Status and distribution of colonial waterbirds in coastal Virginia: 2003 breeding season

B. D. Watts, The Center for Conservation Biology


Colonial waterbirds are highly visible components of coastal avifaunas that share the unusual characteristic of nesting in dense assemblages. 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 these sensitive species requires current status and distribution information. In 1993, the most comprehensive assessment of the colonial waterbird community in coastal Virginia was conducted with the purpose of providing information needed for strategic management and future trend analysis. After the conclusion of the 1993 survey, a consortium of partners collectively agreed to maintain a survey interval of 10 years to monitor the community. The survey effort in 2003 represents this 10-year anniversary. More than 800 surveys were conducted of 446 colonies during the breeding season of 2003. Colonies contained an estimated 79,343 breeding pairs of 24 species. Gulls were the most abundant group with more than 50,000 breeding pairs. Terns and waders accounted for 8,399 and 15,557 pairs respectively. Laughing gulls were several times more abundant than any other species and represented 56.7% of the total waterbird community. Great Blue Herons were the most widely distributed species with more than 200 colonies documented. The barrier island/lagoon system of the Eastern Shore was the most important region for the majority of colonial species encountered. This region supported 22 of the 24 species found in coastal Virginia and accounted for greater than 70% and 35% of all breeding pairs and colonies, respectively. For 18 of the 24 species, the region supported more than 50% of the known coastal population. The colonial waterbird community in coastal Virginia declined by more than 16% during the 10 years between 1993 and 2003. Losses were widespread with 17 of 24 species exhibiting negative trends. The magnitude of declines varied between species with 10 species declining by more than 40% and 4 species declining by more than 70%. Cattle Egrets showed the highest loss rate, declining from an estimated 1,459 to only 166 pairs over the 10-yr period. Seven species increased between 1993 and 2003. Dramatic expansions were documented for White Ibis, Great Black-backed Gull, Double-crested Cormorant, and Brown Pelican. The overall waterbird community declined within all geographic regions except the western shore. Due to the area’s importance to the overall community, declines on the seaside of the Delmarva Peninsula accounted for more than 85% of the broader coastal decline. It remains difficult to separate the relative influences of local conditions from regional population phenomena on population trends of many of these species. Many population increases have followed colonization events where populations do not appear to have reached stable levels. Some species experiencing recent declines have followed population increases associated with earlier colonization events. Caution should be used when attempting to attribute increases or declines solely to local factors. However, all of the species that nest on open barrier beaches experienced substantial declines. Many of these species have nested on the islands as far back as records exist. Their declines appear to be linked to continuing increases in predator populations on the islands. If so, these populations should respond to ongoing programs to manage predation pressure.