The center of the most populated spanwise region that birds inhabited during flight was 0.9 meters, resulting in a wingtip overlap of approximately 0.12 meters. For the ibis, this was approximately 1.2 meters behind the preceding bird, at an angle of 45°.
These birds have a ‘V-favored position’ where they spend most of their time when flying in formation. In northern bald ibis ( Geronticus eremita), the mechanism was revealed by which upwash could be captured. Simultaneously, such positioning also benefits the birds by avoiding the region of downwash, flying into which significantly increases the cost of flight. Theoretical studies have demonstrated that, if birds position themselves optimally in a V-formation, they can take advantage of the upwash from the preceding bird to contribute to lift and reduce power requirements.
This upwash flows outboard of the wing, while there is a region of downwash more centrally behind the main body of the bird. This in turn forms a stream of air trailing from the wingtip and behind into the birds wake, which is commonly referred to as upwash ( Figure 2). As a result, the high-pressure under the wing flows around the tip, and inwards across the dorsal surface of the wing. This pressure difference cannot be maintained beyond the wingtips, as there is no longer a surface to create the pressure gradient. When a bird flies, the lift required for flight is achieved through a pressure difference between the top and bottom of the bird’s wing.
This notion was based on the potential positive aerodynamic interactions that may be taking place between members of the flock. The notion that these distinctive formations provide energetic savings for those individuals not leading the formation was based on applying fixed-wing aerodynamic theory - typically applied to aircraft - to bird flight, even though many birds flap their wings between four and seven times a second. Since then, numerous ideas have been proposed to explain the function of these V-shaped flocks that are a such a familiar sight. Pliny the Elder noted that flocks of geese flew ‘like fast galleys, cleaving the air more easily than if they drove at it with a straight front’. V-formation flightįor centuries, people have been fascinated by V-formation flight. These two flock types are thought to provide different benefits and disadvantages. When birds flock in the air they form either clusters or V-formations. Many studies have demonstrated that individuals who are less well aligned to the group, slower to respond or choose the wrong direction are more likely to be preyed upon. Likewise, for all animals travelling in groups collisions or indecisiveness may lead to individuals being more easily picked out by predators. Therefore, for birds in a flock, decisions need to be made literally on the wing, and at a certain speed. While this speed will vary greatly depending on the size of the bird, all birds will eventually stall below a certain minimum speed. Birds can only fly so slow before they are no longer creating enough lift to keep themselves airborne. However, such signals pertaining to group movement and direction cannot be as subtle or slow for animal groupings moving in air, due to the risk of collision between flock members, or falling out of the sky. For example, herds of ungulates seemingly make group movement decisions through facing the same direction, or through slow directional movements aimed to illicit a response from other herd members. Bird flocks are a particularly exciting study system due to their dynamic nature, the speed at which events and decision making need to occur, and the potential for collisions and injury. How birds assemble and the dynamics within the group have significant implications for individual energy expenditure within the flock. Flapping flight is the most energetically demanding mode of locomotion in vertebrates.
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