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Audubon Society Migration Page | Earth Navigators PBS |
CONTENTS: Monarch Butterflies | Sandhill Cranes | The Serengeti |
Click on images for enlargement.
Each year, 300 million monarch butterflies from most of North America vanish into Mexico in what is undoubtedly the greatest of all migrations. They flutter from as far north as Canada and the United States east of the rocky Mountains to twelve ancestral wintering grounds in the remote mountains of central Mexico. Most monarchs west of the Rockies travel to small groves of trees along the California coast. [ Fall Migration Map] In Mexico, in semi-dormancy, the insects cling together for seven months, so tightly packed on fir branches as to color entire groves of trees in orange and black. The tree branches bend under the weight of 30 to 100 million butterflies packed into a three-acre area. From the air the patch of butterfly forest glows bright orange as if on fire. The monarchs choose their winter roosting sites well. Located on Mexico's volcanic belt at altitudes over 9000 feet, the sheltered trees sit on cool, north-facing slopes where low, moist cloud cover prevents their drying out and the tree canopy protects against winds and frost. Most important, the sites are thermally stable, with temperatures ranging from 42 to 60 degrees F, not cold enough to freeze the butterflies, but not warm enough to speed up their metabolism and thereby waste energy needed for flight and reproduction in the spring.
Unlike most temperate-climate insects, monarchs are unable to withstand freezing temperatures. Yet they are dependent on milkweed plants, which thrive in colder climates, for food and protection. For this reason, monarchs spend part of the year in North America, where 100 different species of milkweeds flourish. [Spring Migration Map] Females lay their eggs on the underside of the toxic leaves. When the tiny caterpillars hatch three to twelve days later, they begin eating the leaves, ingesting the plants' poisonous cardiac glycosides. In just two weeks, the caterpillars grow to 3,000 times their birth weight. During the ensuing weeklong metamorphosis, the milkweed toxins are passed from caterpillar to butterfly. Birds quickly get the message to leave these brightly colored insects alone, as the bad-tasting toxins make them vomit. [See Mimicry]
With their food source in the temperate zone and their winter roosting areas in the tropics, monarchs solved the distance problem by becoming the only species in their family that migrates. No other butterfly migrates like the monarch, traveling farther than any other tropical species, up to three thousand miles. Weighing one-fifth of an ounce, the fragile insects can fly eighty miles a day. They reach their summer destinations by leapfrogging north in successive generations - several generations are born, breed, and die before the last generation reaches Canada in time for the winter migration south. With no previous knowledge of the route or the destination, the last generation fuels up on flower nectar and then flies by instinct 2,000 miles south to Mexico -- right back to the same three acre patch of forest. Propelled on breezes at speeds up to thirty-five miles per hour, these "flying flowers" make the return trip in six weeks.
[The Sandhill Crane Page] [Sandhill Migration Map]
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Flocks of greater sandhill cranes, members of he oldest living family of birds in the world, can be seen flying over the open wetlands of New Mexico's Bosque del Apache National Wildlife Refuge. The refuge is a primary wintering area for more than 12,000 of these magnificent birds, together with thousands of snow geese [Flyway Map], Canada geese, and other waterfowl. Six subspecies of sandhill cranes inhabit North America and Cuba. Of these, three are migratory and inhabit the West, where they winter in New Mexico, Texas, Mexico, and parts of the Gulf Coast. When they migrate, cranes fly in long lines across the sky, using rising thermals to carry them to altitudes as high as 13,000 feet. While aloft, or when alarmed, the cranes' sonorous, trumpeting calls can be heard for several miles. The calls are produced through extraordinarily long windpipes which have been compared to French horns.
One of North America's largest birds (the Whooping Crane is larger), sandhill cranes stand up to four feet tall with a six-to-seven-foot wingspan. For the most part, they are shy, wary birds. Each year, 500,000 migrating sandhill cranes converge along the eighty-mile stretch of the Platte River in Nebraska to rest and refuel before continuing their migration north. With 10,000 to 20,000 birds packed per mile, it is the largest single gathering of cranes in the world, constituting roughly 80 percent of the total sandhill crane population in North America.
During the winter, as they fly between their daily feeding and roosting sites, greater sandhill cranes fill the dawn and dusk skies over the Bosque del Apache. See [Bosque del Apache] [Maps] [Bosque Photos] [bird list] [1997-98 Crane Migration] [NWF Article] In the spring they migrate to Gray's Lake National Wildlife Refuge in Idaho and the greater Yellowstone ecosystem. The cranes pair for life and maintain their monogamous bonds by performing musical duets and elaborate bowing and leaping courtship dances at both their wintering and breeding grounds.
Following their spring migration, the crane couples stake out and defend enormous territories on which they build large mound nests. Both parents incubate two brown-spotted eggs and attend the young. Because northern summers last only three months, young cranes must grow quickly in order to be strong enough to survive the fall migration. Sandhill cranes are especially sensitive to human disturbance and need large open areas of shallow wetlands in order to nest successfully. Even under ideal circumstances, coyotes and ravens prey heavily on the eggs and young.
The Whooping Crane Program [Whooping Crane Page] [Migration Map]
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TAKEN FROM: U.S. FISH AND WILDLIFE SERVICE DIVISION OF ENDANGERED SPECIES
FAMILY: Gruidae
STATUS: Endangered, Federal Register, March 11, 1967 Florida Experimental Population, Federal Register, January 22, 1993
DESCRIPTION: The whooping crane is the tallest North American bird. Males, which may approach 1.5 meters in height, are larger than females. Adults are snowy white except for black primary feathers on the wings and a bare red face and crown. The bill is a dark olive-gray, which becomes lighter during the breeding season. The eyes are yellow and the legs and feet are gray-black. Immatures are a reddish cinnamon color that results in a beautiful mottled appearance as the white feather bases extend. The juvenile plumage is gradually replaced through the winter months and becomes predominantly white by the following spring as the dark red crown and face become apparent. Yearlings achieve the typical adult appearance by late in their second summer or fall. The life span is estimated to be 22 to 24 years in the wild. Whooping cranes are omnivorous feeders. They feed on insects, frogs, rodents, small birds, minnows, and berries in the summer. In the winter, they focus on predominantly animal foods, especially blue crabs and clams. They also, however, forage for acorns, snails, crayfish and insects in upland areas.
REPRODUCTION AND DEVELOPMENT: Whooping cranes are monogamous and form life-long pair bonds but will remate following the death of a mate. Whooping cranes return to the same breeding territory in Wood Buffalo National Park, Canada, in April and nest in the same general area each year. Whooping cranes construct nests of bulrush and lay one to three eggs, but mostly two-egg clutches, in late April and early May. The incubation period is about 29 to 31 days. Whooping cranes will renest if the first clutch is lost or destroyed before mid-incubation. Both sexes share incubation and brood-rearing duties but they seldom fledge more than one chick. Autumn migration begins in mid-September, and most birds arrive on the wintering grounds in Texas by late-October to mid-November. Whooping cranes migrate singly, in pairs, in family groups or in small flocks, and are sometimes accompanied by sandhill cranes. They are diurnal migrants, stopping regularly to rest and feed, and use traditional migration staging areas. On the wintering grounds, pairs and family groups occupy and defend territories. Subadults and unpaired adult whooping cranes will form flocks, which use the same habitats but use areas outside occupied territories. Subadults tend to winter in the area where they were raised their first year, and paired cranes often locate their first winter territories near their parents' winter territory. Spring migration is preceded by dancing, unison calling, and frequent flying. Family groups and pairs are the first to leave the refuge in late-March to mid-April. Juveniles and subadults return to summer in the vicinity of their natal area, but are chased away by the adults during migration or shortly after arrival on the breeding grounds. Only one out of four hatched chicks survive to reach the wintering grounds. Whooping cranes generally do not produce fertile eggs until age 4.
RANGE AND POPULATION LEVEL: The historic range of the whooping crane once extended from the Arctic coast south to central Mexico, and from Utah east to New Jersey, into South Carolina, Georgia, and Florida. The historic breeding range once extended across the north-central United States and in the Canadian provinces, Manitoba, Saskatchewan, and Alberta. A separate non-migratory breeding population occurred in southwestern Louisiana. The current nesting range of the self-sustaining natural wild population is restricted to Wood Buffalo National Park in Canada and the current wintering grounds of this population are restricted to the Texas Gulf Coast at Aransas National Wildlife Refuge and vicinity. In June 1994, this population was estimated at 146 individuals.
In 1975, an effort to establish a second, self-sustaining migratory flock was initiated by transferring wild whooping crane eggs from Wood Buffalo National Park to the nests of greater sandhill cranes at Grays Lake National Wildlife Refuge in Idaho. This Rocky Mountain population peaked at 33 birds in 1985, but declined to only seven birds in 1994. Pilot studies are being conducted to reintroduce whooping cranes to a migratory situation by bonding captive-reared chicks with wild cross-fostered adults, which would teach the chicks appropriate survival techniques and a migration pathway.
Captive populations exist at three facilities, including Patuxent Wildlife Research Center, International Crane Foundation, and the Calgary Zoo. Several whooping cranes are located at the San Antonio Zoological Gardens, Rio Grande Zoo Park, and Grace, Idaho.
Florida Experimental Nonessential Population: [See Worldwatch Institute Crane Research Page] An experimental reintroduction of whooping cranes in Florida was initiated in 1993 to establish a non-migratory population at Kissimmee Prairie. This approach was influenced by the apparent failure to establish a migratory population in the Rocky Mountain reintroduction, probably the result of inappropriate sexual imprinting associated due to cross-fostering with sandhill cranes and the lack of a proven technique for establishing a migratory flock. A nonmigratory population will avoid the hazards of migration, and by inhabiting a more geographically limited area than migratory cranes, individuals can more easily find a compatible mate. Captive-reared chicks were released and monitored, and three additional groups of whooping cranes were released in 1994. Bobcat predation was a problem because young birds did not select appropriate roost sites. By June 1994, there were 15 whooping cranes in Florida, and behavioral observations indicate evidence of pair formation with hope of the establishment of a nesting territory in Spring 1995.
HABITAT: The nesting area in Wood Buffalo National Park is a poorly drained region interspersed with numerous potholes. Bulrush is the dominant emergent in the potholes used for nesting. On the wintering grounds at Aransas National Wildlife Refuge in Texas, whooping cranes use the salt marshes that are dominated by salt grass, saltwort, smooth cordgrass, glasswort, and sea ox-eye. They also forage in the interior portions of the refuge, which are gently rolling, sandy, and are characterized by oak brush, grassland, swales, and ponds. Typical plants include live oak, redbay, Bermuda grass, and bluestem. The Kissimmee Prairie experimental population release site includes flat, open palmetto prairie interspersed with shallow wetlands and lakes. The primary release site has shallow wetlands characterized by pickerel weed, nupher, and maiden cane. Other habitats include dry prairie and flatwoods with saw palmetto, various grasses, scattered slash pine, and scattered strands of cypress.
REASONS FOR CURRENT STATUS: The whooping crane population, estimated at 500 to 700 individuals in 1870 had declined to only 16 individuals in the migratory population by 1941 as a consequence of hunting and specimen collection, human disturbance, and conversion of the primary nesting habitat to hay, pastureland, and grain production. The main threat to whooping cranes in the wild is the potential of a hurricane or contaminant spill destroying their wintering habitat on the Texas coast. Collisions with power lines and fences are known hazards to wild whooping cranes. The primary threats to captive birds are disease and parasites. Bobcat predation has been a serious problem with the Florida experimental population.
MANAGEMENT AND PROTECTION: The self-sustaining wild population is protected on public lands in the nesting area at Wood Buffalo National Park in Canada and on the principal wintering area at Aransas National Wildlife Refuge in Texas. A major traditional migratory stopover is at Salt Plains National Wildlife Refuge in Oklahoma. This population is closely monitored throughout the nesting season, on the wintering grounds, and during migration. The Canadian Wildlife Service and the U.S. Fish and Wildlife Service are involved in recovery efforts under a 1990 Memorandum of Understanding (MOU), "Conservation of the Whooping Crane Related to Coordinated Management Activities."
REFERENCES:
U.S. Fish and Wildlife Service. 1986. Whooping Crane Recovery Plan. U.S. Fish and Wildlife Service, Albuquerque, New Mexico. vi + 283 pp.
**U.S. Fish and Wildlife Service. 1993. Endangered and threatened wildlife and plants; Establishment of an experimental nonessential population of whooping cranes in Florida. Federal Register 58:5647-5658.
For more information please contact:
Whooping Crane Coordinator
U.S. Fish and Wildlife Service
Post Office Box 1306
Albuquerque, New Mexico 87103
Telephone: 505/766-2914
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