 BST
- Catrin Newman