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COSEWIC Update Status Report on the Roseate Tern in Canada 1999

Limiting Factors


Predation during the breeding season

Predation at breeding colonies appears to be the most important factor limiting the distribution and productivity of Roseate Terns in Canada.  Red foxes (Vulpes vulpes) are major predators of tern eggs on the Magdalen Islands (Shaffer and LaPorte 1996), and Northern Ravens (Corvus corax) and American Crows (Corvus brachyrhynchos) have been known to take tern eggs (including Roseate eggs) at several Nova Scotian colonies (Whittam 1997, D’Eon 1997).  Great Horned Owls (Bubo virginianus) are major predators of Roseate Tern adults, chicks, and fledglings in the U.S. (reviewed in Nisbet and Spendelow 1998).  Hunting owls cause adult terns to abandon their nests at night, leading to exposure of embryos and chicks, and greater predation by nocturnal species such as Black-crowned Night-Herons (Nycticorax nycticorax) and ants (Nisbet and Spendelow 1998).  Great Horned Owl predation on adult Common Terns has been reported in Pubnico Harbour, Nova Scotia (D’Eon 1997), but it is not known whether Roseate Terns are also affected.  Nothing is known about potential Black-crowned Night-Heron or ant predation on Roseate Terns in Canada.  There are few Black-crowned Night-Heron colonies in Atlantic Canada, none of which are located near Roseate Tern colonies (D. Amirault pers. comm.).

The major predators at Canadian tern colonies are Herring (Larus argentatus) and Great Black-backed (L. marinus) Gulls.  These species prey on tern eggs, chicks, and adults (Hatch 1970, Nisbet 1981).  A recent study of the Country Island tern colony provides in-depth information on gull predation at this site (Whittam 1997).  Country Island supported almost half of the Canadian Roseate Tern population in 1996 (i.e. 45 pairs; Table 3).  Only one pair nested in 1997, however, and they abandoned their egg after 10 days of incubation.  The combined number of breeding Arctic and Common Terns also dropped by more than half between 1996 (500 pairs) and 1997 (220 pairs).

The reason for this large-scale abandonment was thought to be predation by corvids on tern eggs, and gulls on tern adults, chicks, and eggs.  A Great Black-backed Gull was observed depredating an adult Roseate Tern, and gulls took more than half of all tern chicks hatched from this colony in 1996 (i.e. about 600 chicks).  Only 0.08 Roseate chicks per nest, or about five chicks in total, survived to fledge in 1996.  This level of productivity is well below the level needed to maintain the colony at its current size (Whittam 1997).


Adult overwintering survival

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 (Nisbet and Spendelow 1998).  The average annual survival rate for adult Roseate Terns from four major U.S. colonies is approximately 81-84%, which is low relative to other seabirds (Spendelow and Nichols 1989, Spendelow et al., 1995).  In the 1980s only Bird Island and Great Gull Island produced enough chicks to offset adult mortality (Ratcliffe 1997).  Because adult mortality is rarely observed at breeding colonies, Roseate Terns are probably dying during migration or at their wintering grounds.  Roseate Terns were trapped intensively between 1968-1981 in Guyana for sale at local markets, but this practice has since reportedly stopped (Nisbet 1984).  More information is required to determine the causes of winter mortality (Spendelow et al. 1995).


Extreme weather

Major storms, such as Hurricane “Bob” which passed through the principal staging area for Roseate Terns in August of 1991, may hinder population recovery (Nisbet and Spendelow 1998).  Circumstantial evidence suggests that “Bob” was responsible for the crash in the U.S. Roseate Tern population between 1991 and 1992 (Table 1).  Adult mortality increased from 17% to 33% in 1991-1992.  Furthermore, 80% of fledgings raised in 1991 were lost (Nisbet and Spendelow 1998).  Hurricane “Bob” is the only storm for which tern demographic data are available.  More cases need to be documented to clarify the role of extreme weather in population crashes (Nisbet and Spendelow 1998).


Skewed sex ratio

A shortage of males may limit the productivity of Roseate Terns at some colonies in northeastern North America (Nisbet and Hatch in press).  The sex-ratio of breeders on Bird Island, MA is 127 females: 100 males.  Twenty per cent 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.  Female-female pairs produce 75% fewer fledgings per female than male-female pairs.  As a result, average colony productivity at Bird Island is reduced by about 16%, compared to the value expected if all females had male mates (Nisbet and Hatch in press).  It is not known whether the sex ratio is skewed at hatching or fledging, or is due to subsequent sex-specific mortality (Nisbet and Hatch in press).



While the population of Roseate Terns in northeastern North America has been increasing slowly since 1987, more than 90% of the population is concentrated into five predator-controlled sites in the United States.  Three to four per cent of the northeastern population nests in Canada, where there are currently only three large colonies supporting up to 94% of the Canadian population.  These are The Brothers Islands, Grassy Island and the Country Island complex (Table 2, Fig. 1).  Only The Brothers is known to be a stable breeding colony.  The concentration of the population into few sites is cause for concern, because any disturbance at these sites due to environmental contamination, human interference, a major storm, or an increase in predation pressure could lead to the extirpation of Roseate Terns from Canada.