Black-fronted Tern Chlidonias albostriatus


Justification of Red List Category
This species has a small population which has shown very rapid population reductions across its breeding range exceeding 50% within three generations, and this is predicted to continue in the future owing to ongoing threats. It is therefore classified as Endangered.

Population justification
Previously the population has been estimated at 7,000-10,000 individuals (R. Keedwell in litt. 2006), roughly equivalent to 4,600-6,700 mature individuals. Similarly, based on index counts of rivers, the population was estimated at c.10,000 mature individuals in 2011 (O'Donnell and Hoare 2011). However, data based on genetic samples and evidence of steep ongoing declines (thought to be equivalent to 50-70% within three generations) suggest that the population is less than 10,000 mature individuals at present. Between 2013 and 2015, the effective number of breeders was estimated at around 700 (381-4,229) based on 17 microsatellite loci of 104 chicks (Schlesselmann and Robertson 2020). The effective population size is estimated at 3,216 (1,827-10,611) based on 422 chick and adult samples (Schlesselmann and Robertson 2020). In 2020, the New Zealand Department Conservation estimated there to be 1,000-5,000 mature individuals overall (Robertson et al. 2021).

Trend justification

Breeding populations on braided rivers in the South Island, New Zealand, are assumed to be in decline as their habitat comes under increasing pressure from exotic pests, hydroelectric power development and water abstraction. A meta-analysis of the trends of the breeding population from 1962 to 2008 (O’Donnell and Hoare 2011) showed that rivers on which declines have occurred are characterised by having relatively low flows. At such rates, populations on low-flow rivers (51.4% of Black-fronted Terns counted on the oldest counts) would decline by a further c. 90% within 25 years. Even if populations on larger rivers were stable, it is predicted that the total population would decline by c.50% over the next 25 years. Although there has not been an updated trend analysis, annual reports continue to show that nest survival is poor across many different river systems such as Aparima, Waitaki, Rangitata, Rakaia, Ashley and Hurunui, with relatively high survival at the Tasman and Clarence rivers due to conservation management (A. Schlesselmann in litt. 2022). Given that most unmanaged river systems have poor nesting success and that the managed population only represents 5-10% of the global population, steep declines are thought to be ongoing and Robertson et al. (2021) estimate an ongoing rate of 50-70% within three generations.

Distribution and population

Chlidonias albostriatus breeds in the South Island, New Zealand. It is found from September to February predominantly along the eastern riverbeds from Marlborough to Southland, and on the upper Motueka, Buller and Grey Rivers (Heather and Robertson 1997, Schlesselmann et al. 2017). Birds disperse to the coastline and estuaries in winter, mostly from Stewart Island to the southern North Island, occasionally seen as far north as Kaipara Harbour (Lalas 1979). During winter, the species has also been observed feeding at sea within 10 km of the coast (Heather and Robertson 1997, Taylor 2000).


It breeds on riverbeds, creating simple scrapes in the shingle. It usually lays two eggs (Keedwell 2005). The young fledge after c.30 days. It feeds on freshwater invertebrates and small fish, taken as it forages over channels in gravelly rivers of South Island, occasionally taking earthworms and other invertebrates in terrestrial environments such as pastureland (Lalas 1977) and, when at sea, feeding mainly on crustaceans (Heather and Robertson 1997). Adults in one colony fed their chicks 36-73 common skinks Oligosoma polychroma per hour (O'Donnell and Hoare 2009).


Introduced mustelids Mustela spp., feral cats Felis catus, Brown Rats Rattus norvegicus and House Rats Rattus rattus, Western European Hedgehogs Erinaceus europaeus, Common Brushtail Possums Trichosurus vulpecula, dogs, Australian Magpies Gymnorhina tibicen, as well as native Kelp Gulls Larus dominicanus, Swamp Harriers Circus approximans and South Island Oystercatchers Haematopus finschi prey on eggs, chicks, and/or adults of this species, and major breeding failures have been recently reported at colonies as a result of predation (Keedwell 2005, Steffens et al. 2012, Bell 2017, Schlesselmann et al. 2018).

Cats Felis catus have been confirmed as predators of both adults and chicks in a study of tern nest predation, via video evidence, can also cause desertion without a predation event (Keedwell 2005), DNA evidence also showed cats responsible for the majority of adults killed in one study (Steffens et al. 2012). A single cat has been recorded as causing the demise of 76% of nests and 10% breeding adults in tern colony on Rangitata River (O'Donnell et al. 2010). In one study, despite ongoing control measures to reduce cat predation, including the control of rabbits to reduce hyperpredation, it seems that cats did not avoid the tern colony (Cruz et al. 2014), implying control methods may be ineffective. In the past, Brown Rats Rattus norvegicus have been the principle predator of Black-fronted Terns being listed as the cause of mortality for an estimated 51% predator related deaths in study 1998-2000 (1022 nests, Ohau River) (Keedwell et al. 2002). Rats have been videoed eating carcassess of Black-fronted Tern chicks in the Ohau River (Keedwell 2003). Western European Hedgehogs Erinaceus europaeus are also thought to be able to cause considerable damage to Tern colonies. Hedgehogs exist throughout the species’ range and have been shown to predate nests through DNA and video evidence (Keedwell et al. 2002). DNA combined with video evidence showed that stoats were a major cause of adult and juvenile mortality during breeding - stoats were strongly implicated in the desertion of 57 nests on the same night as 8 carcasses returned stoat DNA (Steffens et al. 2012). DNA evidence has also implicated House Rats Rattus rattus in the predation of tern nests (O'Donnell et al. 2010) and video evidence has shown nest predation by ferrets (Keedwell 2005). It is thought that an individual predator may be able to destroy an entire colony in a relatively short space of time (Keedwell et al. 2002). Predator control is ongoing at many sites; however, many attempts have been criticised as ineffective (O'Donnell et al. 2016).

Swamp Harriers Circus approximans were responsible for the majority of nest predation events monitored by video in a small study on the Wairau River (Steffen et al. 2012), indicating that in certain breeding areas losses to native predators may be significant. Avian predators were not a significant risk to the species in another study, and individual predator behaviour may be the determine the severity of the impact, Swamp Harriers occur throughout the breeding range of Black-fronted Tern. Australian Magpies Gymnorhina tibicen have also been recorded to predate Black-fronted Tern nests, but are not thought to be significant predators (Keedwell and Sanders 1999).

Black-fronted Terns rely on gravel beds around braided river systems to breed and as such, are vulnerable to changes in human land and resource use. Future developments are a major threat, for example, the Wairau River project, which proposes the diversion of water into a man-made canal, would affect 12% of the nesting population, destroying vital tern habitat as some areas of river would be prone to drying up. This would: change hydraulic regimes; encourage weed establishment, reducing breeding ground; alter feeding habitat; and encourage predators to nest sites (Keedwell et al. 2002, Nelson 2006). However, despite still being active, the Wairau project is currently on hold. A meta-analysis of the trends of the breeding population from 1962-2008 (O’Donnell and Hoare 2011) showed that rivers on which declines have occurred are characterised by having relatively low flows. These stabilise islands and increase channelisation, facilitating the spread of alien plant species and increasing mammalian access to nesting areas (Schlesselmann et al. 2018). Invasion of braided river systems by broom and other ‘weeds’ reduces area of gravel beds available for nesting; provides areas for mammalian predators to hide, so increasing adult and nest mortality; and changes river dynamics by constraining river braids into deep, swift channels which are not conducive to suitable gravel beds (Taylor 2000, Keedwell et al. 2002, Duncan et al. 2008). Broom-invaded gravel bed systems recover upon the removal of broom (Caruso 2006). Similar is true for a number of other plants, for example, in Eglinton Valley, Lupins Lupinus spp. were cleared by hand-pulling and within a week, a colony of around 50 Black-fronted Terns had established on one island, with birds continuing to nest with repeated weeding (O'Donnell et al. 2016).

Human disturbance can cause breeding failure, via nests being trampled and crushed by vehicles, and disturbance at wintering sites. High levels of recreational use of rivers often coincide with Black-fronted Tern breeding season potentially reducing egg- and chick-survival (Keedwell 2005). Other human activities, such as dog-walking, are also likely to cause disturbance. Although it is well-documented that the species may be threatened at its roosting sites and during migration by predation, disturbance and development, key information of their migration pathways, migratory connectivity, and threats at wintering sites is still lacking.

Conservation actions

Conservation Actions Underway
Several studies have been completed covering aspects of the species' biology and ecology. Habitat restoration and nest monitoring is carried out by Project River Recovery and Te Manahuna Aoraki Project in a number of major riverbed habitats in the MacKenzie basin, covering part of the range (Cruz et al. 2013). In addition, predator control and/or population monitoring is being carried out in the Clarence, Acheron, Ashley, Rangitata, lower Waitaki, Makarora and Eglinton Rivers. Habitat restoration in the form of clearing islands of introduced vegetation is carried out in the Waitaki and Clarence Rivers (Bell 2017, Schlesselmann et al. 2018). Social attractions in the form of decoys and audio playback has been tested to aid conservation projects showing that black-fronted terns interacted with social attractants, but did not form colonies among the social attractants (Hamblin et al. 2019).

Conservation Actions Proposed
Census and map all breeding colonies, and census winter flocks including genetic monitoring. Examine all proposals for the development of hydroelectric dams or irrigation projects to identify impacts on the species, including reduced flows on potential loss of feeding habitat, increased vegetation establishment, lack of floods causing channelisation (A. Schlesselmann in litt. 2020). Identify if there are any threats during migration, staging, and at wintering sites (A. Schlesselmann in litt. 2020). Carry out nest monitoring to measure fledging success throughout the South Island (including upper and lower reaches of rivers).
Continue adaptive management of predator control and introduced vegetation clearance to increase productivity (A. Schlesselmann in litt. 2020).


29 cm. Small, grey tern with black cap. Grey body and wings. Short, forked tail. White underparts. In flight, contrasting white rump. Bright orange legs, bill. Breeding adult, black cap extending to bill. Thin white line along cheek. Non-breeding adult, grey-flecked crown reduced to arc from eye to eye. Juvenile, similar except black-flecked crown, white chin. Similar spp. Whiskered Tern Chlidonias hybrida has even grey on rump. Little Tern Sternula albifrons has dark primaries on upper wings, yellow bill. Fairy Tern Sternula nereis has yellow-orange bill. White-fronted Tern Sterna striata, Common Tern Sterna hirundo are larger with black bill, legs. Voice Call repetitive kit.


Text account compilers
Martin, R., Vine, J.

Anderson, O., Bell, B.D., Bell, M., Benstead, P., Bird, J., Butchart, S., Calvert, R., Dowding, J.E., Grant, A., Hitchmough, R., Keedwell, R., Mahood, S., McClellan, R., Pilgrim, J., Schlesselmann, A.-K., Stuart, A., Szabo, M., Taylor, G.A. & Taylor, J.

Recommended citation
BirdLife International (2023) Species factsheet: Chlidonias albostriatus. Downloaded from http://datazone.birdlife.org/species/factsheet/black-fronted-tern-chlidonias-albostriatus on 01/06/2023. Recommended citation for factsheets for more than one species: BirdLife International (2023) IUCN Red List for birds. Downloaded from http://datazone.birdlife.org on 01/06/2023.