with the parents within a very few days. The parents lead
the chicks to good foraging areas, often along the shore,
in muddy swales, or in pools and ponds. Both incubation
periods and fledging periods (the time parents continue to
guard the chicks, prior to their being able to fly away) relate
to body size. In larger shorebirds, the incubation period and
fledging period are longer.
Shorebirds are notorious for drawing predators from
their nests with a series of distraction displays that involve
feigning injury, a broken wing, or an inability to fly, only
to fly away when the predator is drawn sufficiently far
from the nest. Even some shorebirds that nest in loose colonies (e.g., stilts) will use distraction displays to entice
predators to leave the area (Gochfeld, 1984).
Foraging
Shorebirds breed on land and usually winter along the
coasts, foraging in estuaries, bays, and along the tide line,
but they also make use of saltmarshes, agricultural land,
and other upland habitats for roosting or foraging at high
tide (Evans-Ogden et al., 2007). They usually locate their
prey visually by plucking it from the water, ground, or
other surface or by probing in the mud. Morphology
affects foraging methods. Shorebird species with larger
bills can eat larger prey items, whereas those with longer
bills can probe deeper in the sand or mud. Those with
longer legs can forage in deeper water (Durell, 2000).
On land and along the shore, shorebirds are generally
omnivorous, eating a wide range of foods including
insects, snails and clams, worms, and other invertebrates,
although some eat fish, fruit, seeds, and even carrion.
Seedsnipes eat only plant material, while at the other
extreme, sheathbills will eat carrion and penguin chicks
(Warnock et al., 2001).
Shorebirds are generally solitary during the breeding
season, when they forage solitarily as well. Some will even
defend foraging territories (Burger and Olla, 1984). During
migration, and on the wintering grounds, shorebirds often
form foraging and roosting flocks of hundreds to thousands.
Particularly dense foraging flocks often occur under circumstances where there is a superabundance of prey that
is renewed regularly, as occurs with rising tides. Shorebirds
forage and roost in dense flocks as an anti-predator strategy.
Foraging in flocks has the advantage of enough eyes to provide early warning of approaching predators, evasive
actions of the flock, and predator swamping (whereby any
one shorebird has a lower probability of being taken if it
is a flock member, rather than being solitary). Predators,
such as hawks, however, can have a negative effect on foraging shorebirds in terms of energy cost and lowered survival (Goss-Custard et al., 2006), and they can affect both
the timing and the routes of migrating shorebirds
(Lank et al., 2003).
Migration and overwintering
Shorebirds breeding in north temperate to Arctic habitats
migrate to warmer climates to overwinter, and well over
half of the shorebird species migrate. For these species,
their strategy is to nest in regions with abundant food supplies during the breeding season, to leave when food is no
longer plentiful, and to migrate to regions with abundant
food for the winter. They usually follow coastlines or go
over the oceans (Morrison, 1984; Warnock et al., 2001).
Except for phalaropes, shorebirds rarely touch the ocean
surface during migration, and many species spend significant time flying over the oceans. Migration is generally
associated with wind patterns, and overwater routes provide
energetic savings compared to following the coast. In the
advent of light-sensitive global location sensors, it has been
possible to document long transoceanic migrations of several species, including nonstop flights of 7,600 km in
Ruddy Turnstone (Arenaria interpres; Minton et al.,
2011). A round-trip migration flight of 26,700 km was
reported in Red Knots (Calidris canutus rufa), with continuous 6-day flights of 8,000 km (Niles et al., 2010, Figure 1).
Shorebird populations
Estimates of shorebird populations are difficult to determine because most nest solitarily or in small breeding colonies and are usually scattered throughout the available
habitat. Estimates can be determined by counting birds
along transects in suitable, known habitats, and by estimating populations on the wintering grounds using aerial
counts (Morrison, 2006). Survival rates can be determined
with mark-recapture rates, either on the breeding grounds
or the wintering grounds (Sandercock, 2003). Determining survival, even for marked birds, is difficult because it
is hard separate whether birds survived, simply returned
elsewhere, were undetected, or the marking method failed
(e.g., bands fell off, batteries died). Understanding population trends of shorebirds, however, requires monitoring
over long periods, which has been done for many species
in North America (Bart and Johnston, 2012) and Australia
(Clemens et al., 2012). An overview of North American
shorebirds shows declines in 80 % of 35 species with data
(Morrison et al., 2001).
Threats and conservation
At least 21 % of the world’s shorebirds (32 of 155) are
listed as species of conservation concerns by BirdLife
International (Piersma et al., 1997), and this number has
increased since then. The main threats to shorebird
populations are habitat loss, human disturbance, commercial harvesting of shorebird prey, hunting, pollution, and
long-term effects from global warming and sea-level rise
(Goss-Custard et al., 2000). While harvesting of the birds
may not be a problem in much of the developed world,
shorebirds are still harvested in some places, such as South
America.
Although Arctic and Antarctic breeding habitat is not
generally threatened (except by climate change, oil development, sea-level rise), habitat for breeding, foraging, and
migrating shorebirds along temperate and tropical regions
is threatened by development, human disturbance, and
586
SHOREBIRDS
the chicks to good foraging areas, often along the shore,
in muddy swales, or in pools and ponds. Both incubation
periods and fledging periods (the time parents continue to
guard the chicks, prior to their being able to fly away) relate
to body size. In larger shorebirds, the incubation period and
fledging period are longer.
Shorebirds are notorious for drawing predators from
their nests with a series of distraction displays that involve
feigning injury, a broken wing, or an inability to fly, only
to fly away when the predator is drawn sufficiently far
from the nest. Even some shorebirds that nest in loose colonies (e.g., stilts) will use distraction displays to entice
predators to leave the area (Gochfeld, 1984).
Foraging
Shorebirds breed on land and usually winter along the
coasts, foraging in estuaries, bays, and along the tide line,
but they also make use of saltmarshes, agricultural land,
and other upland habitats for roosting or foraging at high
tide (Evans-Ogden et al., 2007). They usually locate their
prey visually by plucking it from the water, ground, or
other surface or by probing in the mud. Morphology
affects foraging methods. Shorebird species with larger
bills can eat larger prey items, whereas those with longer
bills can probe deeper in the sand or mud. Those with
longer legs can forage in deeper water (Durell, 2000).
On land and along the shore, shorebirds are generally
omnivorous, eating a wide range of foods including
insects, snails and clams, worms, and other invertebrates,
although some eat fish, fruit, seeds, and even carrion.
Seedsnipes eat only plant material, while at the other
extreme, sheathbills will eat carrion and penguin chicks
(Warnock et al., 2001).
Shorebirds are generally solitary during the breeding
season, when they forage solitarily as well. Some will even
defend foraging territories (Burger and Olla, 1984). During
migration, and on the wintering grounds, shorebirds often
form foraging and roosting flocks of hundreds to thousands.
Particularly dense foraging flocks often occur under circumstances where there is a superabundance of prey that
is renewed regularly, as occurs with rising tides. Shorebirds
forage and roost in dense flocks as an anti-predator strategy.
Foraging in flocks has the advantage of enough eyes to provide early warning of approaching predators, evasive
actions of the flock, and predator swamping (whereby any
one shorebird has a lower probability of being taken if it
is a flock member, rather than being solitary). Predators,
such as hawks, however, can have a negative effect on foraging shorebirds in terms of energy cost and lowered survival (Goss-Custard et al., 2006), and they can affect both
the timing and the routes of migrating shorebirds
(Lank et al., 2003).
Migration and overwintering
Shorebirds breeding in north temperate to Arctic habitats
migrate to warmer climates to overwinter, and well over
half of the shorebird species migrate. For these species,
their strategy is to nest in regions with abundant food supplies during the breeding season, to leave when food is no
longer plentiful, and to migrate to regions with abundant
food for the winter. They usually follow coastlines or go
over the oceans (Morrison, 1984; Warnock et al., 2001).
Except for phalaropes, shorebirds rarely touch the ocean
surface during migration, and many species spend significant time flying over the oceans. Migration is generally
associated with wind patterns, and overwater routes provide
energetic savings compared to following the coast. In the
advent of light-sensitive global location sensors, it has been
possible to document long transoceanic migrations of several species, including nonstop flights of 7,600 km in
Ruddy Turnstone (Arenaria interpres; Minton et al.,
2011). A round-trip migration flight of 26,700 km was
reported in Red Knots (Calidris canutus rufa), with continuous 6-day flights of 8,000 km (Niles et al., 2010, Figure 1).
Shorebird populations
Estimates of shorebird populations are difficult to determine because most nest solitarily or in small breeding colonies and are usually scattered throughout the available
habitat. Estimates can be determined by counting birds
along transects in suitable, known habitats, and by estimating populations on the wintering grounds using aerial
counts (Morrison, 2006). Survival rates can be determined
with mark-recapture rates, either on the breeding grounds
or the wintering grounds (Sandercock, 2003). Determining survival, even for marked birds, is difficult because it
is hard separate whether birds survived, simply returned
elsewhere, were undetected, or the marking method failed
(e.g., bands fell off, batteries died). Understanding population trends of shorebirds, however, requires monitoring
over long periods, which has been done for many species
in North America (Bart and Johnston, 2012) and Australia
(Clemens et al., 2012). An overview of North American
shorebirds shows declines in 80 % of 35 species with data
(Morrison et al., 2001).
Threats and conservation
At least 21 % of the world’s shorebirds (32 of 155) are
listed as species of conservation concerns by BirdLife
International (Piersma et al., 1997), and this number has
increased since then. The main threats to shorebird
populations are habitat loss, human disturbance, commercial harvesting of shorebird prey, hunting, pollution, and
long-term effects from global warming and sea-level rise
(Goss-Custard et al., 2000). While harvesting of the birds
may not be a problem in much of the developed world,
shorebirds are still harvested in some places, such as South
America.
Although Arctic and Antarctic breeding habitat is not
generally threatened (except by climate change, oil development, sea-level rise), habitat for breeding, foraging, and
migrating shorebirds along temperate and tropical regions
is threatened by development, human disturbance, and
586
SHOREBIRDS