September 9, 2011

The Amazing Common Loon (photos by Donald Collins)

The common loon is one of five species of diving predatory birds, inhabits northern freshwater lakes around the world. Loons have an amazing physiology to warrent a Physics Photo of the Week. The beading of water on the loon's head as it emerged from a dive indicates the oil from the bird's preen glands serving to keep the feathers dry. Loon physiology includes many more areas which physics helps to explain:

Loons are excellent flyers

In spite of loons spending most of their time on the water and in spite

of the fact that their wing areas are small in comparison to their weight, loons can fly in excess of 75 km/hr (~50 mile/hr). The photo at right shows a loon flying. Unlike other large birds that can glide and soar, loons must continually flap their wings in order to remain aloft. The photo at right of a loon in flight shows the characteristic open mouth, presumably to enable more rapid air intake. Notice also that the webbed feet appear to serve as the bird's tail - possibly aiding in steering and elevation control. Birds' lung physiology is considerably different from mammals' lungs. To provide more efficient absorption of oxygen to satisfy the large energy demands for flight, the air passes through birds' lungs in only one direction by means of air sacs that serve as pumps both posterior to each lung and anterior to each lung. In contrast humans and mammals have a "tidal" in-out breathing mechansim - the air exits the lungs through the same bronchial tube through which the air enters. Mammals lungs always have some "spent" air in their lungs, wheras birds always have a continuous supply of fresh air.

Loons are excellent swimmers and divers

Loons spend most of their time in and on the water - feeding, hunting, sleeping. Their bones are largely solid (not hollow like those of many birds) hence denser. Their feet are powerful and placed far back on the bird for strong propulsion under water. Their relatively short wings are sometimes used for maneuvering under water. They can stay submerged for several minutes at a time, sometimes reaching depths of 60 meters (200 ft). They switch their muscle energy needs to an anerobic mechanism to enable long dives. However, the heavy bones and short wingspan require that the loons run long distances on top of the water to reach take-off speed. They have been known to be trapped on small ponds that don't have adequate "runways" for take-off. Once airborne, however, they reach remarkable flight speed.

Loons have excellent vision

Loons have excellent vision both in air and underwater. Human's vision is very poor (without goggles) under water. That is because most of the eye focusing is accomplished by the cornea - the curved outer surface of the eye. Underwater, because water and the cornea have nearly the same refractive index, the human cornea loses its refracting power. It has been hypothesized that loons have an inner transparent eyelid (of either a different refractive index or different curvature) that enables sharp underwater vision.

Loons can live in both fresh water and salt water

Loons spend all their winter time in the oceans (when the freshwater lakes are frozen). Therefore they must be able to drink salt water. This ability is provided by means of a gland in the loon's head that filters the salt water (presumably similar to a kidney's function).

Unique vocalizations

Loon sounds are indeed eerie and impressive. The most well-known sound is the wail - a three syllable up and down pitched: "ooooo-eeeeee-oo" sound. These sounds are recorded and available at many web sites (You Tube and the Lab of Ornithology and grainy8films). The "eeeeee" pitch sounds exactly an octave above (twice the frequency) the "ooo" pitch. This may be due in part to birds' voice mechanism - birds don't have vocal cords as mammals do. The bird voicebox is deep in the bronchial tubes called the syrinx. Muscles and ligaments in the walls of the tubes control the frequency of the natural vibrations in the tube walls as air flows through the tubes. The rest of the tube between the syrinx to the nostril/mouth serves as a resonant pipe (as an organ pipe). By exhaling with more force, the tube can vibrate with twice the frequency. This is only an hypothesis about the mechanism of the loon wail and is calling for further research. My wife and I have seen a loon produce the wail sound as we leisurely paddled our canoe past a resting loon. The loon stretched its neck and produced the sound without opening its beak. (I have also found a YouTube video by mudslinger of the same effect. The wail sound appears in the final quarter of the video segment)

Another unanswered question:

Loons spend much of the day and night on northern lakes. The eerie loon call is a symbol of northern aquatic wilderness throughout much of the night. Do loons hunt underwater at night? If so do they echolocate their prey?