Migrant Shorebirds in Panama
Shorebirds, which include the sandpipers, plovers, turnstones, dowitchers, yellowlegs, curlews, godwits, and phalaropes are among the most migratory groups of animals known to science. Thirty-six of the 49 species of shorebirds that breed in North America spend their northern winter in Latin America. Each year millions of individuals make the journey between breeding and wintering grounds. In the spring, they fly north to the habitats of the high arctic. In the fall, they fly south to wetland, grassland, and intertidal habitats distributed throughout Central and South America.
The physical demands of migrating between summer and winter quarters are extreme. For some shorebird species, the round-trip journey covers greater than 30,000 kilometers. Migration over these distances requires a great deal of physical preparation. Following the breeding season, and before departing south, birds may build up as much as fifty percent of their normal body weight in fat reserves. Even so, for many species, the energy requirements needed to reach their destination will exceed this amount several times over. For this reason, distances are typically covered during several nonstop, long-distance flights that are separated by periods of rest and refueling.
For many shorebird species, access to high quality refueling or staging areas is essential to the successful completion of migration. Migrants often arrive on staging areas depleted of energy stores after long periods of nonstop flight. Because adequate staging areas are often separated by long distances, energy stores must be renewed before migration may be resumed. In order for migrants to travel over long distances, a chain of staging areas is required that forms a direct connection between breeding and wintering areas. In many respects, staging areas represent stepping stones. Individually, they allow for the successful completion of a specific segment along the migration pathway. Collectively, these staging areas make migration energetically feasible.
Worldwide, locations with enough prey to refuel large numbers of shorebirds appear to be extremely rare. Because of this, major staging areas have tremendous conservation significance. The concentration of shorebirds within so few sites makes them vulnerable if the health of these sites becomes compromised. Single staging areas often support a high proportion of the individuals for entire species and may ultimately serve to regulate population sizes. Recently, concern for the conservation of migrant shorebirds has been heightened with the finding that populations of some species have declined dramatically since the 1970's. One possible factor contributing to these declines is the loss or degradation of areas used for staging during migration.
The Buff-breasted Sandpiper breeds on the tundra of the high arctic. It migrates south through the interior of North America east of the Rocky Mountains. The Buff-breasted Sandpiper is one of a number of species that migrate through Panama but do not spend the winter. Individuals funnel through Panama in September and October before moving on to wintering grounds on the grasslands of Argentina. While in Panama, Buff-breasteds are typically observed in flocks of 10-20 on short grasslands. Photo by Bryan D. Watts.
Below: The Whimbrel has two distinct breeding populations in North America. The western population breeds in Alaska and the Yukon Territory. The eastern population breeds around Hudson Bay. These two populations apparently migrate and winter on separate coasts.
Above: The Whimbrel is an example of a shorebird species that uses the upper Bay of Panama as a staging area during migration but also as a wintering area for a portion of the population. Migrants begin to move into the upper Bay by late August and most have moved through by late October. While in the upper Bay, most individuals are associated with the intertidal zone but may also be seen in wetland and grassland habitats. Photo by Bryan D. Watts.
Upper Bay of Panama
Panama's position as a land bridge between two huge continents has endowed it with a remarkable diversity of resident plant and animal life. This land bridge also provides one of the major movement corridors within the western hemisphere for intercontinental migrants. Each year millions of birds migrate between their summer breeding grounds in North America and their winter quarters in South America. Like sand through an hour glass, these birds fly from all corners of North America only to converge on the narrow Isthmus of Panama before moving on to the vast reaches of South America.
The country of Panama has an S-shaped land mass that is formed along an east-west axis. Along the Pacific shoreline, the horseshoe-shaped formation nearest to South America, forms the Gulf of Panama. The northern section of the gulf is the Bay of Panama. Two of the most apparent features within the upper Bay include Panama City and the adjacent Panama Canal. Panama City is a modern city with over 600,000 inhabitants. However, moving out from the city either east or west, the coastline becomes increasingly rural and accessible in very few locations. Beyond 10 kilometers, the coastline hosts only occasional fishing villages.
Due to its unusual physical shape and the surrounding climatic conditions, the tidal range within the upper Bay of Panama approaches seven meters. This tidal range combined with the local geology has resulted in the formation of an extraordinary intertidal zone. During periods of low tide, the exposed tidal flats that extend to the east of Panama City may reach several kilometers in width. Many of these tidal flats are associated with mangrove forests and are composed of rich mud formed by fine silt and organic material. West of Panama City, the terrain is more mountainous and the intertidal zone is composed of alternating volcanic rock and tidal flats. Unlike the eastern edge of the Bay, most of the tidal flats are not associated with mangrove forests but are formed from sand and shell fragments.
A view of the shoreline west of Panama City. West of the city, the topography is steep along the coastline. The intertidal zone is generally narrower and the flats are composed of volcanic rock, sand and shell. Photo by Dana S. Bradshaw.
A view of the shoreline east of Panama City. East of the city, the topography along the immediate coastline is a low coastal plain. The intertidal zone is wide and flats are composed of silt, sand, and organic material. Photo by Dana S. Bradshaw.
In addition to the vast intertidal zone, the upper Bay has both wetland and grassland habitats that are also frequently used by shorebirds. Wetland habitats are distributed wherever low areas hold water for a period of weeks. Such areas include marshlands, rice plantations, and margins of streams and ponds. Grassland habitats are concentrated on the military installations surrounding the Pacific entrance to the Canal. Thousands of acres of managed grasslands occur on these installations associated with training areas, activity fields, and airfields. Outside military lands, grassland habitats include pasturelands, agricultural lands, and commercial airfields. Panama hosts a diversity of shorebirds including both migrant and resident species. More than 40 different species of shorebirds may be seen during at least some portion of the year. The upper Bay of Panama, in particular, has long been known to bird watchers as a place where extraordinary numbers of shorebirds congregate during the periods of fall and spring migration. Individuals of some migrant species begin to arrive in the Bay as early as late July and early August. Numbers increase through September and October, only to decrease again through November as individuals of some species continue to move further south. Large numbers of shorebirds remain in the Bay as residents for the entire northern winter. Numbers in the Bay swell again in February and March as birds from more southerly wintering areas begin to move north toward arctic breeding grounds.
A large portion of the shorebird species known from the Bay of Panama were observed during field work in the fall of 1997. These included four species that actually breed in Panama, as well as 30 species that migrate to Panama from various locations in North America. Shorebird concentrations were highest within the intertidal habitats east of Panama City, however, significant numbers were also observed using wetland and grassland habitats.
Large numbers of shorebirds utilize intertidal habitats within the upper Bay of Panama. During the fall of 1997, estimated numbers increased from below 50,000 in late August to over 370,000 by mid-October. Because many birds continue to move through on a weekly basis, peak numbers underestimate the total number of shorebirds using the Bay for the entire fall season. It is estimated that between 1,000,000 and 2,000,000 birds pass through during fall migration within the intertidal zone alone.
Right: Aerial survey team (l-r): Dr. Bryan Watts, Capt. Krish Persaud, and Dana Bradshaw. Low altitude aerial surveys were flown weekly to determine the number and distribution of intertidal shorebirds along a.l00 km shoreline transect. Photo by Marian U. Watts.
Overall, shorebird numbers were dominated by the smaller species. The number of small shorebirds were estimated as a group because they could not be separated to species during aerial surveys. However, repeated surveys within high-tide roost areas revealed that small shorebirds were dominated by Western Sandpipers (78.4%), Semipalmated Sandpipers (13.1%), and Semipalmated Plovers (8.5%). The other most abundant intertidal shorebirds included Millets, Whimbrels, Short-billed Dowitchers, and Black-bellied Plovers. Other less abundant species observed included, among others, American Oystercatcher, Red Knot, Marbled Godwit, Spotted Sandpiper, Wilson's Plover, Sanderling, and Ruddy Turnstone.
Within the intertidal zone, concentrations of shorebirds correspond closely to the distribution of mangrove forests. Over 95 percent of the total shorebirds observed were associated with the extensive intertidal areas east of Panama City. Nearly 80 percent of the birds utilized a 30 kilometer segment of shoreline east of Panama City. Shorebirds within this area reached the extraordinary linear density of over 10,000 birds per kilometer of shoreline. Most of the shorebirds observed along the shoreline west of the city were associated with isolated patches of mangrove forests that form around the mouths of small rivers.
Graph illustrates relative proportion of all birds surveyed within each 10km shoreline segment based on compilation of aerial survey data. Eighty percent of birds were recorded within the first three 10km segments east of Panama City. (Green areas represent remaining mangrove stands. Black shoreline border represents boundaries of aerial survey.)
Short-billed dowitcher in the hand. This species moves through the upper Bay in large numbers from mid-September through early October. Photo by Bart Paxton.
The Western Sandpiper breeds on the tundra of the high arctic in Alaska and Siberia. Breeding generally begins in mid to late May and is concluded for most of the population by mid-July. A portion of the adults begin to stage for fall migration on the intertidal flats of coastal Alaska as early as fate dune. Most of the population migrates south along the Pacific Coast stopping to refuel for short periods of time in a series of staging areas. Some individuals winter along the Pacific Coast as far north as California and Washington. Although large numbers of birds spend the entire northern winter in the Bay of Panama, many individuals continue to move south and winter along the coasts of Colombia, Ecuador, and Peru. In general, males migrate later than females and over shorter distances. Consequently, males tend to spend the winter in more northerly areas. In the early fall, one in eight Western Sandpipers in the Bay of Panama are males. In the late fall, their numbers increase to one in four.
The Western Sandpiper is the most abundant shorebird species that utilizes the upper Bay of Panama. This species may be observed in the Bay during all months of the year. A few early fall migrants begin to arrive in the upper Bay in late July and early August. By mid-August the Bay hosts nearly 40,000 individuals. The population continues to build throughout the early fall reaching a peak of nearly 300,000 by late October. Numbers decline through the late fall as some birds continue to move further south for the winter. By February and March, spring migrants begin to move north and numbers in the Bay swell again, only to decline as birds move through toward northern breeding grounds. A portion of the young birds totaling 10-12,000 do not migrate north to breed during their first year but spend the summer months in Panama.
Right: A female Western Sandpiper in winter plumage forages in shallow water. This species is the most abundant migrant shorebird within the upper Bay of Panama. Photo by Bryan D. Watts. Left: Gender in the Western Sandpiper may be determined by bill length. Males (bird on left) have shorter bills than females (bird on right). In the fall, males migrate later than females and tend to spend the winter further north. Photo by Bart Paxton. Below: Bart Paxton collects Western Sandpipers from mist net at dawn. A banding site was operated 1-2 times per week within a high-tide roost area to monitor population structure, migrant condition, molt patterns, and to collect morphometric data. Photo by Bryan D. Watts.
Many of the intertidal shorebird species utilize wetland habitats for roosting or foraging during high tide periods. In addition to these species, there are a number of shorebird species that utilize wetland habitats almost exclusively. Some of the more common wetland species include Least Sandpiper, Black-necked Stilt, Greater and Lesser Yellowlegs, and Solitary Sandpiper. These species feed on small fish, crabs, shrimp, and other aquatic invertebrates. Foraging in these species is not tidally influenced and they are generally not seen in communal roost areas.
Freshwater wetland in the upper Bay of Panama. Such habitats support large numbers of shorebirds, as well as, waterfowl, herons, and other birds. Photo by Dana S. Bradshaw. Right: Wilson's Phalarope standing in shallow water. This species does not spend the winter in the upper Bay but may be seen in fresh water marshes with open wafer curing the early fall months. Photo by Dana S. Bradshaw. Below: Alberto Castillo with Short-billed Dowitcher.
Photo by Bryan D. Watts.
Above: Wattled Jacana forages in shallow water. The species is a resident in Panama throughout the year and actually builds its nest within the marsh grass. Photo by Bryan D. Watts.
A number of intertidal and wetland shorebird species utilize grassland habitats. One of the most common shorebirds observed on grasslands is the Whimbrel. The Whimbrel and other species may be seen foraging in grasslands after heavy rains. Rain serves to soften the ground and brings prey species closer to the surface. In addition to the intertidal and wetland species, a number of shorebird species use grasslands as their primary habitat. Among others, these species include the Upland Sandpiper, Buff-breasted Sandpiper, Baird's Sandpiper, and Pectoral Sandpiper. These species tend to move through the upper Bay somewhat later in the fall season compared to most other species but do not spend the winter. Some individuals may stage in the upper Bay for 3 weeks or more before moving on to wintering grounds in southern South America.
Below: Willet foraging in a recently mowed field on Howard Air Force Base. Shorebirds use grassland patches most intensively just after mowing operations or rain when invertebrate prey are most accessible. Photo by Bryan D. Watts. Pectoral Sandpiper. Within the upper Bay area, this species feeds primarily on insects and worms on mowed fields. Photo by Bryan D. Watts
Below: Military training areas such as this parachute drop zone on Howard Air Force Base are frequently used by a number of shorebird species. Photo by Bryan D. Watts
Right: Many areas within the military installations provide foraging habitat or grassland shorebirds.
Photo by Bryan D. Watts
Western Sandpiper in the early stages of molt. Individuals arriving early in the season, typically drop 4-6 inner primaries upon arrival into the Bay. Photo by Bryan D. Watts.
During late August and early September when large numbers of migrant shorebirds move into the upper Bay, examinations of high-tide roost areas reveal thousands of feathers. These feathers may nearly cover the ground in areas of one hectare or more where shorebirds gather in large flocks. The appearance of these feathers marks the beginning of the annual replacement of flight feathers for these shorebirds. Each year, as feathers become worn they must be replaced in order for birds to maintain efficient flight. For many bird species, flight feathers are replaced or molted just after the breeding season. For most of the shorebirds using the upper Bay of Panama, feather replacement occurs on the wintering grounds or during the later stages of migration.
Nearly all of the intertidal shorebird species and some of the shorebird species using wetland and grassland habitats show evidence of feather replacement while resident in the upper Bay. These species replace most feathers during the period from early September through early October. By mid-October, most of the shorebirds in the Bay have completed their flight-feather molt. In addition to its role as a staging and wintering area, the Bay appears to have significance as an annual molting ground.
Western Sandpipers within a high-tide roost area. The combined energy requirements of so many birds are high in such a small area.
Photo by Bryan D. Watts.
Shorebirds require large amounts of energy to meet their normal metabolic needs and to travel over long distances. A rich supply of available energy is undoubtedly the primary reason that so many shorebirds gather in the upper Bay of Panama. Just to meet their normal energy requirements, the large number of migrant shorebirds within the 30 kilometer concentration area just east of Panama City would need to extract more than 50 metric tons of marine worms during the month of October alone. A large amount of additional energy is required to successfully complete feather replacement and, for some species, to accumulate fat for further migration. In late September, some Western Sandpipers are actively growing more than 50 percent of their flight feathers simultaneously. Such an intense level of molting is very energetically expensive.
In addition to having such high energetic requirements during the fall months, shorebird species that depend on intertidal habitats face severe constraints on the time that they may actively forage. Aside from the daily time constraints imposed by the tidal cycle, shorebirds face added constraints on foraging time as the fall season progresses. Within the upper Bay, intertidal shorebirds do not tend to forage in the rain but instead move to high ground. Disturbances due to rain increase after early October as the upper Bay moves into the rainiest period of the year. In addition to the increase in rain disturbance, both Merlin and Peregrine Falcons begin to migrate into the Bay in mid-October. These aerial predators hunt shorebirds that are concentrated within the intertidal zone. Continual hunting, keeps the shorebirds churned up and may reduce daylight foraging time by a significant amount. The annual timing of these disturbances seems to highlight the importance of early arrival. Birds that arrive early may replace feathers in the early fall before constraints on the time available for foraging intensify.
Western Sandpipers foraging in a wetland pool. During high tide periods, intertidal shorebirds often forage in alternate habitats such as wetlands and wetlands. Photo by Bryan D. Watts.
Western Sandpiper killed by Peregrine Falcon. This bird was one of 44 kills found within a high-tide study area within a single day. Photo by Bryan D. Watts Heavy rainstorms are frequent in the upper Bay. These storms increase in frequency in October and November interrupting foraging by intertidal shorebirds. Photo by Dana S. Bradshaw.
Throughout the year, the upper Bay of Panama supports large numbers of shorebirds. The role that the Bay plays in the life cycle of these shorebirds is likely to vary widely between species. For some, the Bay is a significant wintering ground. For others, the Bay is an important staging area during fall or spring migration or a place where nonbreeding individuals spend the summer. Currently, information is inadequate either in the Bay itself or within other similar areas to say with certainty how significant the resources within the Bay are to most shorebird species. However, information collected recently is beginning to give a general indication of its importance.
The upper Bay area appears to be a significant wintering area for the Western Sandpiper. Aerial surveys of shorebirds conducted by the Canadian Wildlife Service during the winter of 1993 provide the first opportunity to compare the significance of the Bay of Panama to other wintering areas for shorebirds in Central and South America. During a single flight in January, the Canadian Wildlife Service recorded over 224,000 shorebirds in the upper Bay. Surveys of the remaining Panamanian coastline suggest that nearly 90 percent of the intertidal shorebirds wintering in Panama occur within the upper Bay area. The overwhelming majority of these birds were Western Sandpipers. The fact that the majority of Western Sandpipers that use the upper Bay are adult females gives the site added significance.
In addition to being a winter destination, the upper Bay of Panama appears to be a significant staging area for southbound migrants. The fieldwork conducted during the fall of 1997 indicates that many species that do not spend the winter or spend the winter in relatively small numbers, move through the upper Bay in large numbers. This is particularly true of the intertidal shorebirds that spend the winter along the Pacific Coast of South America but is also true for selected wetland and grassland species.
Beyond the shear number of birds, the upper Bay appears to be a significant molting ground for a number of species. Individuals of some species may complete their molt and remain in the Bay for the winter months. Other individuals may stay within the Bay for extended periods of time to partially or completely finish their molt before moving further south for the winter.
Western Sandpipers rest in a high-tide roost. Photo by Bryan D. Watts.