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Shorebirds
Order Charadriiformes
Shorebirds are most famous for their incredible feats of migration.
Some fly non-stop for 3-4 days, equivalent to a human running continuous
4-minute miles for 60 hours.
Why do shorebirds migrate?
Actually, it is more amazing that all birds don't migrate. Having
wings, it makes sense for birds to follow changes in availability
of food, breeding sites and to avoid predators.
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Mangrove
and wetland wildlife at
Sungei Buloh Nature Park
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Main
features: Generally
small to medium, with long slender legs, necks and beaks;
wings long and narrow for fast flight.
Status in Singapore: A large variety are visiting
migrants.
World distribution: Worldwide
except Antarctic.
Classification: Order Charadriiformes,
a vast assembly of birds. Shorebirds
make up the bulk, and includes curlews, sandpipers, snipe,
oystercatchers, plovers, dotterels, thick-knees, stone
curlews, ibisbill, stilts, avocets, jacanas, coursers,
pratincoles, seedsnipes, sheathbills. The 3 other major
groups are the gulls, terns
and auks. |
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In
the warmer south, food supplies are sufficient for subsistence but not for
breeding. In the north during the summer, there is an explosion of food
resources due to the long daylight hours. Combined with a lack of permanent
predators due to the long winter season, the north is an ideal place for
breeding. Most birds fly north to the Arctic to breed, instead of south
to Antarctic, because there is more land mass in the north.
Dangers in migration: Staging areas
are essential to allow birds to replenish their reserves. Before migrating,
shorebirds undergo hormonal changes which induce them to lay down fat. They
must double their body weight before they can embark on their migration.
Their reliance on staging areas make migratory shorebirds vulnerable. Without
the staging areas, they may not survive to breed.
In nature, most natural mortality of adult shorebirds occurs on the breeding
grounds and is due to bad weather. Bad weather on breeding grounds also
kills many eggs and young, and may sometimes wipe out breeding for the year.
Even under ideal conditions, shorebirds produce few eggs (average 4) and
fewer young make it the journey to the south safely. Away from the breeding
grounds, mortality among adults is very low. 70-95% of adult birds usually
return to breed. Shorebirds also live a long time (large birds up to 30
years, medium 10-20 years, small 4-10 years).
Females usually leave the breeding sites first. Exhausted from producing
eggs, it is an advantage for them to arrive at the staging area early and
have first pick of the food. Males usually leave next, and the juveniles
later. These waves are not due to the lack of food on the northern summer
grounds, but because there is more food at the staging area for those that
arrive first.
For more about the Mysteries of Migration.
Feeding styles: Shorebirds need to
be efficient feeders. They need to eat up to one-third their body weight
every day to fuel their active lifestyle as well as build up fat reserves
for their long migrations. In some, fat reserves can make up to 30% of their
body weight.
Shorebirds can be broadly divided into either "pickers" or "probers".
Pickers search for food by sight and forage in a typical run-and-peck manner.
They run a long distance, up to a few metres, then abruptly stop with their
heads held high, and sometimes end with a peck. Plovers use this method.
They also have good night vision and they feed both at night and during
the day.
Probers usually have long bills and they stick these into the soft mud or
sand to feel for prey. Some "mine" the surface in a "stitching"
or "sewing-machine" manner. Their bills are not rigid insensitive
probes. The Sandpiper's bill tip is mobile and can act as a finger tip to
grasp or grip prey. Under the horny layer, the bill is rich with tactile
organs.
There are, of course, other ingenious bills. Oystercatchers have a triangular
bill that is a cross between a knife and a chisel. They may use these to
either stab into an open bivalve and severe the muscles that close the shells;
or to smash open the shell. Shorebirds with upturned bills use it to "scythe"
the water, sweeping it back and forth to stir up and snag prey.
Shorebirds have other ways to detect or find prey. Snipe are believed to
be able to use their feet to sense underground prey. Many Plovers foot-tremble:
lifting one foot and vibrating the mud with their toes. These apparently
persuade prey to show themselves to the birds. Turnstones, as their names
imply, find food by turning over seaweed and small stones.
Phalaropes have an ingenious way to gather food at their arctic breeding
sites. They spin above the water at one revolution per second creating a
whirlpool beneath them which whips up bottom-dwelling insect larvae. As
these rise to the surface, they are picked off with the Phalaropes' needle-like
beaks.
How can so many birds find food in the same place?
Although huge flocks of different shorebirds may be found in one location,
they divide up the territory among them. There is a horizontal
division: some search above the tideline, others follow the waterline,
yet others in the shallows and some in deeper water still (those with longer
legs). There is also a vertical division:
some pick titbits off the ground, others probe underground-some deeper than
others, yet others prey on creatures living deeper in the water, and others
on titbits floating on the water surface.
Breeding:
Shorebirds nest on treeless habitats, mainly on the Arctic
tundra. During the summer of long days, the tundra supports an explosion
of insects (larvae and adults) and plant food. This allows shorebirds
to quickly raise their young, which often must migrate soon after
they fledge.
For example, Plover chicks hatch fully feathered and able to move
about and feed on their own soon after. The parents, however, still
have to protect them from predators and the elements; keeping them
cool from the sun and warm when it is cold. The breeding success of
Plovers depend on the population cycles of lemmings and voles. When
these creatures are abundant, predators such as Arctic foxes do not
turn to Plovers, their eggs and chicks. |
The
Mysterious Matings of Shorebirds
Most shorebirds are monogamous, although a few form lifelong pair
bonds. A few are more promiscuous. Some sandpipers are polygynous:
one male mating with many females, and the female incubating and raising
the young on her own. Males are territorial and those with good feeding
grounds attract the most females. In this way, the system makes sense.
Some shorebirds are polyandrous: one female mating with more than
one male, and it is the male that incubates and raises the young,
while the female defends the territory and chases off other females.
(Less than 1% of birds practice this, and most are shorebirds: phalaropes,
spotted sandpiper). Yet others practice sequential polyandry: the
female lays one clutch that is incubated by the male, then a second
clutch that she incubates herself (mountain plover and sanderling). |
LINKS
- BirdWatch
Ireland: Migration-changing with the seasons: excellent explanation
of migration with maps of migration in Europe and Africa; and excellent
illustrations of how birds migrate plus details of experiments to highlight
their talents; the effects of man on bird migration.
- Northern
Prairie Wildlife Research Center: Migration of Birds, based on the
book by by Frederick C. Lincoln; Tons of details about bird migration
in the Americas; origin and evolution, how scientists study bird migration,
migration routes and patterns, and lots lots more.
- The
Why Files: lots of details about bird migration in the Americas;
why and how they do it and threats to migratory birds.
- Birding.About.com:
lots of details about why and how birds migrate, with examples, details
and maps about migration in the Americas.
- Audubon
Society: Bird Migration Facts with lots of details on why wetlands
are crucial for migrating birds.
- The
National Aviary: a brief description of why birds migrate.
- US Fish and Wildlife
Service Sister
Shorebird Schools Home Page: fact sheets about shorebirds in general,
and some very interesting facts about shorebird behaviour.
REFERENCES
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