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COSEWIC Update Status Report on the Roseate Tern in Canada 1999

Executive Summary

Roseate Tern

Sterna Dougallii



The Canadian population of Roseate Terns was estimated at 101-125 pairs between 1982-1985, which constituted 3% of the northeastern population (~3100 pairs). While the number of Roseate Terns in Canada had probably always been small, it appeared as though the population had declined since the 1930s.  The two most important factors limiting the distribution and abundance of Roseate Terns in northeastern North America were thought to be trapping of adults on the wintering grounds for sale at local markets, and predation and displacement by gulls on the breeding grounds.  Roseate Terns were designated as threatened by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) in 1986.  Since the original COSEWIC designation, a number of studies have provided new information on Roseate Terns in Canada.


Population size and trend

Estimates of breeding pairs in 1996 and 1997 place the Canadian Roseate Tern population between 88 and 137 pairs breeding in six locations.  Roseates currently nest in only three of nine sites noted in the original status report.  The Brothers, N.S., is the only large colony (i.e. greater than 20 pairs) in Canada known to have maintained a stable population of Roseate Terns for the last 15 years.  Two new relatively large Roseate Tern colonies, Country Island and Grassy Island, were discovered in 1987 and 1993, respectively.  These three large colonies support up to 94% of the Canadian population. Machias Seal Island is the only known breeding site for Roseate Terns in New Brunswick, and only 1-2 pairs breed in this colony annually.  Fewer than five pairs breed on the Magdalen Islands, the only known breeding location for Roseate Terns in Quebec.

The number of breeding pairs in the U.S. has been increasing slowly since 1988. In 1997, 3382 pairs bred in the U.S. Ninety-three per cent of the U.S. population currently nests in the five largest colonies, and 78% nests in the two largest colonies. This concentration makes the species extremely vulnerable to disturbance and was the primary reason for the designation of this population as endangered in the U.S.


Limiting factors

Predation at breeding colonies appears to be the most important factor limiting the distribution and productivity of Roseate Terns in Canada.  The major predators at Canadian Roseate Tern colonies are Herring (Larus argentatus) and Great Black-backed (L. marinus) Gulls.  Adult mortality during migration or at wintering grounds is probably the main factor limiting population size in the U.S., where predators are controlled at most breeding colonies. Roseate Terns were trapped intensively between 1968-1981 in Guyana for sale at local markets, but this practice has since reportedly stopped. More information is required to determine the causes of winter mortality.  A shortage of males may limit the productivity of Roseate Terns at some colonies in northeastern North America.  In one colony, 20% of breeding females do not obtain male mates, and instead pair together to produce supernormal clutches of three to four eggs.  Fertilization is achieved through extra-pair copulations, however female-female pairs produce 75% fewer fledgings per female than male-female pairs. As a result, average colony productivity is reduced by about 16%.



In Canada, Roseate Terns have almost completely abandoned both Sable Island and Country Island, the latter due almost certainly to gull predation.  In the absence of predation, these two islands are high-quality breeding sites, primarily because they are located far from the mainland (160 km and 5 km, respectively) and are thus less susceptible to terrestrial predators and human disturbance.  In general, Roseate Terns in Canada currently breed on small sites close to the mainland.  Colonies that are deemed essential to the survival of this species in Canada need to be monitored yearly and kept free of breeding gulls.  Gulls currently nest on three of the six Canadian Roseate Tern colony sites. In the U.S., 97% of the Roseate Tern population nests at sites that are managed and protected by biologists or wardens.  Predator management is clearly necessary for the survival of this species.



The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) determines the national status of wild species, subspecies, varieties, and nationally significant populations that are considered to be at risk in Canada. Designations are made on all native species for the following taxonomic groups: mammals, birds, reptiles, amphibians, fish, lepidopterans, molluscs, vascular plants, lichens, and mosses.



COSEWIC comprises representatives from each provincial and territorial government wildlife agency, four federal agencies (Canadian Wildlife Service, Parks Canada Agency, Department of Fisheries and Oceans, and the Federal Biosystematic Partnership), three nonjurisdictional members and the co-chairs of the species specialist groups. The committee meets to consider status reports on candidate species.



Species: Any indigenous species, subspecies, variety, or geographically defined population of wild fauna and flora.

Extinct (X): A species that no longer exists.

Extirpated (XT): A species no longer existing in the wild in Canada, but occurring elsewhere.

Endangered (E): A species facing imminent extirpation or extinction.

Threatened (T): A species likely to become endangered if limiting factors are not reversed.

Special Concern (SC)*: A species of special concern because of characteristics that make it particularly sensitive to human activities or natural events.

Not at Risk (NAR)**: A species that has been evaluated and found to be not at risk.

Data Deficient (DD)***: A species for which there is insufficient scientific information to support status designation.

*      Formerly described as "Vulnerable" from 1990 to 1999, or "Rare" prior to 1990.
**    Formerly described as "Not In Any Category", or "No Designation Required."
***  Formerly described as "Indeterminate" from 1994 to 1999 or "ISIBD" (insufficient scientific information on which to base a designation) prior to 1994.


The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) was created in 1977 as a result of a recommendation at the Federal-Provincial Wildlife Conference held in 1976. It arose from the need for a single, official, scientifically sound, national listing of wildlife species at risk. In 1978, COSEWIC designated its first species and produced its first list of Canadian species at risk. Species designated at meetings of the full committee are added to the list.

Environment                              Environnement
Canada                                     Canada

Canadian Wildlife                      Service canadien
Service                                     de la faune

The Canadian Wildlife Service, Environment Canada, provides full administrative and financial support to the COSEWIC Secretariat.