When it comes to diving, penguins probably are the unrivalled champions of all birds. Their body is a work of wonder that nature has endowed for a perfect plunge into the ocean blue: the smooth contours, strong flippers, and small pointed wings. Some of these traits are directly selected for improved diving ability but others may be merely evolutionary artefacts. Many researchers believe that small wings reduce drag underwater and, therefore, are better suited for diving. Indeed, many aquatic birds that dive to forage also have small, pointed wings. But until recently, these were merely perfunctory observations and hearsay, with no concrete evidence of the supposed benefits of small wings. Studying the effects of wing areas on diving is a tricky business; cross-species studies never give fair comparisons. This is why Eli Bridge of the University of Minnesota decided to study the effect of altered wing size on puffins during the bird's drastically brief moulting season (p. 3003).
Bridge's `laboratory' was SeaWorld California where a large number of puffins are housed as part of an exhibit called `The Penguin Encounter'. The puffins live in a naturalistic habitat with a large pool that allows them to dive underwater. Bridge used two pairs of video cameras to film the bird's diving activity by mounting one camera in front of the pool's viewing window, and the other above the pool pointing straight down. In this way, Bridge could plot the bird's movement in three dimensions and calculate diving parameters such as dive speed and angle of descent.
After hundreds of hours of video footage and laborious calculation, Bridge found that instead of improving the bird's diving performance, wing moult had an unexpectedly adverse effect. During moult, the birds dived a shorter distance with each flap of the wings, and energy output from the wing movement, as measured by work per flap, was also reduced, especially when both primary and secondary feathers were missing. However, Bridge discovered that the puffins seemed to compensate for the impairment by more frequent flapping, diving at the same speed as when their plumage was intact. Bridge believes that the moulted bird's impaired diving ability along with the period of flighlessness caused by wing moult could explain the drive to minimize the moulting season and reduce the period when their ability to forage and avoid predators is compromised.
But if reduced wing areas do not improve diving ability, what drives evolution to select for small, pointed wings in many aquatic birds? Apparently birds with small, pointed wings are adept at high-speed, long-distance flight, essential for rapid movement between specialized habitats. But this comes at the cost of manoeuvrability; small, pointed wings cannot generate lift at low speed, so rapid vertical escape takeoffs are impossible. This is not a big problem for most diving birds because their open aquatic habitats prevent close approach by undetected predators. Similarly, when the birds slow down to land, their small wings stall easily and lose lift. Fortunately, high-speed hard landings are more acceptable on water than on solid surfaces. Bridge's findings suggest that the bird's aquatic habitats relax the constraints on the evolution of small, pointed wings. In other words, those birds can afford to trade manoeuvrability for high-speed flight.
- © The Company of Biologists Limited 2004