Written by Bryan Watts & Bart Paxton
May 8, 2009
Colonial waterbirds are highly visible components of coastal avifauna that share the unusual characteristic of nesting in dense assemblages. Due to their close association with aquatic resources, they are often good indicators of wetland and aquatic health. However, one consequence of having large portions of populations nesting in few locations is that even restricted disturbance may have profound consequences on a population level. Development of conservation strategies for the protection of these sensitive species requires current status and distribution information.
Since the 1960s, the Center for Conservation Biology (CCB) has been conducting surveys of colonial waterbirds that have ranged from individual populations to the entire community. Among other objectives, this information has been used to assess population trends and changes in distribution. The most comprehensive of the historic benchmark surveys that included the entire community was conducted in 1993 when CCB lead a consortium of partners to survey the entire Coastal Plain of Virginia. The effort covered 446 colonies supporting an estimated 94,947 pairs of 24 species. The 1993 benchmark survey was repeated in 2003. During the spring of 2007, a decision was made to conduct a third assessment in 2008 on the 5th anniversary of the 2003 survey. Due to funding constraints, the 2008 survey did not cover the widely dispersed colonies of great blue herons and great egrets in the interior of the Chesapeake’s western shore or within the portion of the Coastal Plain south of the James River. For this reason, the 2008 survey produced comparable population assessments for 22 of the 24 colonial waterbird species known to nest in Virginia.
Nearly 550 surveys were conducted of 250 colonies during the breeding season of 2008. Colonies supported an estimated 60,758 breeding pairs of 24 species. Gulls were the most abundant group with more than 40,000 breeding pairs. Terns and waders accounted for 9,455 and 4,763 pairs respectively. Laughing gulls were several times more abundant than any other species and represented 61% of the total waterbird community. The barrier island/lagoon system of the Eastern Shore was the most important region for the majority of colonial waterbird species encountered. This region supported 20 of the 23 species evaluated during the survey and accounted for greater than 74% and 70% of all breeding pairs and colonies, respectively. For 15 of the 23 species, the region supported more than 50% of the known coastal population.
The colonial waterbird community in coastal Virginia assessed in this survey (all species except great blue heron and great egret) declined by 28.9% during the 15 years between 1993 and 2008. Population estimates for 14 of 22 species assessed declined since 1993, and 11 of these have declined since the 2003 survey. Declines varied considerably between species, with 10 species declining more than 40% and 4 species declining more than 70%. Cattle egrets showed the highest population loss rate, declining from an estimated 1,459 to only 120 pairs. Eight species’ populations increased between 1993 and 2008. Dramatic expansions were documented for white ibis, great black-backed gull, double-crested cormorant, and brown pelican.
The seaside of the Delmarva Peninsula is the most important region for colonial waterbirds in Virginia. Despite heroic efforts to manage several species, declines have not been abated in this area. Collectively, the waterbird community has declined 33% between 1993 and 2008. Snowy egret, tricolored heron, cattle egret, green heron, yellow-crowned night heron, glossy ibis, herring gull, laughing gull, gull-billed tern, royal tern, Forster’s tern, common tern, and black skimmer all showed consistent declines across this period. Only species that colonized the area since 1970, including white ibis, great black-backed gull, double-crested cormorant, and brown pelican, have exhibited consistent increases. Ecological differences among waterbird species showing declines suggest a system-wide problem greater than mammalian predators that have been the focus of recent management actions.
Partnering with The Center for Conservation Biology’s (CCB) on the colonial waterbird project are:
The Nature Conservancy (TNC), Virginia Dept. of Game & Inland Fisheries (VDGIF), US Fish & Wildlife Service (USFWS), and the Virginia Coastal Zone Managment Program (CZM).