Sterna sandvicensis
Sandwich tern

Sterna sandvicensis is considered a strictly coastal species although it may fly over large land masses during migration and inhabits some large inland lakes. In the Americas sandwich terns can be found as far north as the state of Virginia in the United States and extend southward to the northernmost parts of Chile on the western coast of South America and down to the southern reaches of Argentina along the east coast of the continent. In the Old World, sandwich terns can be found as far north as southern Sweden, extending south and west along the coast and wrapping around the tip of South Africa. Populations also exist in the Black and Caspian Seas, along the Persian Gulf and in Kenya, Tanzania and Djibouti in eastern Africa. (Birdlife International, 2009)

Sandwich terns inhabit a variety of habitats including sandy or rocky oceanic beaches, oceanic cliff sides, estuaries and large inland lakes. Preferred nesting sites are usually sandy beaches with little or sparse vegetation or bare rock outcrops. (Birdlife International, 2009; Visser and Peterson, 1994)

Sandwich terns are medium-sized terns best identified by having a straight, slender black bill with a yellow tip and a black crown with a short crest. The underparts, rump and forked tail are white while the long, pointed wings and back are ash gray. The legs and webbed feet are black but can be varying degrees of yellow in one subspecies found in South America and the Caribbean. In non-breeding (winter) plumage the crown is mottled black and white to nearly all white and occasionally has black wing markings. In many migratory birds this change in plumage is thought to function as a sign to other birds of the same and similar species that the individual is not receptive to breeding.

Sandwich terns weigh between 130 and 311 g. They range from 36 to 46 cm in length and have a wingspan of 86 to 105 cm. (Birdlife International, 2009; Voelker, 1996)

Sandwich terns mate in monogamous pairs within large, densely packed nesting colonies that can reach numbers in the tens of thousands. This species typically nests alongside other species, most notably the common black-headed gull in western Europe. Often Sterna sandvicensis does not initially colonize a given area, rather it does not arrive until another species has already done so.

Upon arrival to colonial sites, adult birds will begin their rituals of attracting a mate or re-establish a relationship with a previous partner. Courtship displays include a male catching one or more fish and lining them side-by-side in its bill and presenting them to the female. She may immediately consume the fish or the two may engage in an aerial display soon after the initial presentation. Copulation in this family of birds is done by the male mounting behind the female on one side. The female will move her tail to the opposite side and the male will try to engage cloacal contact while using his wings to awkwardly maintain balance. (Birdlife International, 2009; Fasola and Canova, 1992; Fuchs, 1977)

Suitable nesting habitats include sandy or muddy beaches and open rock outcrops. Sandwich terns often return to colonial sites they have previously occupied before unless the habitat is deemed unsuitable. Nest sites are typically higher above the high tide line than the amount of ground actually available. Vegetation is generally avoided and mating pairs may abandon their nest sites if growing vegetation invades. Individual nests are simple concave scrapes in the substrate often lined with excrement.

Female sandwich terns lay 1 to 3 eggs per breeding season that incubate for 21 to 29 days. Birds at higher latitudes often begin laying eggs in May whereas lower latitudes begin in December. At hatching the young weigh 22 to 24 g. After 7 to 14 days some colonies will form crèches where young birds gather in large groups when parents are absent in order to gain safety in numbers. The ability to fly, or fledging, comes after 28 to 35 days. During their first year, young sandwich terns learn to catch food on their own by watching adults and practicing. Practice may include diving for actual food items or inanimate objects in order to gain accuracy. Sandwich terns reach sexual maturity at 3 to 4 years old. (Birdlife International, 2009; Fasola and Canova, 1992; Visser and Peterson, 1994)

After eggs have been laid, both parents share incubating responsibilities. Both parents take turns protecting the nest or foraging for the young. Parents continue to feed young until they are fledged and able to gather food on their own. After 7 to 14 days some colonies will form crèches where young birds gather in large groups when parents are absent in order to gain safety in numbers. (Birdlife International, 2009; Dunn, 1972; Fuchs, 1977; Stienen, et al., 2001)

The oldest recorded sandwich tern lived to be 23 years and 7 months. (Birdlife International, 2009)

These birds are often silent much of the time and are usually only noticed by sight. They can be frequently seen flying along the shallows of a coastline and occasionally dive-bombing prey. When resting they will often form small groups and face away from the wind. If a neighbor gets too close short squabbles tend to unfold until one assumes dominance. This species is migratory, sometimes traveling great distances from breeding to wintering grounds. Migration may be southward or northward depending on the location of breeding grounds. Individuals that breed in western Europe will travel south along the western coast of Africa. Individuals from the Black Sea will stay in the Black Sea or move to the Mediterranean. Caspian Sea individuals winter in the northern Indian Ocean and Persian Gulf. North American individuals travel south to the Caribbean and down through Central and South America. (Birdlife International, 2009)

Traveling 10 to 15 km per day in search of food is common in this species with reports extending as far as 70 km per day. (Birdlife International, 2009)

It may be inferred from the studies on royal terns that nesting adult sandwich terns are able to distinguish their own eggs by visual cues alone. Sandwich terns are able to recognize their own young by visual cues and vocalizations. Young are often called out of a crèche by parents, showing that the young can also identify their parents by vocalization. Sandwich terns perceive their environment through visual, tactile, auditory, and chemical stimuli. (Hutchison, et al., 1968)

Sandwich terns are known to eat reef silverside, dwarf round herring, Atlantic thread herring, hardhead silverside, scaly sardines, common anchovies, mackerel, jacks, flying fish, damselfish, parrotfish, Atlantic herring, sprat, lesser sand eel and squid. The adults feed chicks different prey species than they themselves prefer to eat. It is not clear if this is due to different dietary needs of young versus adults. In instances where this species has become accustomed to human interaction, adults will occasionally accept bread. (Shealer, 1998; Stienen and Brenninkmeijer, 2002)

These birds typically do not form large feeding flocks but will gather in small groups or hunt alone. They fly several meters above the water while foraging for prey. Once prey item is spotted they first hover above with beaks aimed directly downward in order to set their aim then dive-bomb into the water. First-winter birds are often less successful in catching food for themselves than older adults. (Dunn, 1972; Shealer, 1998)

Short-eared owls, carrion crows, herring gulls, great black-backed gulls, foxes, and stoats are predators of chicks while common black-headed gulls prey mostly on eggs. Sandwich terns display little aggressive anti-predator behaviors. Densely situated nesting sites with more aggressive species of bird tend to alleviate the necessity for defensive behaviors. If necessary, sandwich terns will show defensive displays involving spreading their wings and raising their bodies to appear larger and mobbing or dive-bombing potential predators. Most of this aggressive behavior is seen during the breeding season when chicks are beginning to hatch. (Fuchs, 1977)

There appears to be a mutualism between sandwich terns and common black-headed gulls where the latter provides protection from predators while occasionally eating the eggs and stealing prey items of the former. (Stienen and Brenninkmeijer, 2002; Stienen, et al., 2001)

Inshore fisherman will often look for diving terns in order to locate small baitfish.

There are no known adverse effects of sandwich terns on humans.

This is a species of Least Concern from the International Union of the Conservation of Nature due to its large numbers and geographic distribution. The suitability of a given habitat for creating nesting colonies of sandwich terns is vulnerable to destruction by natural phenomena events (ie: hurricanes, storms) and vegetation overgrowth. Habitat destruction by humans is known to adversely affect the ability of this species and many others that depend on undisturbed beach to breed. Overuse by humans visiting beaches can cause whole colonies to abandon the area en masse and minimize re-use in future years. (Visser and Peterson, 1994)

In 2009 a DNA study separated the three subspecies of Sterna sandvicensis into two distinct species. The North American forms are combined as one under the name Thalasseus acuflavidus and the European form as Thalasseus sandvicensis. As of yet this suggestion has not been fully adopted by the scientific community. (Efe, et al., 2009)