North American shorebirds have declined significantly over the past several decades. As a result, ornithologists have carried out numerous studies to identify key habitat for preservation and develop conservation strategies. Most of this research has been conducted on northern breeding grounds or on wintering grounds in South America, but there have been few studies in the Caribbean. However, a significant number of migrating shorebirds stop at the salt flats at Cabo Rojo National Wildlife Refuge, in southeastern Puerto Rico, the most important shorebird site on the island and one of the most important in the Caribbean. During fall migration, daily counts of up to 10,000 shorebirds have been recorded. As a result, Cabo Rojo has been designated an Important Bird Area and a site of the Western Hemisphere Shorebird Reserve Network.
The Cabo Rojo salt flats consist of two lagoons, parts of which are used for commercial salt production pursuant to a special permit from the U.S. Fish and Wildlife Service (FWS). The activities are conducted in accordance with the needs of shorebirds and have been ongoing for more than a century. FWS believes that the salt production is helpful to birds, as it prevents salt buildup in lagoon beds.
Nearly 30 species of shorebirds have been observed at Cabo Rojo. Small calidrid sandpipers (Least, Western, Semipalmated, and White-rumped), Stilt Sandpiper, and Lesser Yellowlegs are the most abundant migratory species, though approximately 20 others have been seen. Led by Jaime Collazo, of the U.S. Geological Survey and North Carolina State University, researchers including Morgan A. Parks have conducted shorebird surveys at the salt flats during three multi-year periods (1985-1992, 1999-2007, and 2013-2014), allowing them to compare population numbers over a nearly 30-year period. They have also comprehensively studied the wintering ecology of the Semipalmated Sandpiper.
Their results, which were published in a recent paper, are alarming though consistent with larger scale trends. Virtually all species declined more than 70 percent between 1985-1992 and 2013-2014. For example, both small calidrid sandpipers (grouped together due to difficulty of identification at a distance) and Lesser Yellowlegs declined by 81 percent. Stilt Sandpiper declined by 57 percent. Moreover, eight rarer species observed in the earlier periods were not even seen in the later period.
Researchers also conducted surveys in nearby wetlands to determine if some birds had simply shifted to other sites, but even when these neighboring areas were considered, the declines were significant.
It is unclear whether the declines are due to conditions at Cabo Rojo or factors elsewhere, including during migration. However, research on Semipalmated Sandpipers found that they stayed at Cabo Rojo for an average of 110 days in the fall and average body condition improved during that time. Survival rates were also comparable to other sites.
Although researchers found that certain prey items have become less common, the studies of Semipalmated Sandpipers suggest that habitat quality at Cabo Rojo remains high. However, other shorebird species may have been adversely impacted by the declines of the prey items.
The studies at Cabo Rojo add to the mounting evidence that many shorebird species are experiencing broad, significant, and ongoing declines, including at the most important U.S. shorebird site in the Caribbean.
Source: Bird Watching, 19 December 2016