‘Bird flight and co-operative aerodynamics’, a talk by Dr Steve Portugal
Many creatures form groups to confuse predators, harass them and lessen the chance of each individual being caught. Birds flying in a flock at perhaps 60 mph cannot stop or change course immediately. Leaders must take decisions and communicate them to the others. There are two main flock formations depending on the size of the species. Birds larger than a black-headed gull normally fly in a V formation, others in a tight or loose cluster.
Birds flying in a V generally fly fast and are not very manoeuvrable but they can:
• keep in visual contact with the group;
• see the leader who knows the route;
• see predators further off;
• take advantage of the energetic benefit which the formation provides.
Large birds, like planes, create a vortex as they push air out of the way. Behind the wing is “downwash” where the air is pushed down. This is a bad place for the following bird to be. However, little vortices coming off the wingtips create “upwash”, and a bird which can take advantage of that is in a good place and can save energy. We have been aware of the theory for many years but lacked the means to test it. However, the aviation industry became interested and realised that a study of birds could benefit them. Birds can carry loggers which record the necessary information but these must be retrieved at the end of the flight. Steve solved this problem by studying hand-reared Northern Bald Ibises from the Waldrapp Scheme which aims to re-establish a migratory population in Central Europe. The chicks imprint on human foster parents who spend all day, every day with them. To teach them their migration route the foster parent flies in a micro-plane which the youngsters follow.
The study showed that the youngsters soon discovered the ideal positions in the V to benefit from the upwash. While each bird had its preferred position, right side, left side, front or back they all took a turn at the front where the bird works hardest. A bird which had done a long stint at the front one day was allowed to spend more time at the back the following day but a bird which had not done its share is “attacked” on the ground. Planes have tried to follow the V formation but have not been very successful! During the flight the social status of individuals became apparent. When young Merlin was tired and flew down into a conifer tree the others went on but when Archimedes went down all followed and waited until he was ready to continue. (Merlin was collected by the ground team.) Bald Ibises have friends who they spend most of their time with on the ground.
Pigeons fly in cluster flocks. These are much more densely packed and there is an energetic cost but those at the front don’t have to work as hard as those at the back. They decide the speed and direction of flight. Those further back must avoid collisions and maintain their position. This raises questions which Steve’s work with domestic pigeons set out to answer. When released at a new site the flock experimented a little on the first few flights to determine the best route home and then kept to it. They appeared to use big country pubs as landmarks and would roost overnight in a pub garden if necessary. Birds that started in the front remained there and other birds were always at the back. When the front group was released on its own, they came home but when the back group was released without any “front” birds they were utterly lost even if they had flown the route nearly 200 times. They had not been paying attention to where they were going. For about 14 minutes they flew as a group but then individuals began to peel off. One sat in a tree for 8 hours, another took 9 days to complete a 12 or 13 km flight! When they were released from the same spot a second time their performance was even worse. Why do birds opt for the back where they have to work harder? A batch of young pigeons was let out for a day: half flew off to explore the neighbourhood, the others stayed at home. The birds who went exploring became “front” birds, the stay-at-homes became back of flock birds. It apparently boils down to the individual pigeon’s personality.
We thank Steve for a really fascinating and most enjoyable talk giving us insight into aspects of bird behaviour which it would not have been possible to study without the new technology which is now available. For more information on the Waldrapp scheme go to www.waldrapp.eu.
Helen Baker