Sandwich tern (Sterna sandvicensis)

Synonyms: Thalasseus sandvicensis
  
French: Sterne caugek
KingdomAnimalia
PhylumChordata
ClassAves
OrderCharadriiformes
FamilyLaridae
GenusSterna (1)
SizeLength: 34 - 45 cm (2)
Weight180 - 300 g (2)

The sandwich tern is classified as Least Concern (LC) on the IUCN Red List (1).

A medium-sized pale tern, the sandwich tern (Sterna sandvicensis) is considered to be one of six species of ‘crested’ tern, which, according to some taxonomists, should be grouped separately from the typical black-capped terns into the revised genus Thalasseus (3).

Generally a rather slim bird with long, pointed wings and a forked tail, the sandwich tern has a distinctive black crest of spiky, slightly shaggy-looking feathers that extend from the back of the crown (2) (4) (5) (6). The back and upperwings are uniformly pale ash-grey, except for the darker outer primaries. The rump and tail are white, and the underparts are white, with dark trailing edges to the primaries. The legs are black (2) (4) (5) (7). The sandwich tern is the only species of the crested tern group to have a long, slender, black bill with a yellow tip (2) (4) (5) (6).

The sandwich tern has an entirely black cap and crest during breeding, which gradually becomes whiter on the forehead as the season progresses. Outside of the breeding season the forehead is almost completely white, but the black crest on the rear of the crown is retained year-round (2) (5) (7). The male and female sandwich terns are very similar in appearance (2). However, the juvenile sandwich tern has a dusky or brownish crown and blackish mottled markings on the back, upperwing coverts and tail. The bill is much shorter and thicker than that of the adults, and often lacks the yellow tip (2) (7).

Three subspecies of the sandwich tern are recognised. Sterna sandvicensis sandvicensis and Sterna sandvicensis acuflavida, also known as Cabot’s tern, are very similar in appearance. The third subspecies, Sterna sandvicensis eurygnatha, also called the Cayenne tern and sometimes treated as a separate species, has a rather distinctive yellow bill, often with a dark base, rather than the black bill with yellow tip that is typical of the sandwich tern (2) (4). 

The sandwich tern is distributed throughout Europe, Africa, western Asia and the Americas (8).

It breeds along European coasts, east to the Caspian Sea, and in America from Virginia to Texas, as well as the Yucatan Peninsula in Mexico. In the Caribbean the sandwich tern breeds on the Lesser Antilles, Puerto Rico, and the U.S. and British Virgin Islands. In South America, it is found breeding in Venezuela, French Guiana, Brazil and Argentina (2) (4) (5) (8).

The sandwich tern winters on coasts in southern Europe from the Caspian, Black and Mediterranean Seas, to the coasts of western and southern Africa, the Red Sea and the Arabian Peninsula. It may also be found wintering in northwest India and Sri Lanka. In the Americas, the sandwich tern winters from Texas to southern Argentina on the Atlantic coast, and in Mexico, Peru and Chile on the Pacific coast (2) (4) (5) (8).

The subspecies Sterna sandvicensis acuflavida breeds in North America along the Atlantic and Gulf coasts to Belize, the Bahamas, Greater Antilles and the eastern Caribbean. It winters in southern Florida and along the Gulf Coast, south to the Greater Antilles, Colombia, Ecuador, and Peru. Sterna sandvicensis sandvicensis breeds in Europe, east to the Caspian Sea, and winters in the Mediterranean, western and southern Africa, northwest India, and Sri Lanka. Sterna sandvicensis eurygnatha ranges from Puerto Rico to islands in the southern Caribbean, and along eastern South America to Argentina (2).  

The sandwich tern is primarily found in coastal areas, including beaches, bays, estuaries, mudflats, inlets and lagoons (5) (7) (8). It generally occurs where there is immediate access to clear waters with shallow sandy substrates that are rich in surface-level fish (8).

It typically nests in areas of unvegetated sand on open beaches, exposed low, flat, sandy islands close to the shore, or on gravelly, muddy or bare coral substrate. It may also nest on artificial (man-made) islands (2) (4) (5) (8). Outside of the breeding season, the sandwich tern is frequently found on sandy or rocky beach fronts, mangrove flats, estuaries, harbours and bays (4) (8). It is often seen resting on docks or pilings (2).

A particularly gregarious species, the sandwich tern often forms feeding flocks where prey is abundant or concentrated (8). It feeds almost entirely on small fish, squid and crustaceans (2) (4), which it catches by plunging into the water from a height of five to seven metres (2). When foraging over shallow water, the sandwich tern may hover stationary over the water while it searches for prey, before ascending and rapidly diving to feed. It may also snatch its prey from the surface of the water, by flying low and skimming the waves (2). The sandwich tern has been recorded feeding over fishing nets, close to sea lions and over porpoise pods, where it scavenges small fish (4).

The sandwich tern usually breeds in dense colonies with other terns, such as the royal tern (Sterna maxima), the roseate tern (Sterna dougallii), the common tern (Sterna hirundo) and the Arctic tern (Sterna paradisaea). Colonies are also typically found in association with other seabirds, including the black-headed gull (Larus ridibundus), the laughing gull (Larus atricilla), and the black skimmer (Rynchops niger) (2) (8). The timing of breeding is variable with location, but is typically around April to May in the U.S. and the Caribbean, May and June in Europe, June in Brazil and December in Argentina (2) (4).

Generally, the nest is a shallow scrape which is excavated by both adults (2) (8). The male and female sandwich tern walk together to potential nest sites, alternately making shallow scrapes by planting the breast on the substrate and kicking away the sand or gravel behind with the feet (2). The chosen nest is gradually lined with shell, excrement, vegetation or other materials throughout the breeding season (2) (4).

The female sandwich tern lays a clutch of 1 or 2 eggs, which are incubated for around 21 to 29 days. Both adults share the incubation duties equally, except for in the first few days after laying when the female is fed at the nest site by the male (2) (4). Once hatched, the young may gather together in a group, called a ‘crèche’, which is attended by one or several adults, and remain there until fledging at around 27 to 29 days. In general, sandwich tern chicks are whitish, buff, grey or brown, with many black speckles (4). The chicks are fed by both adults, and continue to be fed after fledging by one of the adult sandwich terns, probably the female (2). 

The sandwich tern is particularly vulnerable to human disturbance, especially near breeding colonies, and to the loss and degradation of breeding habitats through inundation, wind-blown sand, erosion and coastal development. It is also known to be sensitive to disturbance from coastal wind farms (5) (8). Pollution is thought to be a problem in some areas, while egg collection at breeding colonies poses a threat to this species in the tropics (2) (8).

Additional threats to the sandwich tern include predators and extreme high tides during the nesting season (5).

The sandwich tern is not currently thought to be in danger of extinction. Populations of the sandwich tern appear to be relatively stable or increasing, and the breeding range has expanded in recent years (2) (5).

A range of management activities, which include the designating refuge areas, protecting habitat and breeding colonies, and creating artificial nesting sites, have increased the number of breeding pairs at many locations, such as in Florida, North Carolina, Lousiana in the U.S., in Puerto Rico, and in Britain. Population surveys, conservation research and public education and involvement programmes have also been included in management activities (2) (5). 

The sandwich tern is afforded some legal protection by its inclusion on several key pieces of legislation. It is listed on Appendix I of the EU Birds Directive (9), and is included in Appendix II of the Bern Convention on the Conservation of European Wildlife and Natural Habitats (10), the Agreement on the Conservation of African-Eurasian Migratory Waterbirds (11) and the Convention on Migratory Species (12).

For more information about the sandwich tern:

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  1. IUCN Red List (March, 2011)
    http://www.iucnredlist.org/
  2. Shealer, D. (1999). Sandwich tern (Thalasseus sandvicensis). In: Poole, A. (Ed.) The Birds of North America Online. Cornell Lab of Ornithology,   Ithaca. Available at:
    http://bna.birds.cornell.edu/bna/species/405
  3. Bridge, E.S., Jones, A.W. and Baker, A.J. (2005) A phylogenetic framework for the terns (Sternini) inferred from mtDNA sequences: implications for taxonomy and plumage evolution. Molecular Phylogenetics and Evolution, 35: 459-469.
  4. Avibirds European Online Bird Guide - Sandwich tern (March, 2011)
    http://www.avibirds.com/euhtml/Sandwich_Tern.html
  5. Florida Natural Areas Inventory. (2001) Field Guide to the Rare Animals of Florida: Sandwich tern (Sterna sandvicensis). Florida Natural Areas Inventory, Florida. Available at:
    http://www.fnai.org/FieldGuide/pdf/Sterna_sandvicensis.PDF
  6. Bird Guides - Sandwich tern (March, 2011)
    http://www.birdguides.com/species/species.asp?sp=061085
  7. World Register of Marine Species - Sandwich tern (March, 2011)
    http://www.marinespecies.org/aphia.php?p=taxdetails&id=137166
  8. BirdLife International - Sandwich tern (March, 2011)
    http://www.birdlife.org/datazone/speciesfactsheet.php?id=3265
  9. EC Birds Directive (March, 2011)
    http://www.jncc.gov.uk/page-1373
  10. Council of Europe: Bern Convention (March, 2011)
    http://conventions.coe.int/Treaty/EN/Treaties/Html/104.htm
  11. Agreement on the Conservation of African-Eurasian Migratory Waterbirds (March, 2011)
    http://www.unep-aewa.org/
  12. Convention on the Conservation of Migratory Species of Wild Animals (March, 2011)
    http://www.cms.int/