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Kerguelen Tern

Sterna virgata

Number Of Mature
Individuals (Regional)

100

Regional
Population Trend

Decreasing

en

2025
Regional Category

Endangered

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CONTENTS
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    Overview

    Names

    IOC English Name:

    Kerguelen Tern

    SA & IOC Scientific Name:

    Sterna virgata

    BirdLife International Taxonomy (scientific name):

    Sterna virgata

    Order:

    CHARADRIIFORMES

    Family:

    Laridae

    Species name author:

    Cabanis 1875

    Afrikaans:

    Kergeulense Sterretjie

    Sesotho (South Africa):

    Sesotho (Lesotho):

    Siswati:

    Zulu:

    insukakude yaseMarion

    Current Assessment Status

    2025 Regional Category [Criteria]

    EN [D]

    2024 Global Category [Criteria]

    NT [D] (BirdLife International 2018)

    Population size (Regional)

    Up to 50 pairs

    Population size (Global)

    Currently unknown, 3500 – 6500 individuals (Jouventin et al. 1988)

    Distribution size (EOO) (Regional) km2

    720 (breeding), 1400 (breeding foraging area)

    Distribution size (EOO) (Global) km2

    398 000 (breeding/resident)

    Distribution size (AOO) (Regional) km2

    334 (based on colony size), c. 2 (nesting area)

    Generation time

    11 years (BirdLife International 2018)

    Status change reason

    No change

    Migrant (in the region)

    Native, breeding visitor

    Regional endemic

    No

    Historic Listing Information

    2000 Regional Status

    EN [A2e; B1+3d; D]

    2015 Regional Status

    EN [D]

    Status change reason (if applicable)

    Not change

    2015 Population size (Regional)

    c. 50 pairs

    2015 Global Status

    NT [D]

    Reason for Inclusion

    Reason for Inclusion in the Assessment

    The Kerguelen Tern Sterna virgata breeds on the Prince Edward Islands (PEIs). In addition, the species was listed as regionally Endangered in 2015 and globally Near Threatened in 2018.

    Category Justification

    Category Justification

    The Kerguelen Tern is one of the world’s most restricted terns, occurring only on sub-Antarctic islands in the southwestern Indian Ocean. With a regional population estimate of up to 50 pairs, it satisfies Criterion D (population of <250 mature individuals) for classification as regionally Endangered. In addition to its small population size, the species meets Criterion D2 for a regionally Vulnerable (population with a restricted Area of Occupancy (AOO) or with the number of breeding locations ≤5, with a plausible future threat that could drive the population to Critically Endangered or Extinct in a very short time).

    Population Justification

    The global population of Kerguelen Terns was estimated at 3500 – 6500 individuals, roughly equivalent to 2300 – 4300 mature individuals (BirdLife International 2018). Weimerskirch et al. (1989) gauged the Kerguelen Islands’ population of the subspecies S. v. virgata to be c. 1750 mature individuals, while Weimerskirch and Stahl (1988) estimated there were 300 – 400 mature individuals at Crozet Islands. Ryan (1987) recorded 45 Kerguelen Terns at PEI and 11 at Marion Island. At PEI, in 2002 and 2008, two pairs and one pair, respectively, were observed breeding (Whittington et al. 2009). At Marion Island, in 11 seasons between 1997/98 and 2008/09 the number of pairs breeding fluctuated from 12–56 pairs, with a mean and standard deviation of 24 ± 11 pairs and no apparent trend (Crawford et al. 2004, Whittington et al. 2009). At Marion Island, in seven seasons between 2015/16 and 2020/21, the number of pairs breeding fluctuated from 4–38 pairs, with a mean ± SD of 19 ± 12 pairs (Department of Forestry, Fisheries and Environment, unpubl data). Given the difficulty of locating nests over wide areas at Marion Island and the infrequent visits to PEI, it is thought that up to 50 pairs (equivalent to 100 mature individuals) may breed at the PEIs.

    Trend Justification

    The global population trend is believed to be stable (BirdLife International 2018). Regionally, numbers observed breeding at Marion Island have shown considerable inter-annual fluctuations but have been below 40 pairs since 1997/98. Fifty or more pairs bred in 1974 and 1977 (Williams et al. 1979) and 1997/98 (Brooke 1984, Whittington et al. 2009) but <10 pairs in 1979 (Brooke 1984) and 5–10 pairs in 1984 (Ryan 1987). The population at PEI decreased from an estimated 25–30 pairs in 1984 (Ryan 1987) to two pairs in 2001 (Ryan et al. 2003) and a single pair in 2008 (Whittington et al. 2009). Most recently, the number of breeding pairs fluctuated between 4–38 pairs from 2015/16 to 2020/21. No pairs were observed at PEI during a summer survey in 2011, which took place after peak breeding (Taylor and Wanless 2015).

    Biology & Ecology

    Taxonomy

    Kerguelen Tern is separated into two subspecies: S. v. mercuri at the Crozet Islands (and presumably the PEI), and the nominate subspecies at Kerguelen (Gomez and Voisin 2002). The two subspecies are separated based upon morphological characters (Gomez and Voisin 2002).

    Identification

    This small, dark tern has a graceful, buoyant flight similar to Chlidonias marsh terns. It features a narrow white cheek stripe separating its black cap from a grey neck, with a grey body and white underwings. The bill is dark red, and the legs and feet are bright red-orange. It differs from the Antarctic Tern Sterna vittata by its darker grey colour, finer bill, greyer underwings, and a shallower tail fork. Non-breeding adults have a grizzled forehead, paler underparts, and a patchy grey wash on the underparts, with a reddish-black bill.

    Distribution

    The Kerguelen Tern is one of the world’s rarest terns, and is restricted to the Prince Edward, Crozet and Kerguelen islands in the southern Indian Ocean (Gomez and Voisin 2002). Taking into account the large expanses of sea between the Kerguelen and Crozet islands (1480 km), and the Crozet and Prince Edward islands (925 km), as well as the sedentary nature of the species (Weimerskirch and Stahl 1988), exchanges between the Kerguelen Tern populations must be limited (Gomez and Voisin 2002). Kerguelen Terns remain at the PEIs year-round (Ryan and Bester 2008). In any year the area occupied for breeding by these terns will be <1 km2 at both Prince Edward and Marion islands; hence the specific breeding area will be <2 km2. Assuming that they feed across both Prince Edward (45 km2) and Marion (290 km2) islands, their inland foraging would cover 335 km2. Prince Edward and Marion islands have coastlines of c. 72 and c. 34 km, respectively. If it is assumed that the terns feed within 10 km of the islands, then their marine feeding area would have an extent of c. 1060 km2. Hence the extent of their feeding area would be c. 1400 km2.

    Ecology

    The Kerguelen Tern is a summer-breeding, surface-nesting species (Cooper and Brown 1990). Courtship takes place from August to September (Ryan 1987), with egg-laying spanning the period from November to January (Whittington et al. 2009). Breeding sites located on Prince Edward Island were on well-vegetated lava flow ridges (Ryan 1987). They are restricted to the coastal margins of the islands, and mainly along coasts with offshore Macrocystis beds (P Ryan, pers comm). At sea, the species is an inshore, surface feeder, with prey consisting of fish and crustaceans (Ryan 1987, Sagar 1991). Individuals also feed over damp Tillaea meadows (Berruti and Harris 1976) and inland crater lakes (Whittington et al. 2009). Studies at the Crozet and Kerguelen groups of islands demonstrated a catholic diet including fish, crustaceans, mollusks, holothurians, spiders, insects and earthworms (Weimerskirch and Stahl 1988).

    Threats & Conservation

    Threats

    The breeding populations of both Kerguelen and Antarctic terns at île aux Cochons (Crozet Archipelago) were exterminated by feral cats. However, terns were not recorded in the diet of cats at Marion Island (van Rensburg 1985), although this may have been due to the small number of terns there in the early 1980s. Avian disease recently killed substantial numbers of other tern species in South Africa (Khomenko et al. 2018). Assuming limited exchange between the PEIs and the Crozet and Kerguelen groups of islands, the Kerguelen Tern population at the PEIs may be too small to maintain genetic diversity.

    Conservation Measures Underway

    There are no species-specific conservation measures underway. The PEIs are listed as a Ramsar Wetland of International Importance, and as a Special Nature Reserve under South Africa’s Environment Conservation Act (No 73 of 1989). They are managed under the Prince Edward Islands Management Plan of 1996. Several general conservation measures are in place at the PEIs, where a marine protected area was proclaimed in 2013.

    Conservation Measures Proposed

    The vulnerability of small populations to random, catastrophic events makes Kerguelen Terns at the PEIs inherently at risk to events such and oil spills, disease outbreaks and mortality brought about by severe weather. Accordingly, this species should be granted high conservation priority.

    Research Priorities and Questions

    • The geographic isolation and small population size of the regional population may have led to genetic divergence from the populations at the Crozet and Kerguelen groups of islands. An investigation into the phylogenetic relationships between these populations is warranted.
    • Areas where Kerguelen Terns are known to have bred at Marion and PEI should be mapped. Except during limited research initiatives, active nests should be avoided completely, including during conservation initiatives such as attempts to rid Marion Island of house mice.
    • An up-to-date estimate of Kerguelen Tern numbers at PEI during their main breeding period is much needed.
    • Demographic information is required for Kerguelen Terns breeding at the PEIs, including breeding success, age at breeding and adult survival. Banding fledglings of known sex would also allow estimates of sex-specific survival rates. Handling of birds should be minimised, e.g. through deployment of colour-coded bands.
    • Regular checks on the sex ratios of chicks produced at Marion Island would give an indication of possible drift towards a skewed sex ratio.
    • Foraging ecology and links between Kerguelen Terns and other inshore foraging species (including Antarctic Tern and Kelp Gull Larus dominicanus) could be investigated at Marion Island as a means to understand possible causes for the low numbers of birds at the PEIs.

    Contributors & References

    Assessor/s

    Azwianewi B Makhado, Phil A Whittington, Robert JM Crawford

    Reviewer/s

    Peter Ryan

    References

    Berruti A, Harris A. 1976. Breeding schedules of Antarctic and Kerguelen terns at Marion Island. Notornis 23: 243–245.

    BirdLife International. 2018. Sterna virgata. The IUCN Red List of Threatened Species 2018: e.T22694641A132564557. https://dx.doi.org/10.2305/IUCN.UK.2018-2.RLTS.T22694641A132564557.en. [Accessed on 13 February 2025].

    Brooke RK. 1984. South African Red Data Book – Birds. South African National Scientific Programmes Report 97: 1–213.

    Cooper J, Brown CR. 1990. Ornithological research at the sub-Antarctic Prince Edward Islands: a review of achievements. South African Journal of Antarctic Research 20: 40–57.

    Crawford RJM, Cooper J, Dyer BM, Greyling MD, Klages NTW, Ryan PG, Petersen SL, Underhill LG, Upfold L, Wilkinson W, De Villiers MS, Du Plessis S, Du Toit M, Leshoro TM, Makhado AB, Mason M, Merkle D, Tshingana D, Ward VL, Whittington PA. 2004. Populations of surface-nesting seabirds at Marion Island, 1994/95–2002/03. African Journal of Marine Science 25: 427–440.

    Gomez V, Voisin J-F. 2002. Spectrometry validates subspeciation in the Kerguelen Tern Sterna virgata. Marine Ornithology 30: 19–24.

    Jouventin P, Stahl J-C, Weimerskirch, H. 1988. La conservation des oiseaux des Terres Australes et Antarctiques Françaises. In: Thibault J-C, Guyot, I (eds), Livre rouge des oiseaux menacés des régions françaises d’Outre-Mer. Conseil International pour la Protection des Oiseaux, Saint-Cloud, France. pp 225-251.

    Khomenko S, Abolnik C, Roberts L, Waller L, Shaw K, Monne I, Taylor J, Dhingra M, Pittiglio C, Mugyeom M, Roche X, Fredrick K, Kamata A, Okuthe S, Kone P, Wiersma L, Von Dobschuetz S, Soumare B, Makonnen Y, Morzaria S, Lubroth J. 2018. 2016–2018 spread of H5N8 highly pathogenic avian influenza (HPAI) in sub-Saharan Africa: epidemiological and ecological observations. FOCUS ON 12.

    Ryan PG. 1987. The distribution, population size and foraging behaviour of Kerguelen terns at the Prince Edward Islands. South African Journal of Antarctic Research 17: 163–166.

    Ryan PG, Bester MN. 2008. Pelagic predators. In: Chown SL, Froneman PW (eds), The Prince Edward Islands: land-sea interactions in a changing system. Stellenbosch, South Africa: Sun Media. 121–164.

    Ryan PG, Cooper J, Dyer BM, Underhill LG, Crawford RJM, Bester MN. 2003. Counts of surface-nesting seabirds breeding at sub-Antarctic Prince Edward Island, summer 2001/02. African Journal of Marine Science 25: 441–451.

    Sagar PM. 1991. Aspects of the breeding and feeding of Kerguelen and Antarctic terns at the Kerguelen Islands. Notornis 38: 191–198.

    Taylor MR, Wanless RM. 2015. Kerguelen Tern Sterna virgata. In: Taylor MR, Peacock F, Wanless RM (eds), The 2015 Eskom Red Data Book of Birds of South Africa, Lesotho and Swaziland. Johannesburg, South Africa: BirdLife South Africa. pp 166–168.

    van Rensburg PJJ. 1985. The feeding ecology of a decreasing feral house cat, Felis catus, population at Marion Island. In: Antarctic Nutrient Cycles and Food Webs. Berlin, Germany: Springer. 620–624.

    Weimerskirch H, Stahl JC. 1988. The breeding and feeding ecology of the Kerguelen tern. Ornis scandinavica 19: 199–204.

    Weimerskirch H, Zotier R, Jouventin P. 1989. The avifauna of the Kerguelen Islands. Emu 89: 15–29.

    Whittington PA, Crawford RJM, Dyer BM, Ryan PG. 2009. Estimates of numbers of kelp gulls and Kerguelen and Antarctic terns breeding at the Prince Edward Islands, 1996/97–2008/09. African Journal of Marine Science 31: 439–444.

    Williams AJ, Siegfried WR, Burger AE, Berruti A. 1979. The Prince Edward Islands: a sanctuary for seabirds in the Southern Ocean. Biological Conservation 15: 59–71.

    Citation

    Makhado AB, Whittington PA, Crawford RJM 2025. Kerguelen Tern. In: Lee ATK, Rose S, Banda S, Bezeng SB, Maphalala MI, Maphisa DH, Smit-Robinson H (eds), The 2025 Red Data Book of Birds of South Africa, Lesotho and Eswatini. Johannesburg, South Africa: BirdLife South Africa. Available at: https://www.birdlife.org.za/red-list/kerguelen-tern/

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