As birds diverged from their early ancestors, many exploited aquatic habitats by wading or swimming. Others took to diving.
Diving creates a problem for birds: They need a continuous supply of oxygen and must get rid of carbon dioxide, but diving requires breathing to stop. This would appear to be a paradox — but don’t sell birds short.
For starters, divers have greater blood volume and store more oxygen, as oxyhemoglobin in blood and oxymyoglobin in muscle, than non-divers. Tufted Ducks, for example, have 70 percent more oxygen per kilogram of body weight than Mallards. Carbon-dioxide buildup stimulates birds to breathe and can ultimately force breathing — death for a diver. To counter this problem, divers have a better buffering system that allows them to accumulate more carbon dioxide before breathing.
About six seconds into a dive, a reflex causes general metabolism to decrease. The heart rate slows by about 50 percent, and blood is shunted from the skin, viscera, and musculature — body parts that can tolerate limited oxygen — to the heart and brain, which require a constant supply. The actions help conserve a limited oxygen supply. If required, rapid muscular activity can be accomplished anaerobically. Lactic acid that builds up will be removed metabolically after oxygen is restored.
Plunge divers, such as terns, kingfishers, and gannets (above), spot fish from the air and dive into the water headfirst, sometimes from great distances. If the plunges are successful, the birds catch the fish in their bills. If they miss, they swim quickly to the surface and swallow only their pride.
See photos of kingfishers and gannets.
Pursuit divers chase fish underwater. Because their hips are narrow and their body is cylindrical, they move efficiently through the water. Their legs are placed far back on their body, where they function as propellers and rudders. The best examples of pursuit divers are loons and penguins.
Another problem for diving and underwater swimming is that, during the early evolution of birds, natural selection favored low weight and high buoyancy. Such traits are good for most lifestyles but costly to divers, which want to devote their energy to pursuing prey, not to counteracting buoyancy. So they want to be heavy.
Divers are heavier than non-divers because their long bones are filled with marrow, their muscle mass is greater (especially in the legs), and most have heavier plumages, which they make waterproof with oil from their enlarged uropygial, or preen, gland.
Other adaptations for divers are smaller wings, laterally compressed lower legs, larger feet, and toes that collapse on the return stroke. Each reduces friction with the water. Sometimes the adaptations create tradeoffs.
Consider dabbling and diving ducks. Dabblers, such as pintails and teal, spring directly into the air, while divers, such as scaup and goldeneyes, have to run on the water to gain enough ground speed for lift. Divers’ small wings are an advantage for diving and underwater swimming, but they are a detriment to flight because the heavy wing loading forces the birds to make many more rapid, energy-expending wing beats.
Legs like oars
Loons are among the best divers. Most of their dives are shallow, but they have reached depths of 180 feet and stayed underwater for 15 minutes. Two of their adaptations are unique to birds. When swimming, they do not suspend their legs under the body. Rather, they extend them laterally, like oars. This arrangement makes steering easy and allows for maximum thrust without interference from the turbulence created by the other foot.
Unlike other birds, the lower leg of loons has an elongate process that extends prominently above the knee. Called the cnemial crest, the ridge expands the surface area to which the large thigh muscles attach, increasing leverage and giving loons powerful leg extension.
Of all birds, penguins are the most perfectly adapted for a swimming-and-diving lifestyle. Their bodies are smooth and streamlined, and their layered, hair-like feathers are continuous over the entire body. Feathers on other birds are arranged in tracts.
Penguin wings are small, flat flippers, and the wing joints are immoveable. The birds literally fly through the water. The legs are attached at the end of the body. The birds use them as rudders for steering. Penguins have a thick layer of subcutaneous fat that provides great insulation, and they are known to swallow pebbles, presumably to serve as ballast that would make diving and swimming more efficient.
While most penguin dives are short and fairly shallow, Emperor Penguins have been observed underwater for 23 minutes and at depths of more than 1,800 feet. During such dives, their heart rates can decrease to five beats per minute. This is an enigma. Scientists have not figured out how Emperor Penguins can survive events such as these, when oxygen partial pressure would seem too low to combine with hemoglobin, and blood flow is too slow to prevent tissue death. Yet, somehow, the penguins make it work.
The ability of birds to dive and swim gracefully underwater, and to cheat death by not breathing, is an additional example of their amazing behavior.
This article from Eldon Greij’s column “Amazing Birds” appeared in the September/October 2015 issue of BirdWatching.
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