In addition, proponents and land managers should refer to the Recovery Plan (where available) or the Conservation Advice (where available) for recovery, mitigation and conservation information.
|EPBC Act Listing Status||
as Sterna anaethetus
Listed migratory - CAMBA as Sterna anaethetus, JAMBA as Sterna anaethetus
|Adopted/Made Recovery Plans|
|Policy Statements and Guidelines||
Marine bioregional plan for the North Marine Region (Department of Sustainability, Environment, Water, Population and Communities (DSEWPaC), 2012x) [Admin Guideline].
Marine bioregional plan for the South-west Marine Region (Department of Sustainability, Environment, Water, Population and Communities (DSEWPaC), 2012z) [Admin Guideline].
Offshore and foraging pelagic seabirds - A Vulnerability Assessment for the Great Barrier Reef (Great Barrier Reef Marine Park Authority (GBRMPA), 2011h) [Admin Guideline].
Federal Register of
List of Migratory Species (13/07/2000) (Commonwealth of Australia, 2000b) [Legislative Instrument] as Sterna anaethetus.
Declaration under section 248 of the Environment Protection and Biodiversity Conservation Act 1999 - List of Marine Species (Commonwealth of Australia, 2000c) [Legislative Instrument] as Sterna anaethetus.
|Scientific name||Onychoprion anaethetus |
|Species author||(Scopoli, 1786)|
|Reference||Christidis, L. & Boles, W.E. (2008) Systematics and Taxonomy of Australian Birds, p 144|
|Other names||Sterna anaethetus |
This is an indicative distribution map of the present distribution of the species based on best available knowledge. See map caveat for more information.
Scientific Name: Onychoprion anaethetus
Common Name: Bridled Tern
Other English names: Brown-winged Tern, Atlantic Bridled Tern, Red Sea Bridled Tern, Panayan Tern, Dog Tern, Smaller Sooty Tern and, in common with Sooty Tern, Egg Bird and Booby (Avibase 2008c; Gill & Wright 2006, 2008; Gochfeld & Burger 1996; Haney et al. 1999; Higgins & Davies 1996).
The Bridled Tern has long been placed in the genus Sterna (typical terns). Recent taxonomic research using mitochondrial DNA (Bridge et al. 2005) supported the subdivision of Sterna and recognized the genus Onychoprion for the brown-backed terns, which includes the Bridled and Sooty (O. fuscata) Terns (Christidis & Boles 2008).
The Bridled Tern is a conventionally accepted species (Christidis & Boles 2008; Higgins & Davies 1996).
The Bridled Tern is an elegant, medium-sized tern (total length 3032 cm; mean adult weight 110180 g [Higgins & Davies 1996]) with a fairly sturdy, grey-black bill (about the same length as the head), long slender wings and a long, deeply forked tail. Sexes do not differ in plumage or size (though females from Western Australia appear to have shorter bills, though sample sizes are small) (Higgins & Davies 1996).
Breeding. Adults in breeding plumage have a neat black cap, hindneck and loral stripe, isolating a conspicuous white forehead and supercilium that extends past the rear of the eye (distinguishing Bridled from the closely similar Sooty Tern), where it tapers to a point; the rest of the head and neck is white. The rest of the upperparts are dark brownish-grey, with white outer edges to the tail, a white leading edge to the innerwing and basal outerwing, and contrastingly dark-brown remiges. The underbody is pale grey, palest on the breast, and the underwing is mostly white with a dark-grey leading edge (on the outermost primary) and complete dark-grey trailing edge. In breeding plumage, the elongated outermost rectrices form long tail-streamers. With wear, the plumage becomes almost white below, and browner on the mantle, and birds develop a pale, off-white collar across the hindneck. The eyes are black-brown and the legs and feet dark grey.
Non-breeding. Adults are like breeding birds but the cap is dark brown (not black) with white spotting and speckling to the crown, lores and behind and below the eye; the upperparts are much browner throughout, with fine white margins to feathers; the underbody is white; and the tail-streamers are short (about 30 & 40 mm shorter than in adult breeding).
Juveniles. Juveniles are like duller versions of the adult, with conspicuous pale scaling and streaking to the upperparts. The cap is dark brown, finely streaked white or buff on the crown and nape and diffusely spotted buff behind and below the eyes, with a greyish collar that extends from the hindneck onto the sides of the breast. The supercilium is off-white and less distinct than in the adult. The rest of the upperparts are predominantly grey with buff scaling and black crescents on the mantle, buff scaling to the upperwing-coverts, and a dark-brown subterminal tail-band and buff tip to tail; the primaries and secondaries are black fringed with grey. The underbody is off-white with dusky flanks and mottling to the undertail-coverts, and the underwing is largely off-white with a dark leading edge to the outerwing-coverts and a broad dusky trailing edge. The bill is stubbier than in adults, and there are no elongated tail-streamers.
First year immatures in non-breeding. Plumage is like the juvenile but with better defined white forehead, whitish lores speckled with dark brown, greyer upperparts with finer and greyish scaling, whiter flanks and the tail, while shorter than in adult, lacks the pale tip of juvenile (Gochfeld & Burger 1996; Higgins & Davies 1996).
Bridled Terns are typically gregarious, breeding in small to large colonies (of several hundred to thousands of birds), but are more often seen in loose, small to moderately large flocks (typically up to 25 birds in Western Australia) or singly when foraging and during non-breeding periods. The species roosts in groups at colonies. Will forage in mixed flocks with Black Noddies (Anous minutus), Arctic Jaegers (Stercorarius parasiticus), White-winged Black Terns (Chlidonias leucoptera), Common Terns (S. hirundo), Sooty Terns, Roseate Terns (S. dougallii) and Fairy Terns (Sternula nereis). They are typically quiet at sea but call much at breeding colonies (Coates & Bishop 1997; Haney et al. 1999; Higgins & Davies 1996).
In Australia, Bridled Terns are widespread, breeding on offshore islands in western, northern and north-eastern Australia, extending from Cape Leeuwin in the south-west, around northern Australia to north-eastern and mid-eastern Queensland, extending through the Great Barrier Reef and Coral Sea as far south as Lady Elliott Island (approximately 24° S). Exceptionally, a pair bred in South Australia, within a large colony of Crested Terns (Thalasseus bergii), on Baudin Rocks, in 1968 and 1969. Further, the species breeds at one mainland site in far-southern Western Australia (at Knobby Head near Cape Hamelin).
The species forages in offshore, continental shelf waters and is only rarely recorded along mainland coasts, even those adjacent or close to breeding colonies (though note breeding on mainland in Western Australia just mentioned). At least, the southern populations migrate north after breeding. The species is a vagrant to southern and south-eastern Australian waters outside the breeding range (Barrett et al. 2003; Blakers et al. 1984; Bonnin 1968, 1969, 1982; Higgins & Davies 1996; Hulsman & Langham 1985; Johnstone & Storr 1998).
In Western Australia, breeding is widespread from islands off Cape Leeuwin (extending round the southern coast to Seal Rocks) north to Shark Bay and in Pilbara region and Kimberley Division. At sea, distribution extends from Cape Leeuwin north to Dirk Hartog Island, with isolated mainland coastal records at Point Maud and Ningaloo, and from Barrow Island to the Dampier Archipelago, and at sea off the Kimberley coast from waters west of the Dampier Peninsula to Ashmore Reef and Joseph Bonaparte Gulf (Barrett et al. 2003; Blakers et al. 1984; Higgins & Davies 1996; Johnstone & Storr 1998). In the Northern Territory, most breeding colonies are in the eastern portion of the territory, with main colonies being off north-eastern Arnhem Land, and on south-eastern Groote Eylandt and the Sir Edward Pellew Group.
At sea, the species is an occasional visitor to mainland coasts at Darwin and is found infrequently in the south-western Arafura Sea, but is more widespread from Wessel Islands to the western and southern Gulf of Carpentaria but not recorded in western Cape York Peninsula (Barrett et al. 2003; Blakers et al. 1984; Chatto 2001; Higgins & Davies 1996; Johnstone & Storr 1998; Hulsman & Langham 1985).
In eastern Queensland, breeding colonies are widely distributed on islands in all regions except the south-east, including in the Coral Sea (at Chilcott Islet, in Coringa Islets, and suspected at North-East Cay, on Herald Cays). The species is widespread at sea off north-eastern and mid-eastern Queensland, including the Torres Strait, south to approximately 24° S (Barrett et al. 2003; Blakers et al. 1984; Draffan et al. 1983; Higgins & Davies 1996; James & Reid 1999; Johnstone & Storr 1998).
The species is primarily a vagrant to waters of southern and south-eastern Australia (Barrett et al. 2003; Blakers et al. 1984; Green 1983; Higgins & Davies 1996). The species is recorded sporadically on the Queensland coast south of 24° S, rarely south to Fraser and Stradbroke Islands (Barrett et al. 2003; Blakers et al. 1984; Higgins & Davies 1996). In NSW, there is only a single confirmed sighting, off Wollongong, on November 1994 (Brandis 1996; McAllan & Bruce 1988; Morris 1996). In South Australia, Bridled Terns were first recorded in January 1968, when a pair was found breeding, with up to four birds recorded seven times up until 1975, but not since (Bonnin 1968, 1969, 1982; Higgins & Davies 1996). Elsewhere within Australian territory, a single specimen was collected on Cocos-Keeling Islands in July 1941 (Gibson-Hill 1949b, 1950c), apparently of subspecies O. a. antarctica (which is the sole known record of this subspecies in Australian territory) (Clayton et al. 2006; Higgins & Davies 1996), though the possibility that it is subspecies O.a. fuligula (as recognised here) needs investigation.
There is no estimate of the extent of occurrence of Bridled Terns in Australia. Estimated global extent of occurrence is between 100 000 and 1 000 000 km² (BirdLife International 2007n). The source of this estimate is not known, and there are no available data to indicate past declines or future changes.
The estimated area of occupancy of Bridled Terns in Australia is 17 500 km².
In a summary of known Australian breeding sites, Higgins and Davies (1996) listed 125 islands or island groups throughout the Australian range where breeding is known to occur, listing 52 islands in Western Australia, three in the Northern Territory and 70 in Queensland. However, breeding recorded on multiple islands within many of the island groups listed, for example, in a recent survey of the Houtman Abrolhos, the species was recorded on 97 of the 146 islands surveyed (Burbidge & Fuller 2004). Dunlop (1997) states the species breeds on about 150 continental islands in Western Australia, and Johnstone and Storr (1998) list at least 138 rocks, islets, islands and island groups in Western Australia. Recent surveys of the Northern Territory recorded confirmed breeding in 43 colonies (Chatto 2001). In a summary of the status of Queensland seabirds, King (1993) listed 39 of the major seabird breeding islands in Queensland as confirmed breeding sites for Bridled Terns (and another possible site).
There are no known captive populations of this species and the species has not been reintroduced into the wild in Australia or elsewhere.
The species is also widely distributed in tropical and subtropical waters of the western and eastern Indian Ocean, western Pacific Ocean and western Atlantic Ocean and with several more isolated populations in the eastern Atlantic Ocean.
Worldwide, the Bridled Tern occupies tropical and subtropical waters and coastlines, with several apparently discrete populations, which are treated as subspecies.
The nominate subspecies O. a. anaethetus is widely distributed at sea in the eastern Indian and western Pacific Oceans, ranging from the seas of western Indonesia (west as far as seas around western Sumatra) and western Australia, north-east through the South China Sea and along the coast of South-East Asia to southern Japan, the Philippines and Palau, and east through the Indonesian Archipelago and the seas of northern and north-eastern Australia and New Guinea, to Tonga and Samoa. Within this range the species breeds widely from Indonesia, Australia and New Guinea to Taiwan, the Philippines and Ryuku and Volcano Islands of Japan, and east to western Polynesia (including Fiji, Tonga, Samoa and American Samoa) (AOU 1998; Gochfeld & Burger 1996; Higgins & Davies 1996; Johnstone & Storr 1998; Rinke et al. 1992; Watling 2001; White & Bruce 1986).
Subspecies O.a. nelsoni occupies seas of the eastern Pacific Ocean, from Nayarit, central-western Mexico, south to Ecuador, with breeding recorded from Mexico, north-western Costa Rica and south-western Ecuador (AOU 1998; Gochfeld & Burger 1996).
Subspecies O.a. recognita occurs in the western Atlantic Ocean - Caribbean region, ranging from the West Indies north along the coast of the United States from Florida to Carolina (and occasionally Maine) and the northern coast of the Gulf of Mexico, and south to the Yucatan Peninsula, Belize, Panama and northern Venezuela. Breeding is recorded in the Florida Keys, the Bahamas, Cuba, Cayman Islands, Jamaica, Hispaniola, Puerto Rico, Virgin Islands, Lesser Antilles and off the Yucatan Peninsula, Belize and Venezuela (AOU 1998; Gochfeld & Burger 1996; Higgins & Davies 1996).
Subspecies O.a. melanoptera is found off western Africa in the eastern Atlantic Ocean, breeding off Mauritania and on islands of the Gulf of Guinea (AOU 1998; Gochfeld & Burger 1996; Higgins & Davies 1996).
Subspecies O. a. fuligula is widely distributed in the western Indian Ocean, from the eastern coast of Africa, from as far south as eastern Mozambique and Mauritius, north to the Red Sea, the Persian Gulf and Arabian Sea, and north-east to western India and Sri Lanka. Within this range, Bridled Terns breed at sites in eastern coastal Africa, the Red Sea and Persian Gulf, and in western India (Ali & Ripley 1969; AOU 1998; Gochfeld & Burger 1996; Higgins & Davies 1996).
Subspecies O. a. antarctica breeds on Madagascar, Aldabra, the Seychelles and Mascarenes through the Maldives to the Andaman Islands. Some birds from Seychelles have been recorded in east Africa (Ali & Ripley 1969; AOU 1998; Gochfeld & Burger 1996; Higgins & Davies 1996).
The species has been recorded as a vagrant to many regions, including New Zealand and Vanuatu, Arkansas in the United States, the Caribbean coast of Costa Rica, Newfoundland, Great Britain, south-western coastal Europe and Cape Horn (AOU 1998; Bregulla 1992; Cramp 1985; Gochfeld & Burger 1996; Higgins & Davies 1996).
There is no accurate estimate of the global population of Bridled Terns, though Gochfeld and Burger (1996) suggest the total world population probably exceeds 200 000 birds and Wetlands International (2006) gives the estimated total population of O.a. antarctica as 500 000 birds and that of O.a. recognita as 12 00018 000 birds. Globally, it is listed as Least Concern (BirdLife International 2007n). However, some local populations are apparently under some threat. The species is considered at risk in Tonga and Wallis and Futuna; of conservation concern in Fiji and Samoa; and data deficient in Tuvalu (where its status is not known, with only an old and unconfirmed breeding record) (Watling 2001).
The proportion of the global population of Bridled Terns that occur in Australia is not certainly known. The estimated total minimum and maximum breeding population in Australia is 20 04057 870 breeding pairs (Ross et al. 1995). The Australian populations are of the subspecies O. a. anaethetus, which is widespread in the eastern Indian and western Pacific Oceans. However, both adults and young show high levels of fidelity to natal colonies (Dunlop & Johnstone 1994; Higgins & Davies 1996), which would limit genetic interchange between local populations. Populations disperse from sites after breeding (Higgins & Davies 1996), with banding and recoveries showing some individuals move large distances. For example, birds banded south-western Australia have been recovered in Borneo and the Philippines (Higgins & Davies 1996), indicating mixing of populations in non-breeding periods.
The Australian population of Bridled Terns is reasonably well surveyed as a result of ongoing efforts to describe the seabirds of Australia's islands (published in the Seabird Islands series, in the journal Corella [Australian Bird Study Association], for examples, see Dunlop et al. 1988a; King et al. 1983, 1985; Walker 1989). Higgins and Davies (1996) list 125 islands or island groups where breeding is known to occur. More recent surveys (Chatto 2001) have greatly increased the number of islands on which the species is known to breed. Some breeding colonies of Western Australia have been the subject of recent, and ongoing, study. On Penguin Island, south-western Western Australia, birds were first banded in January 1983 and mark-release-recapture operations have been conducted annually from the 198687 breeding season up to the 200607 season, and over 5000 Terns have been banded (Dunlop & Pedelty 2007; Dunlop et al. 1988a). Further, counts were made of non-burrowing seabirds, including Bridled Terns, on 146 islands of the Houtman Abrolhos in summer 1999, repeating counts made in 199193 (Burbidge & Fuller 2004).
While counts have been conducted on many of the breeding islands of the species, in few cases are there follow-up surveys or data. Further, whereas breeding sites are reasonably well known throughout the Australian range, and numbers fairly accurately known for some breeding colonies (as above), the overall Australian population is not well known because rapid counts may significantly underestimate breeding numbers of the species given that it nests in concealed sites.
There is no estimate of global population size, but the total population is thought not to approach the thresholds for the population size criterion of the IUCN Red List (<10 000 mature individuals in conjunction with appropriate decline rates and subpopulation qualifiers) (BirdLife International 2007n). The species is generally considered common to moderately common (for example, in Western Australia) (Coates & Bishop 1997; Johnstone & Storr 1998), though the species is described as uncommon in at least parts of its range (Stotz et al. 1996).
The total population in Western Australia is estimated to be at least 30 00040 000 pairs and apparently increasing (Dunlop & Johnstone 1994). The total breeding population in the Northern Territory is roughly estimated to be approximately 60 000 breeding birds (Chatto 1998, 1999, 2001). There is no estimate of total breeding population for Queensland.
Six subspecies of Bridled Terns are recognized. These subspecies appear to be geographically isolated (Gochfeld & Burger 1996; Haney et al. 1999; Higgins & Davies 1996) and it is likely that there is little genetic interchange between them. Recoveries of banded birds in Australia indicate high fidelity to natal colonies of both young and adults (Dunlop & Johnstone 1994; Higgins & Davies 1996).
There are no data on global population trends of Bridled Terns but the species is not believed to approach the thresholds for the population decline criterion of the IUCN Red List (>30% decline in 10 years or three generations) (BirdLife International 2007n).
In Australia, the species has markedly extended its range southwards in recent years. In Western Australia, it was not recorded breeding south of the Houtman Abrolhos in 1843, but a few nests were noted on islets off Rottnest Island in 1889. The species has extended its range well south of the Houtman Abrolhos since then, south as far as Seal Island, east of Cape Leeuwin, and populations in this area continue to increase (Dunlop & Johnstone 1994; Higgins & Davies 1996; Johnstone & Storr 1998; Surman & Wooller 2000). In Queensland, while counts show considerable fluctuation, there is some evidence of an increase on and around Lizard Island (Blaber et al. 1998).
Bridled Terns appear to not usually undergo extreme fluctuations in numbers, extent of occurrence or extent of occupancy. However, populations in southern Western Australia appear to be continuing to increase and in Queensland, where numbers are generally stable, fluctuations can occur, with some evidence of an increase on and around Lizard Island (Blaber et al. 1998).
Outside Australia, it is thought at least some West Indies populations fluctuate widely and a few have increased since the 1970s (Haney et al. 1999), although overall the numbers involved are often small.
There are no data on generation length for the Bridled Tern.
Whereas there are many known breeding islands and island groups in Australia, there are few large breeding colonies. Colonies of more than 500 breeding pairs or nests include:
- Penguin Island, 30004000 established breeding pairs in the 200607 season (Dunlop & Pedelty 2007; Dunlop & Rippey 2006), having increased from 200300 pairs in 1983 (Dunlop 1996; Garavanta & Wooller 2000)
- Lancelin Island, approximately 2000 pairs in the 1990s, with an earlier estimate of 200500 breeding pairs in 1976 (Abbott 1978; Higgins & Davies 1996)
- North Fisherman Island, 2000 pairs nested annually 197176 (Johnstone 1978a), and on South Fisherman Island, approximately 500 pairs nested annually 197176 (Johnstone 1978b)
- On the Houtman Abrolhos, 6368 breeding pairs in 1999, spread over 97 islands, including 600 pairs on Beacon Island (Wallabi Group), 500 on Leo Island (Easter Group), and 2500 on Gun Island (Pelsaert Group) (Burbidge & Fuller 2004); total breeding population of the Abrolhos in 199193 was estimated at 7046 breeding pairs, spread over 66 islands (Fuller et al. 1994)
- Lowendal Islands, >10 000 pairs, with 30004000 pairs on Bridled Island (Higgins & Davies 1996)
- On Ashmore Reef, more than 300 birds present and nesting on October-November 1999 (Hobcroft 2000b).
Most colonies consist of 100500 birds, with some containing 10005000 birds. The largest colony, on Three Hummocks Island, was estimated to have up to 30 000 birds but possibly more than 50 000 birds were present. On Higginson Islet, north-eastern Arnhem Land, estimated several thousand pairs nesting during 1993 & 1997. On Low Rock, south-western Gulf of Carpentaria, more than 1000 birds were recorded in late September 1994 (Chatto 1998, 1999, 2001).
- Restoration Rock, Cape York Peninsula, approximately 1000 nesting pairs in 1999 (James 2002)
- Pandora Cay, >500 nests in 1980, but with fewer than 50 nests recorded in 1976 and again in 1982 (King et al. 1983)
- Pelican Island, up to 500 nests (King et al. 1985)
- Three Isles, estimated 10001500 adults, February 1993 (McLean 1996)
- South Barnard Island, >4000 to 6000 adults present in 1989 (Walker & Oldroyd 1991)
- Eshelby Island, 25004000 pairs, 1984-85 and 1985-86 (Walker & Hegerl 1986)
- Bell Cay, 300500 nests, 1976 (Walker & Jones 1986a)
- Lady Elliott Island, >500 nesting, 1986-87 and 1987-88 (Walker 1989).
No hybridisation between Bridled Terns and other species has been reported (Gochfeld & Burger 1996; Haney et al.1999; Higgins & Davies 1996; McCarthy 2006).
Given the widespread distribution of the species on islands in western, northern and north-eastern Australia, many of which are reserved, it is recorded in many conservation reserves. For example, most of the seabird breeding islands within the Great Barrier Reef system are state national parks, and the Great Barrier Reef and its waters are within the Great Barrier Reef Marine Park (GBRMP), which is managed by the Commonwealth Great Barrier Reef Marine Park Authority (GBRMPA) (Stokes et al. 1996). Most of the islands on which the species breeds in the Northern Territory are within Indigenous Australian land, though some colonies are within conservation reserves (Chatto 2001), though none appears to be actively managed for this species.
Bridled Terns occupy tropical and subtropical seas, breeding on islands, including vegetated coral cays, rocky continental islands and rock stacks (Chatto 2001; Dunlop & Jenkins 1992; Dunlop, J.N in Higgins & Davies 1996). Bridled Terns are only rarely found in inshore continental waters and along mainland coastlines, though the species is reported to breed on the mainland of far southern Western Australia (Higgins & Davies 1996; Johnstone & Storr 1998).
Bridled Terns breed on islands. Nests are usually found in rocky areas or on coral, concealed in crevices or caves up to 1.5 m deep, under rocks, among talus or coral rubble, on ledges of cliffs, or on the ground beneath low shrubs, roots of Pandanus, vines or among grasses. Occasionally nests occur on shingle or sandy beaches or, rarely, in the open, on pigface (a plant) or on rock (AOU 1998; Chatto 2001; Dunlop & Jenkins 1992; Higgins & Davies 1996; Hulsman & Langham 1985; Serventy et al. 1971).
During the breeding season in south-western Australia, birds forage over offshore, mid- and outer continental shelf waters, usually within approximately 70 km of breeding colonies but mostly within 2040 km of a colony (Dunlop 1997; Dunlop & Jenkins 1992; Higgins & Davies 1996). Off south-western Australia, there was no direct relationship between the foraging zones of Bridled Terns and the water masses of the Leeuwin Current (Dunlop 1997). Birds breeding on islands of the Great Barrier Reef tend to forage 812 km form their colonies, occasionally within 23 km, especially in the late afternoon. They have been recorded foraging in swell near the surf-zone, in straits between islands and on reefs in very shallow water (Higgins & Davies 1996; Hulsman 1974, 1988; Hulsman & Langham 1985). The species often forages over rafts of Sargassum, particularly in the latter stages of the breeding season. The availability of Sargassum rafts, and other flotsam, in the marine environment may be a key factor influencing the breeding and wintering distribution of this species (Dunlop 1997; Garavanta & Wooller 2000; Higgins & Davies 1996; Hulsman 1988; Surman & Wooller 2000).
The Bridled Tern roosts ashore when breeding. Frequency of use of different sites varies with time of breeding cycle following return from non-breeding areas, and with time of day. At breeding colonies, birds roost or loaf on branches of shrubs or low trees (such as Pisonia, Argusia), on rocks, less often on the ground among vegetation or rubble or on the shoreline. However, at the start of the breeding season and when the chicks are older ( about 40 days old), birds roost or loaf in groups during the day on sandbanks or beaches and the like, distant from final breeding sites. They also roost on artificial structures, including posts and buoys (Chatto 2001; Higgins & Davies 1996; Hulsman & Langham 1985; Serventy et al. 1971).
Roosting behaviour away from breeding colonies is poorly known, but birds appear not to roost ashore. They readily rest on floating material at sea, including coconuts and fishing boats, and such material is probably important for roosting at sea (Dunlop & Johnstone 1994; Higgins & Davies 1996). However, Chatto (2001) located apparent night roosts on some outer islands that may have been used as shore roost by non-breeding birds. For logistical reasons these island roosts were not checked on or after dusk and confirmation is needed.
There is no indication of the use of refuge habitats by Bridled Terns.
Bridled Terns are not known to rely on any threatened ecological community in Australia, nor are they specifically associated with any threatened species though they potentially share habitat with such species. Bridled Terns will associate with or nest close to other species of terns, including Crested, Caspian (Hydroprogne caspia), Roseate Terns, Black-naped Terns (Sterna sumatrana), as well as Silver Gulls (Larus novaehollandiae), Wedge-tailed Shearwaters (Puffinus pacificus ) (Chatto 2001; Higgins & Davies 1996), other terns (including noddies) and shearwaters (Haney et al. 1999).
On Penguin Island, south-western Australia, most Bridled Terns return to their natal colonies in their third year. Age of first breeding appears to depend on both physiological maturity and obtaining a nesting site, but most usually breed in their fourth year (Dunlop & Jenkins 1992, 1994; Dunlop & Pedelty 2007).
Using records of Bridled Terns banded as adults (of >5000 banded Terns), the 20 longest intervals between banding and most recent recapture range from 13.4 to 19.8 years, giving projected age-classes of 16+ to 23+ years. For ten Terns banded as pulli or fledglings, the longest intervals between banding and recapture range from 14.7 to 18.9 years, with known ages of 1519 years (Dunlop & Pedelty 2007).
Bridled Terns are recorded breeding as single pairs or in small to large but loose colonies. They tend to breed in smaller colonies than most other colonial terns though some large colonies are known.
Breeding season varies geographically. Breeding is mainly recorded in austral spring-summer in western and eastern Australia, but the season is possibly longer, or breeding takes place year-round, in colonies of northern Australia and some colonies of north-eastern Queensland. However, northernmost Australian colonies are poorly known. Within colonies, breeding tends to be protracted and not highly synchronized. In Western Australia, birds breed late spring to summer, with eggs recorded from mid-October to late January, and young from mid-December to early March. However, at least occasional winter breeding is known, for example in Houtman Abrolhos juveniles were seen in mid-August 1980 on Newman Island, which must have been from eggs laid in late June or early July (Garavanta & Wooller 2000; Higgins & Davies 1996; Storr et al. 1986; Surman 1994).
In the Northern Territory, breeding is recorded March to November-December. On Higginson Islet, north-eastern Arnhem Land, the season is protracted, with breeding recorded nearly all year, and certainly from March to November with the main period April-June. On some islands, or in some years, breeding was concentrated in a short season, but on other islands breeding was recorded in most months, with other colonies not being consistent in their breeding times (Chatto 1998; 2001).
In north-eastern Queensland most breeding occurs in spring-summer but at some sites breeding appears to occur throughout the year. Eggs have been recorded October to February (Blaber et al. 1998; Higgins & Davies 1996; Hulsman & Langham 1985; Hulsman et al. 1999; Smith & Ogilvie 1989; Walker & Jones 1986a), though clutches also recorded July and on Quoin and Raine Islands breeding is recorded throughout the year (King & Buckley 1985; Walker & Jones 1986a).
In South Australia, a single pair at Baudin Rocks was found with eggs in late December in one season and young in early January in another (Bonnin 1968, 1969).
The clutch-size is one. Clutches of two or three are occasionally reported but they almost certainly arise from loose eggs rolling into other nests (Garavanta & Wooller 2000; Higgins & Davies 1996; Hulsman & Langham 1985). Laying in a colony may be protracted over 3040 days or somewhat synchronised over a period of approximately 15 days. Birds may re-lay after failure (Dunlop & Jenkins 1992; Higgins & Davies 1996; Garavanta & Wooller 2000).
Breeding success varies. On One Tree Island, Queensland, over three seasons (n = from 164 eggs) overall success (young fledged) was 69.5%, varying annually from 17.6% to 96.2%; over two seasons, (n = 113 eggs), hatching success was 94.7% and overall success was 92.9% (Hulsman 1977; Hulsman & Langham 1985). On Penguin Island, Western Australia, in one season, hatching success was 72% and overall success at least 35%, and probably closer to 61% (Garavanta & Wooller 2000).
Bridled Terns nest on the ground, usually under vegetation or in rocky areas that provide shelter, such as beneath overlapping slabs of coral rubble or ledges, or in small caves, crevices or holes, and thus require shrubs or rocky substrate that provide cover over nests. Breeding colonies also need to be near a reliable food supply (within approximately 70 km of their colony). Vegetation utilised can be very thick, such as clumps of prickly pear Opuntia, provided there is access to nesting sites. Otherwise without such good cover, they are vulnerable to predation of their eggs by Silver Gulls and Buff-Banded Rails (Gallirallus philippensis), or of their chicks by Silver Gulls. Cover also provides them with protection from the sun and the chill factor associated with rain and wind. These increase their chances of survival (K. Hulsman 2002, pers. comm.). Ground-nesting makes the chicks vulnerable to drowning in storm-surges during cyclones (Hulsman 1977; Hulsman & Langham 1985; Langham 1986) and makes eggs and young vulnerable to ground-dwelling predators such as rats and lizards, as well as avian predators (Garavanta & Wooller 2000; Higgins & Davies 1996; Warham 1962). Hulsman and Langham (1985) also indicate three other features of the breeding biology of the species that increase susceptibility of eggs and young to predation:
- non-synchronous laying, removing benefits of synchronous breeders swamping potential predators
- adults do not remove egg-shells after hatching, which may attract predators to nesting sites
- the slow growth-rates of chicks, and hence longer fledging period and thus an increased period when young are vulnerable to predation.
Bridled Terns feed on a range of species of fish, crustaceans, cephalopods and insects. In Australia, they feed almost entirely on fish, though they also take crustaceans and aquatic insects. They often feed on schools of fish forced to the surface by other predators. This is reflected in the species caught as these fish occur within the upper few centimetres of the surface. The presence of gastropods and crabs in the diet indicates that they do forage on reefs in very shallow water, but the numerical and biomass dominance of pelagic fish species caught indicate that they mostly forage at sea. Even the reef-dwelling fish that they catch are invariably pre-juvenile individuals that are transported by currents before finding a place to settle (Dunlop 1997; Higgins & Davies 1996; K. Hulsman. 2002, pers. comm.).
Recorded prey items include: molluscs (including gastropods and cephalopods); crustaceans (including prawns and decapod crabs); insects (including Hemiptera, Lepidoptera, Hymenoptera and Coleoptera); and fish (including Acanthuridae, Aluteridae, Apogonidae, Atherinidae, Balistidae, Blenniidae, Carangidae, Clupeidae, Engraulididae, Exocoetidae, Gonorhynchidae, Hemiramphidae, Labridae, Monocanthidae, Mullidae, Pomacentridae, Pomotomidae, Priacanthidae, Scombridae, Syngnathidae and Tetraodontidae) (Barker & Vestjens 1989; Dunlop 1997; Higgins & Davies 1996).
Bridled Terns forage singly or in small loose flocks, mainly by contact dipping, with birds hovering or gliding close to the surface of the sea and swooping down and immersing only the head and breast when attacking prey, which are usually taken from the top few centimetres of the sea surface (<20 cm). Birds occasionally surface-plunge from 213 m above the water at angles of 20°90° and rarely submerging entirely (Cramp 1985; Dunlop 1997; Higgins & Davies 1996; Hulsman 1988). During the breeding season in south-western Australia, birds usually foraged within 70 km of their breeding colonies (Dunlop 1997). On One Tree Island, Great Barrier Reef, Bridled Terns mostly foraged within 15 km of colonies but are recorded up to 28 km from colonies (Higgins & Davies 1996; Hulsman & Langham 1985). Foraging is apparently mostly diurnal, birds possibly moving between breeding colonies and feeding areas at night (Hulsman 1974; Hulsman & Langham 1985).
Bridled Terns are migratory or partly migratory in Australia. On islands off south-western and eastern Australia, birds breed in the austral spring-summer, disperse annually from breeding islands after completion of breeding, then return to breeding sites in the austral spring. Though moving away from Australian coastal waters during the non-breeding season, the non-breeding range is not well known, though at least some of the population moves to the Sulawesi Sea. The movement patterns of birds breeding in northern Australian are essentially unknown (Chatto 2001; Coates & Bishop 1997; Higgins & Davies 1996; Johnstone & Storr 1998) but breeding is known to occur through much of the year, including the austral autumn-winter (Chatto 1998, 2001).
In Western Australia, almost all Bridled Terns return to breeding colonies between late September and mid-October and normally leave from early to mid-April, although they leave some colonies in some years as late as mid-May. Birds are usually absent from breeding colonies and adjacent seas from early May to mid-September. Populations breeding in south-western Australia appear to migrate north to offshore waters of the north-western Sulawesi Sea, between approximatley 4° and 7° N, with adults present there June-August. On northern passage from Western Australia, Bridled Terns appear to move through the Timor Sea during April, with loose flocks, including that season's young, observed moving north through the Lombok and Lintah Straits and Sabu and Banda Seas, in late April to late May, en route to the north-western Sulawesi Sea. A peak in numbers in the Timor Sea from late September to early October thought to be of birds on return passage to southern Western Australia. Bridled Terns are also recorded off the coast of the Kimberley Division from late September, and off the coast of the Pilbara region from late October, with arrival in more southern breeding colonies also during September and October. Some immatures may remain in wintering areas during their first summer (Dunlop & Johnstone 1994; Dunlop & Pedelty 2007; Dunlop & Rippey 2006; Dunlop et al. 1988b; Johnstone et al. 1993; Serventy et al. 1971).
Movement associated with breeding colonies in Queensland is less well known, and movements associated with Northern Territory and other northern Australia colonies is virtually unknown, though at some colonies breeding occurs year-round, indicating probable lack of migration. In Queensland, most birds leave breeding colonies from February to April, with at least some birds moving north. There is one recovery record of a first-year bird from Papua (Indonesia). Most birds return to breeding colonies during October or November (Higgins & Davies 1996; Hulsman & Langham 1985), though they are known to arrive at Booby Island, in Torres Strait, in August (Garnett et al. 1988) and first arrive at One Tree Island at night about mid-September. Birds are not seen during the day until October (Higgins & Davies 1996; Hulsman & Langham 1985).
Fidelity to breeding islands is high, and on islands where breeding is seasonal, birds return to their natal colony year after year (Dunlop & Jenkins 1992). Natal fidelity results in genetic isolation of colonies (Dunlop and Jenkins 1992) as shown by the continued colour difference between chicks on One Tree Island and Lady Musgrave Island in the Great Barrier Reef. Chicks on One Tree Island are dark grey, matching the colour of the coral slabs on which they nest, whereas chicks found at Lady Musgrave Island, approximately 20 km to the south of One Tree, are white, matching the white sand on which they usually nest. If gene flow were occurring, one would expect the occasional white hatchling to appear at One Tree and the occasional grey one at Lady Musgrave, but this has not been seen or reported despite many visits to these colonies over the past 30 years (K. Hulsman 2002, pers. comm.).
Bridled Terns are distinctive and conspicuous, especially the head-pattern (most noticeably the extent of the white supercilium), and are unlikely to be confused with other species, especially in breeding colonies. However at sea, adults may be confused with Sooty Terns but can generally be distinguished by a two-tone pattern between the black head and nape contrasting with the greyer mantle and brownish wings (the dorsum is uniformly black in Sooty Terns), the pattern of the underwing, the grey wash to the underparts and lighter more reactive patterns of flight (Dunlop et al. 1988b).
Surveys of breeding birds have been conducted by researchers moving on foot, searching for nests or nesting colonies (Burbidge & Fuller 2004) and by aerial survey (Chatto 2001). Surveys need to be conducted during the breeding season, which varies regionally. Determination of numbers can be hindered by the concealed nature of nests, and numbers in large colonies need to be estimated (Burbidge & Fuller 2004; King et al. 1985). The islands on which the species breeds require access by boat (Burbidge & Fuller 2004) or, where access by boat is difficult or impossible, by helicopter (Chatto 1998). Standard seabird-counting methods would be used in surveying birds at sea from vessels.
Human disturbance can be a threat in some colonies of Bridled Terns, causing adults to leave nests and exposing eggs and chicks to potential predation. This is a greater threat in places where Terns nest in the open or in sparse vegetation (for example, Lady Musgrave Island, Queensland) than where they nest among or under vegetation (Garavanta & Wooller 2000; K. Hulsman 2002, pers. comm.). Birds may also break eggs when scrambling from nests (Garavanta & Wooller 2000).
Predators include King's Skink (Egernia kingii) (on Penguin Island), White-bellied Sea-Eagles (Haliaeetus leucogaster), Silver Gulls and Red Foxes (Vulpes vulpes); Buff-banded Rails, and rats (Rattus spp.) that probably take eggs (Chatto 1998; Garavanta & Wooller 2000; Higgins & Davies 1996).
On some islands in south-western Western Australia, Bridled Terns are said to compete with Rock Doves (Columba livia ) for nesting sites in small caves in limestone cliffs (Higgins & Davies 1996).
Cyclones and storms can be a threat, with young drowning from storm-surges and some chicks may starve after cyclones if adult foraging efficiency is reduced (Hulsman 1977; Hulsman & Langham 1985; Langham 1986; Chatto 1999). On Gannet Cay, Great Barrier Reef, after a cyclone destroyed plant cover in the summer of 1980, the species was not recorded on the island until April 1986 (Walker & Jones 1986c) though any subsequent absence is not known.
To reduce the human disturbance at breeding colonies of Bridled Terns where people visit currently or will in future, it is recommended that birds are gradually habituated to the presence of people. This may be achieved by gradually allowing access in consistently sized groups and creating a predictable pattern of human movements, such as restricting people to boardwalks (Dunlop 1996; Stokes et al. 1996; WBM Oceanics & Claridge 1997).
There are no mitigation measures known specifically for Bridled Terns. Stokes and colleagues (1996) outline potential measures for mitigating human disturbance on seabird breeding islands within the Great Barrier Reef Marine Park. Some significant seabird breeding islands are totally closed to human visitation, with access by people usually only permitted for research purposes. For example, Eshelby Island, off Bowen, is permanently closed except for permitted research and maintenance activities to protect the largest colony of Bridled Terns on the Great Barrier Reef (WBM Oceanics & Claridge 1997).
There have been several important studies on various aspects of the biology of Bridled Terns in Australia, conducted primarily in south-western Australia and on islands of the Great Barrier Reef. Important studies in south-western Australia include Dunlop (1996, 1997), Dunlop and Jenkins (1992, 1994), Dunlop and Johnstone (1994), Dunlop and Pedelty (2007), Dunlop and Rippey (2006), Dunlop and Wooller (1986, 1990), Dunlop and colleagues (1988a, 1988b), Garavanta and Wooller (2000), and Surman and Wooller (2000). Important studies conducted in the Great Barrier Reef include Hulsman (1974, 1977, 1987, 1988), Hulsman and Langham (1985), Langham (1984), and Stokes and colleagues (1996).
There have been many studies overseas, much of which summarized in major works on the birds of these regions (Cramp 1985; Gochfeld & Burger 1996; Haney et al. 1999; Urban et al. 1986).
No species-specific management documents are known for the Bridled Tern but guidelines have been prepared for managing human visitation to seabird breeding islands (Stokes et al. 1996; WBM Oceanics & Claridge 1997). Detailed summaries of current knowledge of the species in Australasia are found in Higgins and Davies (1996) and Johnstone and Storr (1998), and international summaries in Cramp (1985), Gochfeld and Burger (1996), Haney and colleagues (1999), and Urban and colleagues (1986).
Marine bioregional plans have been developed for four of Australia's marine regions - South-west, North-west, North and Temperate East. Marine Bioregional Plans will help improve the way decisions are made under the EPBC Act, particularly in relation to the protection of marine biodiversity and the sustainable use of our oceans and their resources by our marine-based industries. Marine Bioregional Plans improve our understanding of Australia's oceans by presenting a consolidated picture of the biophysical characteristics and diversity of marine life. They describe the marine environment and conservation values of each marine region, set out broad biodiversity objectives, identify regional priorities and outline strategies and actions to address these priorities. Click here for more information about marine bioregional plans.
The Bridled Tern has been identified as a conservation value in the South-west (DSEWPaC 2012z) and North (DSEWPaC 2012x) marine regions. See Schedule 2 of the South-west Marine Bioregional Plan (DSEWPaC 2012z) and the North Marine Bioregional Plan (DSEWPaC 2012x) for regional advice. Maps of Biologically Important Areas have been developed for bridled tern in the North (DSEWPaC 2012x) and South-west (DSEWPaC 2012z) marine regions and may provide additional relevant information. Go to the conservation values atlas to view the locations of these Biologically Important Areas. The "species group report card - seabirds" for the South-west (DSEWPaC 2012z) and North (DSEWPaC 2012x) marine regions provide additional information.
The following table lists known and perceived threats to this species. Threats are based on the International Union for Conservation of Nature and Natural Resources (IUCN) threat classification version 1.1.
|Threat Class||Threatening Species||References|
|Climate Change and Severe Weather:Storms and Flooding:Natural events such as storms and cyclones leading to habitat destruction and flora/fauna mortality||Sterna anaethetus in Species Profile and Threats (SPRAT) database (Department of the Environment and Heritage, 2006xw) [Internet].|
|Climate Change and Severe Weather:Storms and Flooding:Storm damage||Sterna anaethetus in Species Profile and Threats (SPRAT) database (Department of the Environment and Heritage, 2006xw) [Internet].|
|Human Intrusions and Disturbance:Human Intrusions and Disturbance:Human induced disturbance due to unspecified activities||Sterna anaethetus in Species Profile and Threats (SPRAT) database (Department of the Environment and Heritage, 2006xw) [Internet].|
|Invasive and Other Problematic Species and Genes:Invasive Non-Native/Alien Species:Competition and/or predation||Vulpes vulpes (Red Fox, Fox)|
|Invasive and Other Problematic Species and Genes:Invasive Non-Native/Alien Species:Competition and/or predation by rats|
|Invasive and Other Problematic Species and Genes:Invasive and Other Problematic Species and Genes:unspecified|
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This database is designed to provide statutory, biological and ecological information on species and ecological communities, migratory species, marine species, and species and species products subject to international trade and commercial use protected under the Environment Protection and Biodiversity Conservation Act 1999 (the EPBC Act). It has been compiled from a range of sources including listing advice, recovery plans, published literature and individual experts. While reasonable efforts have been made to ensure the accuracy of the information, no guarantee is given, nor responsibility taken, by the Commonwealth for its accuracy, currency or completeness. The Commonwealth does not accept any responsibility for any loss or damage that may be occasioned directly or indirectly through the use of, or reliance on, the information contained in this database. The information contained in this database does not necessarily represent the views of the Commonwealth. This database is not intended to be a complete source of information on the matters it deals with. Individuals and organisations should consider all the available information, including that available from other sources, in deciding whether there is a need to make a referral or apply for a permit or exemption under the EPBC Act.
Citation: Department of the Environment (2014). Onychoprion anaethetus in Species Profile and Threats Database, Department of the Environment, Canberra. Available from: http://www.environment.gov.au/sprat. Accessed Fri, 18 Apr 2014 09:45:26 +1000.