Copyright © 2011 by University of Nebraska–Lincoln, all rights reserved. Redistribution or republication in any medium, except as allowed under the Fair Use provisions of U.S. copyright law, requires express written consent from the editors and advance notification of the publisher, the University of Nebraska–Lincoln.
Bird migrations are regular two-way movements of birds that have evolved to assure more space and food for raising offspring as well as increased overwinter survival. Internal rhythms, often related to the reproductive cycle, are triggered by photoperiod (changing amount of daylight), prompting the migration. Weather conditions usually influence date of departure and pace of advancement toward a destination. The result is one of the Great Plains's most spectacular natural events, as millions of birds move northward to breeding grounds and southward to wintering sites over large expanses of grasslands, wetlands, and grain fields.
The endangered whooping crane (Grus
americana) is probably the most renowned
Plains migrant. All of the world's naturally occurring
whoopers (about 170 individuals) migrate
between Arkansas National Wildlife Refuge,
along the Texas coast, and Wood Buffalo
National Park, along the borders of Alberta,
Saskatchewan, and the Northwest Territories,
using a pathway that covers the north to south
extent of the Plains region. Most migrant
waterfowl common to the Plains, such as the
lesser snow goose (Chen caerulescens), white-fronted
goose (Anser albifrons), northern pintail
(Anas acuta), and mallard (A. platyrhynchos),
follow the Central Flyway, a locational
characterization of seasonal waterfowl movements
used for international management of
migratory game birds. The flight strategies of
other Plains migrants, such as raptors (burrowing
owl [Speotyto cunicularia] and ferruginous
hawk [Buteo regalis]), shorebirds
(upland sandpiper [Bartramia longicauda],
long-billed curlew [Numenius americanus],
and red-necked phalarope [Libipes lobatus]),
and songbirds (Harris's sparrow [Zonotricha
querula] and clay-colored sparrow [Spizella
pallida]), are more temporally and spatially
diverse.
Bald eagles (Haliaeetus leucocephalus) are the
first migrators during early spring. The next
arrivals are sandhill cranes (Grus canadensis),
ducks, and geese. During later spring, shorebirds
are followed by songbirds. Some migrant
songbirds, such as the dickcissel (Spiza americana),
arrive on breeding grounds in the
Northern Plains while individuals of the same
species have already completed their nesting in
the Southern Plains and started their migration
to wintering grounds. Arctic nesters such as the
Harris's sparrow move to the Central Plains for
the entire winter, whereas the snow bunting
(Plectrophenax nivalis) moves in and out of the
Central Plains in response to severe weather.
Many Plains migrants use different routes
during spring and fall, possibly to optimize
feeding opportunities or avoid harsh winds.
For example, the American golden plover
(Pluvialis dominica) breeds from the northern
coast of Alaska to the eastern shores of northern
Canada, flies southward along the Atlantic
coast to its wintering area in southern South
America, then forms a migration loop as it
returns to its breeding grounds by flying over
the Great Plains.
Distances that birds migrate vary within
and among species. Individuals that nest in
the southernmost part of the lark bunting's
(Calamospiza melanocorys) breeding range
migrate shorter distances to wintering locations
(less than fifty miles) than those in the
northern part of the breeding grounds (hundreds
of miles). Blue-winged teal (Anas discors)
that nest close to the Arctic Circle and
winter in Argentina move a distance of about
7,000 miles. Long-distance fliers rely on a variety
of cues for navigation, such as the position
of celestial bodies, the magnetic field of
the earth, and landscape features. For example,
shorebirds and cranes closely follow paths
that lie over a series of basin wetlands that
extend from the Texas coast to the Northwest
Territories.
Long-distance migrants may fly without stopping. Others may spend a few hours, days, or even weeks at a particular location for rest and refueling. Birds that fly long distances before stopping rely on fat reserves for energy or winds associated with weather systems. Shorebirds are especially known for moving great distances in short time periods. The American golden plover for example, can move 4,000 miles in less than three days with the help of winds in the direction of flight. Most Great Plains migrants are not equipped to make such marathon flights. Instead, they make regular refueling and drinking stops.
Locations where migrants land for short duration are referred to as stopover sites. Solitary birds or family groups usually use these sites. Examples are the seasonally flooded fields and shallow wetlands used by whooping cranes for feeding and resting. Habitats used by migrating birds for longer time periods to replenish energy for farther movement or gain weight for reproduction are sometimes referred to as staging areas. Large groups of Plains migrants are often associated with wetland-related staging areas, many of which are protected as state or national conservation areas, such as Bosque del Apache, New Mexico, Cheyenne Bottoms Wildlife Management Area, Kansas, J. Clark Salyer National Wildlife Refuge, North Dakota, and Delta Marsh, Manitoba, Canada. Arguably the best location in the world for viewing bird migrations is the Central Platte River Valley and Rainwater Basin of south-central Nebraska, which attracts up to three million geese, fifty-two species of waterbirds, and the world's largest concentration of cranes (500,000) in spring.
Since 1900, habitat alterations, such as channel flow reduction, wetland drainage, and conversion of grassland to cropland, have rendered migrant species more susceptible to diseases and natural disasters such as drought. Traditionally, conservation efforts for Plains species emphasized breeding and wintering locations. Current studies emphasize understanding habitat requirements during migration and how to most effectively protect migrant species.
See also SPORTS AND RECREATION: National Wildlife Refuges / WATER: Cheyenne Bottoms;
Platte River Whooping Crane Maintenance Trust.