The wandering albatross, with a wingspan of 9 feet, spends much of its life soaring over the sea in search of food. It might fly a thousand miles for just one meal. Its prey, tiny shrimp-like creatures known as krill, together with the fish that eat krill, occurs in large patches, separated by vast expanses of gray featureless ocean. How do albatrosses and their relatives, the petrels, find their food when faced with nothing but water, water everywhere?
Gabrielle Nevitt, a biologist with the department of Neurobiology, Physiology and Behavior at UC Davis, recently discovered a piece to the puzzle. We don't normally think of birds as having a keen sense of smell, but the albatrosses and petrels, known collectively as tubenoses, have complex odor-detecting anatomy in their noses and brains. Scientists have long suspected that the birds may hunt by smell. The question is, what specific scents are they using to find their food?
Nevitt spent months in the stormy Southern Ocean, conducting experiments aboard a British Antarctic Survey ice-breaker. She poured variously-scented slicks of vegetable oil into the ocean, and aerosols into the air, and then observed which odors attracted the birds. It turned out that the crucial ingredient is a gas known as dimethyl-sulfide, or DMS. This naturally occurring odor is given off by microscopic plant plankton when it is being eaten by krill. To us, it smells like rotting seaweed and is vaguely nauseating, but to a hungry seabird, it's like the Golden Arches - a sure sign of lunch.
Oddly, not all tubenoses seem equally sensitive to the aroma. While storm petrels, for example, tracked the odor like bloodhounds, three species of albatrosses showed no preference for DMS-laced oil slicks. Either they cannot detect the gas, or they use other cues to hunt. Nevitt suggests that albatrosses, who forage during the day, may depend more on visual signs of food, such as gatherings of other seabirds on the water, silvery flashes of fish, or feeding groups of seals and whales that also eat krill. In contrast, storm petrels often hunt at night when smell would be more important.
The ability to sniff out plant plankton might also help explain how birds like the storm petrel manage to navigate such huge distances with no apparent "landmarks". Plankton tends to be concentrated in upwelling zones, which in turn are associated with oceanic features such as sea mountains and ridges. The result is a "smellscape" in the air that corresponds to the underwater landscape. For some seabirds, following their noses, might be as good as reading a map.
We are a long way from fully understanding how tubenoses navigate the high seas and find their food. Unfortunately, some species of these magnificent birds might not be around long enough for us to learn their secrets. Nevitt returned from Antarctica with disturbing news. Fishing vessels that use longlines to fish pose a serious threat to seabirds. The submerged lines can be up to 80 miles long, with baited hooks attached along their length. Albatrosses and petrels are attracted to the bait, get hooked, and are pulled under and drowned as the lines sink. Each longline may hold from 3000 - 10,000 hooks.
The number of albatrosses and petrels lost each year to longlines runs well into the tens of thousands, a mortality level that some species cannot withstand. It takes an albatross 10-15 years to mature, and they produce only one chick every one to two years.
The longline problem is solvable. According to Dr. Peter Hodum, of Avian Sciences at UCD, it is a technical matter whose solution lies in designing baits that seabirds cannot detect or retrieve - ones that sink immediately, for example. Dolphin-safe tuna fishing was developed when the public pressed for change. Bird-safe fishing techniques are just as attainable.
Gabrielle Nevitt, Peter Hodum and their colleagues at UC, Davis
are bringing to light not only biological mysteries, but also
the plight of these magnificent birds. As is often the case, the
scientists who work with wild animals turn out to be the best
watchdogs against threats to their survival.
Dept. of Anthropology, University of California, Davis, CA 95616
e-mail: kjstewart@ucdavis.edu