Shorebirds especially hold a record for long distance
migration. They are
an interesting group of birds because they breed and overwinter
in vastly different places, traveling long distances during the
spring and fall. They may overwinter in Mexico or Central America
and breed in Canada or Alaska. In between these two phases of their
lives, they must fly and rest and refuel; fly and rest and refuel.
One may ask why don't they just stay in Panama or
Mexico, two of their overwintering stops, the entire year? This
is a good question that can best be explained by a crowding phenomenon.
Food and space are of course always at a premium, whether one is
a human, a monkey or a bird. During the breeding season, places
that during the winter can feed an adult male and female, must now
provide food for young also. Often, there is not enough food to
go around during the breeding season at these overwintering sites.
Over a period of thousands of years, some birds
flew north by chance at the breeding time, and it just so happened
that there, they found more food and less competitors than at their
overwintering grounds. As a result, they were able to produce more
offspring than those who stayed behind, and the northward- tendency
was born. We do not know exactly how this occurred, or where the
first northward sites to breed were, but we do know that these groups
of birds did develop a pattern of movement from overwintering sites
to breeding grounds and back that we now call migration.
Thus today, the majority of shorebird species migrate
from their breeding grounds to their overwintering grounds in the
fall and return to them in the spring. Breeding grounds have an
abundance of food for the birds. With the large amount of daylight
in the north during the summer, plant populations grow, and with
them, the insect populations who eat the plants. Shorebirds feast
on these¡ªeither in the adult form or when they are larvae
in the many pothole ponds throughout the tundra. It is this food
that sustains them and their offspring through the short summer,
giving them fat stores for the long migration south in the early
Most people are familiar with shorebirds, for this
group will feed in coastal mudflats, coastal estuaries or rocky
shores along the North, Central or South American coast, or at inland
ponds, lakes and reservoirs at some point during the annual cycle:
breeding, overwintering or migrating. Thus, shorebirds are an ideal
group to study, for they are widespread, often abundant, and very
predictable in their schedule.
Scientists have been conducting surveys on shorebirds
for over 30 years, in places as far apart as the Yukon-Kuskokwim
Delta and the North Slope in Alaska, the Fraser River Delta and
Vancouver Island in British Columbia, San Francisco Bay in California,
Bahia Santa Maria in Mexico and Panama Bay, just outside Panama
City. These are all major breeding, overwintering or stopover sites
along the west coast of North and Central America.
Shorebirds that nest in Alaska use a large part
of the western hemisphere for stopover sites. The stops are transient,
and shorebirds touch down on North American soil for only a brief
time during their annual cycle, yet all sites are important. They
are links in a migratory chain. Shorebirds can only fly so far before
they need to refuel. However, the long-distance migrations to and
from their breeding grounds, a tendency to aggregate, and a dependence
on wetlands, have placed many shorebirds at risk.
These long distance migrants have wintering, migrating
and breeding ranges that extend over vast areas. During their annual
spring and fall migrations, they are dependent on a variety of critical
habitats like wetlands, estu¬aries, beaches, or rocky shorelines
that are typically limited in size and distribution. Concentration
in some of these areas makes shorebirds vulnerable to environmental
disturbance, e.g. 10% of their population stops at Bah¨ªa
Santa Maria in Sinaloa Mexico.
Because of the tremendous energy demands of these
long distance migratory flights, stopover habitats and resources
for rest and refueling are critical to the survival and successful
reproduction of shorebirds.
Different species of shorebirds have different habitat
needs. These stopover points are "migratory bottlenecks"
that the birds must fly through twice each year. The comparatively
limited availability of these areas, and the prey resources that
they must share, present energetic demands on shorebirds that influence
their migration, reproduction, and even survival.
If contamination, habitat degradation, disturbance
etc. is at these stopover sites, then the birds may fail to breed,
don¡¯t migrate, or become adversely affected by lack of proper
Recent census data from North America have revealed
widespread declining trends, giving ample reasons for concern about
population health for many species in this group, including birds
like sandpipers, knots and plovers. For example, semipalmated sandpipers
in the east and mid-western parts of Canada and the United States
have shown annual declines of negative 2 to 4 percent per year.
Western sandpipers on the west coast have shown similar declines.
Western sandpipers are on the American Bird Conservancy¡¯s
Green List at a ¡° Moderately Abundant Species with Declines
or High Threats¡± and this include birds that are still found
in relatively high numbers, but are declining at an alarming rate.
Population surveys of many shorebird species show
that they co-occur with western sandpipers at some of the same wetlands;
thus, any information that we can find from this very common species
will apply to a myriad of other Neotropical migrant shorebird species.
Western sandpipers are one of the most common and
numerous shorebird species. They are found at most wetlands in the
western hemisphere. Even these populations have been declining.
Because they are so common, they are a good model for all shorebirds.
Western sandpipers (Calidris mauri) overwinter
in Central and South America and breed in Alaska. They traverse
the continental United States in spring (northward) and late summer
and fall (southward). On their northward migration along the west
coast, they stage in large numbers on large open mudflats, at estuaries
or alkali flats, where they probe or glean for invertebrates. The
intermountain population, on its northward journey, uses alkali
flats or river banks, ponds or reservoirs where the water level
fluctuates, thus exposing prey-rich mud. Much of the populations
of western sandpipers and many other shorebirds overwinter or pass
through Panama Bay.
Panama is a land bridge between two continents, providing one of
the major movement corridors within the western hemisphere for intercontinental
migrants. Millions of shorebirds migrate through Panama to and from
their overwintering and breeding grounds between North and South
America; others overwinter there.
Between the breeding grounds in Alaska and the overwintering
grounds in Panama Bay, there are 16 major wetlands that have been
set aside as part of the Western Hemisphere Reserve Shorebird Network.
These wetlands are protected by law, and are recognized internationally
Because of the decline of many populations of shorebirds
throughout North America, the Shorebird Group of the Americas, a
consortium of university, government and nonprofit organizations,
has come together to address this problem systematically. They selected
the Western Sandpiper, Calidris mauri, as the first species to begin
the study on migration patterns and changes in population numbers.
This species was chosen in part because they are present along most
wetlands in all north American flyways, they are generalists, eating
many different kinds of food, and because of this, are not as limited
as specialists in the kinds of habitat they may choose. Also, their
population is fairly large compared to that of other shorebird species,
so they are easy to find during surveys. There also have been many
studies on their migration, and the biology is well-known, so historical
stopover, overwintering and breeding sites, as well as historical
numbers from these sites, are well-know also.
Understanding western sandpiper use of wetlands
and its migratory ecology will help to advance understanding of
various other species of shorebirds with similar ecology, and will
help to address a broad range of problems in the conservation of
migratory birds in general, as well as their North American Pacific
coastal and inland habitats. Because western sandpipers occurs from
Alaska to Central and South America, multi-national research teams
have come together to study it throughout its range.
Cooperative international partnerships are the key
to preserving migration corridors. The environment functions as
a whole, and parts of it cannot be isolated. The many partners in
this project displayed in our two power point presentations coordinate
management of the habitat and protection for these migrants on a
broad scale to ensure widespread conservation and maintenance of
these stopover sites.
Below, we present hypotheses for the causes of decline: