| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
| |
|
|
|
WILD LIVES Xantus's Murrelets Go to Sea A little ball of fluff masked and caped in black down, its underside stark white, clambers out from the boulder it hatched behind two days earlier. Following the shrill, broken twitters of its parents, who scramble just ahead of the young bird, it fumbles along a steep, rocky slope leading to a cliff on California’s Santa Barbara Island. The adults reach the edge of the cliff and take off, flapping fervently down to the sea. The chick, about the size of a Hostess cupcake, has never flown. But hardwired to follow its parents’ chirps, it reaches the ledge, and steps over. Bouncing down the cliff, bill-over-webbed toes, the one- to two-ounce wad of feathers tumbles softly to the crashing, white surf that foams 100 feet below. There, the unscathed young Xantus’s murrelet reunites with its parents, and together they paddle out to sea under cover of darkness. One of the world’s rarest seabirds, the Xantus’s murrelet (Synthliboramphus hypoleucus) plunges into life with a splash, and lives inconspicuously far out at sea. Found in a very narrow range off the Pacific Coast of North America, the birds feed 60 or more miles out on the ocean, returning to land only to breed. Although their lives at sea remain largely a mystery, the dark, dangerous waters are a safe haven for the small, vulnerable birds. Swimming partly submerged in the water, necks outstretched like periscopes, their black backs are barely visible against the dark water. They do not flock, and adults are rarely found in groups larger than two. These black-and-white members of the alcid family, which includes other murrelets, the larger murres, puffins, and other seabirds, are expert divers and “fly” underwater, penguin fashion, to feed. Propelling themselves with short, strong strokes of their wings and steering with webbed feet, adults catch crustaceans and small fishes such as anchovies and larval rockfishes. Miniature, fuzzy replicas of adults, chicks are born wildly precocious and can dive and swim at least several yards underwater the instant after they shuffle—or belly flop—into the ocean. Although born swimmers, they must learn how to hunt. So families remain as a unit, the parents feeding one or two young until they can survive on their own. After about two months, young are able to fly, and they leave their parents. Unlike some alcids, which often run across the water like geese to initiate take-off, Xantus’s murrelets can rocket into flight straight up from the water’s surface. When independent juveniles reach sexual maturity, probably at two or three years of age, their biological clocks lead them near rocky islands off southern California and northwestern Baja California, where they congregate and seek out mates. The birds choose mates before approaching an island. Then, a breeding pair searches for a nest site in darkness, when their natural predators—gulls, owls, and peregrine falcons—are less likely to capture them. Throughout their island stay, these cautious birds will continue to leave and return to their nests only at nighttime. Often settling on steep hillsides or cliffs, they seek out inconspicuous nest sites in crevices, deep within caves, or underneath dense shrubs such as silverlace, cholla, or wild buckwheat. Like most alcids, these birds don’t expend energy searching for sticks, leaves, or any other nesting material. Any depression in the soil or rock that keeps their eggs from rolling away will do. Once the site is chosen, the pair will return every year, probably for life. Despite such secretive habits, Xantus’s murrelets are at risk of being eaten out of existence. The world breeding population, a mere 10 to 20 thousand individuals, nests in only four main colonies, one on California’s Channel Islands and three off Baja California. While feral cats are wiping out nearly all of Baja’s colonies, black rats introduced to California’s Anacapa Island, a series of three islets off the coast near Los Angeles, are rapidly devouring the murrelets. The rats can easily negotiate steep, rocky cliffs and squeeze through tight cracks in the rock to reach the birds’ carefully concealed nests, where they feast on eggs and young and aggressively drive away the adults. But the future of Anacapa’s murrelets is brightening, thanks to an aggressive National Park Service plan to remove the exotic rodents. Unlike rats, people cannot access the murrelets’ rugged and steep nesting habitat to place traps, so the Service plans to drop specially designed pellets of rodent poison onto the island by helicopter. To minimize the impact on other island occupants, the inch-long pellets have been dyed green or blue—colors birds don’t associate with food. Moreover, the poison has a delayed effect on the rats. After ingesting a lethal dose, they normally seek out their underground burrows, dying out of reach of hawks, falcons, and scavengers such as common ravens that may visit the island. Preliminary research has shown that 80 to 100 percent of poisoned rats die in their burrows. Slated to begin this November, the helicopter drops target Anacapa at a time when rat populations are lowest, because of a dwindling food supply, and all the seabirds have left for the winter. The Service hopes to eliminate the rats from all three islets by the end of 2002. This should dramatically increase the birds’ nesting success in the United States by providing safe habitat for their eggs. An adult female Xantus’s murrelet typically lays two eggs, which, relative to body mass, are among the largest of any bird. Each egg is about two inches long and weighs nearly a quarter of the six-ounce adult bird. Laying one is comparable to a 125-pound woman giving birth to a nearly 28-pound baby. Producing these large eggs demands a lot of resources from the mother’s body, so after laying one, the female must wait about eight days before the second is fully developed. Not until this second egg is laid will the parents start incubating the eggs. About 30 days later, the two chicks hatch usually within hours of each other. Over the next two days, the young won’t eat, losing up to eight percent of their body weight. Then, lighter than they’ll ever be, they follow their twittering parents to the edge of their nest and make their dramatic dive into life. Dave Brian Butvill is Assistant Editor of California Wild. |