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Roseate Tern (Sterna Dougallii)

Background

Species information

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Status: Assessed by COSEWIC as Endangered in Canada in 1999 (COSEWIC 2003), reclassified from its previous (1986) listing of Threatened. Also Endangered as of 2000 under the Nova Scotia Endangered Species Act (Endangered Species Act 1998, c. 11, s. 1.) and the northeastern population is listed as Endangered in the United States (USFWS 1987).

Reason for status: The shift in status resulted from reevaluating the population’s size and distribution according to revisions to criteria made between the latest (1999) and previous (1986) status reports. During that time, the population remained small and was mainly concentrated at three colonies, only one of which had been stable in that period. One of these colonies (Grassy Island) has since been abandoned (Leonard et al. 2004). Predation and disturbance result in poor productivity and recruitment.

Occurrence: Mainly Nova Scotia, with a few pairs in Québec and New Brunswick.

 

Distribution

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Global range:Breeds worldwide, usually on marine islands (see map in Appendix B). In North America, a northeastern population breeds from the Gulf of St. Lawrence (Magdalen Islands) to New York; a disjunct Caribbean population breeds from Florida and the Bahamas to the Lesser Antilles.

Canadian range: Since 1982, have bred at about 28 sites, most of which are coastal islands in Nova Scotia, and of which 12 or fewer sites are occupied in any given year (see map in Appendix B). The location of colonies changes unpredictably among years, and may not always be known even with extensive searches. Only three colonies are known to have had more than 20 pairs in the last 10 years: The Brothers (33-86 pairs), Grassy Island (0-30 pairs), and Country Island (0-53 pairs). In some years, have bred at a variable subset of other sites, including three of the Magdalen Islands, Québec, Machias Seal Island, NB, and about 21 other sites in Nova Scotia (Whittam 1999, Leonard et al. 2004; Appendix B).

Percentage of global distribution in Canada: Pairs breeding in Canada constitute 3-4% of the northwestern Atlantic population, and less than 1% of the (poorly) estimated world population (Gochfeld et al. 1998).

 

Population sizes and trends

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Population estimates are imprecise, but from 1982-1985 to 1999, the number of breeding pairs was probably roughly stable at about 100 pairs (100-121 in 1982-1985 [Kirkham and Nettleship 1986] to 123-149 in 1999 [Whittam 1999]), although their distribution was certainly variable. In 2000, only Country Island and The Brothers were surveyed, but together these sites contained the equivalent of at least 95 % of the suspected breeding population of <149 breeding pairs (Leonard et al. 2004). Between 1988 and 1991 the northeastern population in the US increased from 2743 to 3430 pairs (USFWS 1998) which corresponds to an annual population growth of 4.6%. After a decline of about 17% between 1991 and 1992, which was attributed to Hurricane Bob (Nisbet and Spendelow 1999; Lebreton et al. 2003), the population increased from 2743 to 4310 pairs between 1992 and 2000 (USFWS 1998; B. Blodget pers. comm.), which is a mean population growth of 5.8% per year. This increase appears to have been achieved both by intensive management of existing colonies and by restoration of additional colonies (Nisbet and Spendelow 1999). However, the population had declined to about 3500 pairs by 2002.

 

Biologically limiting factors

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Roseate Terns have a low annual adult survival rate for a seabird (83 %), lay one small clutch per year (mean clutch size = 1.7 eggs/ pair), and usually do not breed until their third year (Spendelow et al. 2002). Even when reproductive output is high (> 1.1 young per year), as in some US colonies, survival to first breeding is low (about 37% to age 3 [Lebreton et al. 2003]). Colony sites may also be limited, even without displacement by gulls (see below), by their need for specific foraging sites, i.e., shoals free of competition from Common Terns (Nisbet and Spendelow 1999).

 

Threats

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Predation at breeding colonies by Herring and Great Black-backed Gulls was certainly the main factor limiting productivity at one Canadian colony (Country Island), and displacement and predation by these gulls are thought to be the main factor limiting distribution in Canada (Lock et al. 1993; Whittam 1999). Especially at sites near the mainland, other predators (e.g., foxes on the Magdalen Islands) are also a threat. Recent research in the United States and United Kingdom has shown that high post-fledging mortality, and a shortage of males threaten recovery even after predators are controlled (Nisbet and Spendelow 1999).

 

Habitat requirements

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Habitat use

Roseate Terns in Canada nest in colonies almost exclusively on small islands with low vegetation, but will occasionally nest on mainland spits (Whittam 1999). They generally select nest sites with vegetated cover but will also nest under beach debris and driftwood (T.C. D’Eon, pers. comm.), and in tires and nest boxes if provided (Spendelow 1982). The most important habitat feature in northeastern North America for breeding Roseate Terns appears to be the presence of breeding Common Terns (Gochfeld et al. 1998), as they have not been known to nest at sites without them. Terns require colony sites that are relatively free from predators, and will abandon a colony after a season of heavy predation (Nisbet 1981; Whittam 1997). Roseate Terns breeding in North America are limited by the number of available predator-free (or predator-controlled) colony sites that are also in close proximity to good foraging sites.

Roseate Terns generally forage in shallow areas close to shore, near shoals and tide rips (Gochfeld et al. 1998), although little is known about their foraging ecology in Canada. After fledging in early August, juvenile Roseate Terns from the northeastern population disperse with their parents to staging areas. There is also little known about staging habitat for Canadian birds, although in 2002 two Roseate Terns banded as chicks on The Brothers were sighted at Great Gull Island, New York within a month of fledging (H. Hays, pers. comm.). Roseate Terns migrate south in late August and early September. They arrive in South America by October, where they have been recovered and recaptured along the north coast from western Colombia to eastern Brazil, between 11°S and 18°S (Hays et al. 1997).

Habitat trends

Since the early 1900s, tern colonies across the northeastern US and Canada have been abandoned, apparently displaced by the spread of large Larus gulls throughout the region (Kress et al. 1983) and possibly deterred by  nocturnal predation by owls (Nisbet and Welton 1984). Recent apparent declines in gull populations, suggest that this trend may have stopped or is being reversed (Boyne et al. 2001b; Boyne and Hudson 2002). No apparent effect on the availability of habitat has been detected, however (Whittam 1999).

 

Ecological role

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In Canada, Roseate Terns are predators of young fish, mostly sand lance (Ammodytes spp.), herring (Clupea spp.), and white hake (Urophycis tenuis; Whittam 1999). Given their sparse distribution in Canada, their impact on these fish populations is negligible.

Roseate Terns are preyed upon by various species, especially by Great Horned Owls (Bubo virginianus) and other birds of prey, large gulls, corvids, and mammals which prey on eggs and chicks (Gochfeld et al. 1998). Again, given their rarity, the importance of Roseate Terns to any of these predator populations is negligible.

 

Importance to people

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The socio-economic value of the Roseate Tern is principally derived from its aesthetics rather than its ecology, but is nevertheless important. Historically, it was valued for the millinery trade, which accounted for precipitous declines in terns in general in the 19th century. Recently, it has become an icon for conservation efforts; for example, it is the emblem of Bird Life International, the Association of Field Ornithologists, and the Atlantic Canada Conservation Data Centre. Sightings of Roseate Terns are prized by ecotourists and birdwatchers. Next to the Piping Plover (Charadrius melodus), it is probably the second best known coastal rarity in Atlantic Canada. As such, it serves as a focus of coastal conservation efforts and a source of local pride in those efforts.

 

Anticipated conflicts or challenges

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The main challenges to recovery are: 1) the expense of keeping wardens on colony sites for the duration of breeding, 2) nourishing sustained partnerships between government and public stewards, when government funding can be irregular and public involvement relies on enthusiastic individuals, 3) at some sites, conflict between recreational use of islands and the need for protecting colonies, 4) the importance of mortality at wintering sites, which, from Canada, can be addressed only indirectly, and 5) the ethical and public relations challenges of predator control.

 

Knowledge gaps

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Research conducted in the United States, especially by the Roseate Tern Metapopulation Project, has made the Roseate Tern into one of the best studied bird species in North America (Nisbet and Spendelow 1999). Knowledge specific to Canadian sites is poor, partly because sample sizes are inevitably low, but partly also because a sustained, systematic effort has been lacking. This closing section highlights the most important gaps in our knowledge about the Canadian population.

Survey requirements 

Thanks largely to recent aerial surveys, overall data on where Roseate Terns have bred in Canada are good and could yield useful data on their habitat requirements and selection of sites for recovery. Several improvements to surveys are critical:

  1. set schedule for surveys, ensuring they are conducted at regular intervals and are complete
  2.  include scheduled revisits on the ground to confirm breeding and judge  success
  3. determine adult movements among colony sites and across the US border (requires banding and recapture/ re-sighting efforts);
  4. determine sensitivity of surveys for analysis of population trends

Biological/ecological research requirements

The most important research questions are:

  1. what are the limiting factors in colonies free of predation?
  2. what do Roseate Terns at each colony eat and where do they forage, both for their chicks’ and their own food?
  3. what factors account for colony abandonment and tenure?

Threat clarification

Trends in threats to Roseate Terns are poorly understood. The most important questions for Canadian research are:

  1. what is the distribution of gulls near Roseate Tern colonies?
  2. are gulls increasing or decreasing, and what factors account for the trend?
  3. what is the impact of other predators (e.g., mink, owls)?
  4. what is the pattern of recreational use on islands used by Roseate Terns?
  5. to what extent do Common Terns displace or out compete Roseate Terns at different foraging sites?

 

Ecological and technical feasibility of species recovery

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The Roseate Tern has demographics that dictate a slow recovery and breeds at colony sites that frequently change with no apparent reason. Restoration requires a sustained effort against displacement and predation by gulls and other predators, which are adaptable and well established over a wide area and require a constant and sustained human presence to deter. Post-fledging mortality, most of which occurs away from the breeding colonies and much of which probably occurs well south of Canada, may be the most important factor limiting recovery.

On the positive side, breeding occurs at small, well circumscribed islands, allowing an effective focus of management efforts. Adults are long-lived, and thus are likely to remember successful management efforts and reward them by returning to breed (Spendelow et al. 1995, Nisbet and Spendelow 1999). In the absence of predation and irregular or intense human disturbance, productivity is particularly high for a seabird.

Thus, if sustained presence of experienced personnel can be maintained at suitable breeding colonies, recovery is likely.

 

Recommended approach / scale for recovery

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Conservation of Roseate Terns will simultaneously enhance conservation of Arctic and Common Terns, although a multi-species recovery strategy is not appropriate because the latter two species are not currently listed as at risk. Roseate Tern recovery will be implemented using a single-species approach but it is understood that the conservation of the Roseate Tern relies on, and will enhance, the conservation of Common and Artic tern populations.