Four factors conspire to generate problems for shorebird populations: long-distance migration, low reproductive rates, incompatibility between human and shorebird use of land, and concentrations of large numbers of shorebirds in a few key areas.
Problem #1: Most of the Western Hemisphere shorebird species migrate from wintering sites in Central and South America to breeding grounds in North America. The birds fly non-stop from one staging area to the next, traveling up to three to four days at a time and covering several thousand miles. At the staging areas, food must be extraordinarily plentiful, easily obtained, and available in areas with little disturbance. Few places meet these criteria, so staging areas are critically important.
Problem #2: Because of their short breeding season, most shorebirds make only one breeding attempt with a typical clutch of four eggs. High predation rates mean that most eggs do not hatch. Many hatchlings fail to fledge (leave the nest). Survivorship among adult shorebirds, between 70% and 95% per year, compensates for these low reproductive rates. This makes population size vulnerable, however, to unusually high non-breeding or migratory mortality.
Problem #3: Many shorebird sites along the migration routes have been used by people for construction, commerce, agriculture, and recreation. Development and land reclamation along coastal beaches and wetlands decreases the availability of valuable feeding and nesting habitats. Construction of jetties, riprap, or sea walls to prevent beachfront erosion similarly destroys critical shorebird habitat.
Problem #4: Because large proportions of their populations gather in single sites at the same time, shorebirds become vulnerable to single catastrophic events. The mouth of the Delaware Bay provides an entrance for one of the largest oil shipping ports on the East coast. An oil spill during spring migration would result in direct mortality and contamination of food resources.
Scientists and wildlife managers are concerned that declining horseshoe crab populations may adversely affect shorebirds because horseshoe crab eggs are their primary food resource in the Delaware Bay staging area. During their stopover, the six most abundant shorebird species can consume almost 539 metric tons of horseshoe crab eggs! At least 1.8 million female horseshoe crabs must spawn on the shores of Delaware Bay to provide this food surplus.
The graph to the right compares horseshoe crab spawning estimates (peak number of horseshoe crabs) with migratory shorebird abundance in Delaware Bay. Following several years (1990 and 1991) of high spawning estimates and horseshoe crab egg availability, shorebird abundance began to increase in 1993 from a previous low. Between 1992 and 1995, however, there was a decline in the number of horseshoe crabs spawning on Delaware Bay beaches. Fewer horseshoe crab eggs were available to foraging shorebirds. Following the path of declining horseshoe estimates, the numbers of migratory birds began to fall again in 1994. Between 1994 and 1995, the number of migratory shorebirds using Delaware Bay beaches during spring migration decreased more than 50%. Again, following the subsequent increases in the horseshoe crab spawning estimates (1996-1999), numbers of migratory shorebirds started rising again in 1997.
Because their short bills are incapable of boring deep beneath the sand, most shorebirds feed on surface eggs, those that are deposited within 0-5 cm of the surface. Recent studies by Dr. Mark Botton at Fordham University and Dr. Robert Loveland at Rutgers University illustrate a downward trend in the density and abundance of surface eggs. If fewer eggs are available, shorebirds will have greater competition for the limited food. If alternate food sources are not available, the population may undergo a decline. If the relationship between shorebird population size and numbers of horseshoe crab eggs available for food is as strong as researchers believe it is, horseshoe crab conservation is essential for protecting the future of migrating shorebird populations.
Realizing that migratory shorebird conservation is not a local issue, but requires a coordinated management network which links critical sites, the Western Hemisphere Shorebird Reserve Network (WHSRN) was formed in 1985. This network unites wildlife agencies, private conservation groups, and other organizations to support local wetland conservation initiatives. There are three categories of sites in the reserve network: Hemispheric, International, and Regional. To qualify as a Hemisphere reserve, at least 500,000 shorebirds, or 30% of a species population along a specific flyway must utilize the resources of the site per year. The lower portion of the Delaware Bay became the first Hemisphere Site in the reserve system in November of 1985.
Membership in WHSRN is essentially voluntary. Management priorities remain the prerogative of the landowner. WHSRN provides support to the reserves in five areas: training biologists and managers, providing technical assistance in habitat management, education and outreach, local and regional monitoring of shorebirds, and assistance in identifying funding sources for reserve projects. By 2001, there were over 165 organizations in the network, and 46 official sites in seven countries stretching from Tierra del Fuego to Alaska. Click here to find out more about the WHSRN.
Despite management of migratory shorebird populations and protection of wetlands habitat over the past 15 years, populations of several migratory shorebird species including semipalmated sandpipers and sanderlings are believed to be declining. However, new policies developed for the horseshoe crab fishery may indirectly help migratory shorebirds who stop over at Delaware Bay. Click here to find out about horseshoe crab management practices and plans.
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