North American Roseate Tern Populations
In the United States alone there are currently 90 different avian species listed as either endangered or threatened under the Endangered Species Act (USFWS 2009). Among these is the roseate tern (Sterna dougallii), a small, plunge-diving seabird. In 1986 the Northeastern Atlantic breeding population was federally classified as endangered in the United States, and since then numerous studies have been conducted concerning the threats facing the species, as well as how to effectively manage and enhance the existing population (Heinemann 1992). The roseate tern is also listed as threatened by the Canadian Wildlife Service, and effective population management requires both the United States and Canada working in tandem to protect critical habitat (Gochfield et el 1998). Roseate tern mortality rates are higher than those of most other terns and seabirds, and the average annual survival rate of roseate terns is lower than that of other marine bird species in the taxonomic orders Procellariiformes, Pelecaniformes, and Charadriiformes (Spendelow & Nichols 1989, Nisbet2 1992). Despite conservation efforts, the Northeastern Atlantic breeding population continues to decrease in size, and it is feared that it will soon reach a critically low level, after which there will be no hope of recovery (Mostello 2007).
Main Colony Sites
The total North American breeding population is estimated at approximately 3300 pairs (Mostello 2007). The two largest colony sites, Bird Island and Great Gull Island, experience high annual reproductive success and may be serving as source populations for nearby, less productive colonies (Shealer 1995).
Falkner Island, Connecticut
Falkner Island supports approximately five to six percent of the total Northeast Atlantic roseate tern population (Nisbet 1992). In 1997 it was estimated that only about 136 pairs of roseate terns breed at the Falkner Island site, meaning that this subpopulation of roseates is at a critically low level (Spendelow et el 2001, Borkhataria 1997). Falkner Island is home to the Stewart B. McKinney National Wildlife Refuge, and supports the largest colony of roseate terns breeding on federal land (Spendelow 2002, Nisbet 1992). However, it is thought that the Falkner Island population is not maintaining itself, but rather is primarily supplemented by immigrants from Great Gull Island (Shealer 1995). The average clutch size on Falkner Island is approximately 0.901 fledglings per breeding pair, suggesting that a sizable portion of the breeding pairs at this site have no reproductive success (Spendelow2 et el 2005). It is thought that the main cause of roseate tern population declines at Falkner Island are due to night heron activity (the night heron is one of the primary predators of roseate tern chicks) (Spendelow 2002). Falkner Island is also an unstable site geographically, with the island’s clay cliffs being slowly but steadily eroded away in heavy rainstorms (Nisbet 1992). Losing roseate terns at Falkner Island would be a major setback for the roseate tern recovery plan (Spendelow 2002).
Bird Island, Massachusetts
In 1997 it was estimated that there were approximately 1100 breeding pairs of roseate terns on Bird Island, making it home to approximately 40 percent of the regional population (Szcys et el 2001). The Bird Island colony is extremely productive with approximately 87 percent of all eggs laid hatching, and the average clutch size being 1.7 eggs (Szcys et el 2001). The neighboring Cape Cod vicinity is the single most important foraging ground for terns nesting at Bird Island, and birds in this location travel anywhere from 8 to 20 kilometers away from their colony site in order to feed (Nisbet 1992, Heinemann 1992). Unlike Falkner Island, Bird Island is open to the public, and has very low predation rates (Nisbet 1992).
Occasionally, the entire island is washed over during severe storms and hurricanes, and rows of granite blocks have been placed in order to prevent further erosion of the island (Nisbet 1992). Vegetation at this particular location tends to be very dense, and the breeding colony has been enhanced by the use of wooden nestboxes (Nisbet & Hatch 1999, Nisbet 1992). The dominant vegetation on Bird Island includes black mustard (Brassica nigra), ragweed (Ambrosia artemisifolia), lambs quarter (Chenopodium alba), smartweed (Polygonum pensylvanica), and morning glory (Convolvubus speium) (Nisbet 1992).
Great Gull Island, New York
In 1991 it was estimated that approximately 1300 pairs of roseate terns nest on Great Gull Island, making it the largest single colony site for the Northeastern Atlantic breeding population (Nisbet 1992). Great Gull Island is characterized by very high tidal currents, but large boulders protect the island from erosion (Nisbet 1992). Most roseates will nest near boulders on the steep slopes of the island, making many nests difficult to locate. Public access to this site is prohibited, as the area is protected as a research station (Nisbet 1992). Average productivity at this colony site is approximately 1.2 fledglings per breeding pair, making it a fairly productive colony site (Nisbet 1992).
Cedar Beach, New York
The Cedar Beach site supports about three percent of the Northeast Atlantic population with approximately 90 breeding pairs of roseate terns (Nisbet 1992). The Cedar Beach colony is located on a sandy area between lines of sand dunes on a barrier beach, and the roseate nesting area is partially covered by beach grass (Ammophila breviligulata) (Nisbet 1992). Birds nesting at this site feed at a tidal inlet approximately five kilometers away from the colony (Nisbet 1992). Average productivity at this colony site is usually between 0.5 and 1.1 fledglings per pair, with approximately 11 percent of the chicks at Cedar Beach dying before fledging (Safina 1988, Nisbet 1992). This high number of fatalities could be due to predation by herring gulls (Larus argentatus), northern harriers (Circus cyaneus) and feral cats (Felis catus) (Nisbet 1992). The beach adjacent to the colony site is also a popular vacation destination, and is used daily by thousands of people during the roseate breeding season. The presence of people, however, appears to have little effect on roseate tern productivity, and public access to the immediate areas in which the terns nest is prohibited (Nisbet 1992).
Other Colony Sites
There are approximately 30 smaller colony sites, with the number of breeding pairs inhabiting each ranging from one to 40 (Nisbet 1992).
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