Other Environmental Themes

Flight Efficiency

For birds, long-distance migration is a constant balancing act: They need to reach their destinations with the minimum energy expenditure, carrying within their bodies the right amount of energy stores, while stopping for food or rest only when necessary. One well-known trick for saving energy is to fly in formation, an aerodynamically-preferred arrangement that confers numerous benefits, including group cohesion, protection from predators, and unobstructed views for all. Flocks keep in close visual contact, which facilitates communications. Animals can also help each other navigate, with experienced migrators showing youngsters the correct travel routes and best places to stop.

In a 2001 Nature report, French researchers measured wing-beat frequency and heart rate for great white pelicans in Senegal. When flying in a V formation, a pelican's heart rate was 11 to 14 percent lower than when flying solo. The birds reduced their energy output, again by 11 to 14 percent, by gliding for a greater proportion of their flight time.

Another report published that same year reached a surprising conclusion: Catharus thrushes, migrating northward over the Midwest in spring, spent twice as much energy during stopovers as they did in actual flight. Stopovers are an especially big energy drain in cool weather.

A 2003 study of red knots probed a related issue: How much "fuel" should a bird take on during long-distance migrations? Birds pay an energy cost for toting around too much fuel, as more fuel means more weight. Carrying too little fuel, on the other hand, is clearly inefficient, as it would entail frequent stops for food. The Swedish and Dutch researchers found that the penalty for flying with additional fuel was much smaller than previously assumed. Although metabolic requirements increased with fuel load, the mechanical output increased even more. The most probable reason, they say, is that flight muscles simply become more efficient when a bird is operating on a full tank. This could explain why shorebirds like red knots make long, nonstop flights, often travelling 2,500 miles or more without pause.