Code
CCBTR-14-03
Publication Date
2014
Abstract
Colonial waterbirds are highly visible components of coastal avifauna 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 the fall of 1992, a consortium of agencies and individuals agreed that a comprehensive monitoring program for the Virginia colonial waterbird community was needed and that assessments should be made on decadal intervals for trend analyses. Surveys were conducted during the breeding seasons of 1993 and 2003. The 2013 survey reported here is the third in the time series. All of these surveys have systematically covered all 24 species of colonial waterbirds throughout the Coastal Plain province of Virginia. Nearly 800 surveys were conducted of 496 waterbird colonies during the breeding season of 2013. Colonies supported an estimated 60,604 breeding pairs of 24 species. Gulls were the most abundant group with more than 28,000 breeding pairs. Waders and terns accounted for 14,117 and 10,993 pairs respectively. Laughing gulls were the most abundant species representing nearly 40% of the total waterbird community. The barrier island/lagoon system of the Eastern Shore was the most important region for the majority of colonial species encountered. In 2013, this region supported 23 of the 24 species evaluated. The Eastern Shore accounted for 54.7% and 27.2% of all breeding pairs and colonies respectively. For 15 of the 24 species, the region supported more than 50% of the known coastal population. The colonial waterbird community in coastal Virginia declined by 36.2% in the years between 1993 and 2013. Population estimates for 19 (79%) of 24 species assessed declined between 1993 and 2013. Declines varied considerably between species with 10 species declining more than 40% and 5 species declining more than 60%. Cattle Egrets showed the highest loss rate (-96.2%), declining from an estimated 1,459 to only 56 pairs. Eight species increased between 1993 and 2008. Dramatic expansions were documented for White Ibis, Great Black-backed Gull, Double-crested Cormorant, and Brown Pelican. Over the past 20 years, three major forces appear to be shaping the colonial waterbird community in Virginia. These include 1) regional shifts in population centers that are driving population increases in Virginia, 2) habitat degradation related to sea-level rise, and 3) recovery of the bald eagle population. With the exception of Great Egrets, all species that have increased over the past 20 years have experienced ongoing range expansions and are riding a population wave that is progressing through Virginia. This includes Great Black-backed Gull, Double-crested Cormorant, Brown Pelican, and White Ibis. Most of the decline in medium-sized waders is being driven by habitat loss related to erosion of islands. This erosion results from sea-level rise, is ongoing and represents a significant threat to these populations. Several ground-nesting seabirds are likely more directly impacted by increased restriction in viable habitat and demographic impacts related to frequent flooding. The most notable example is the Laughing Gull that has experienced a catastrophic decline in just 10 years. Finally, the dramatic recovery of the Bald Eagle within the Chesapeake Bay has resulted in the species breeding within more than 25% of large wader colonies and may be responsible for the fragmentation of historic colonies and the beginnings of a population decline. This factor may ultimately impact other populations in the future.
Topic
Abundance/Distribution; Breeding/Demography/Population Dynamics
Species
Colonial Waterbirds
Source
The Center for Conservation Biology Technical Report Series, CCBTR-14-03. College of William and Mary & Virginia Commonwealth University, Williamsburg, VA.
Pages
23
Recommended Citation
Watts, B. D. and B. J. Paxton. 2014. Status and Distribution of Colonial Waterbirds in Coastal Virginia: 2013 Breeding Season. The Center for Conservation Biology Technical Report Series. College of William and Mary & Virginia Commonwealth University, Williamsburg, VA. 23 pp.
