Thalasseus sandvicensis

Thalasseus sandvicensis

The Sandwich Tern (Thalasseus sandvicensis, Latham, 1787) is a bird belonging to the Laridae family.

Sistematica –
Eukaryota domain,
Kingdom Animalia,
Phylum Chordata,
Subphylum Vertebrata,
Superclass Tetrapoda,
Aves class,
Subclass Neornithes,
Superorder Neognathae,
Order Charadriiformes,
Suborder Lari,
Laridae family,
Subfamily Sterninae,
Genus Thalasseus,
Species T. sandvicensis.
The following terms are synonymous:
– Sterna boysii Latham;
– Sterna boysii Latham, J, 1790;
– Sterna cantiaca Gmelin, JF, 1789;
– Sterna sandvicensis Forster, T, 1817;
– Sterna sandvicensis Latham, 1787;
– Sterna sandvicensis sandvicensis Latham, 1787;
– Sterna stubberica Otto, BC, 1790;
– Thalasseus sandvicensis Stotz et al., 1996;
– Thalasseus sandvicensis sandvicensis (Latham, 1787).

Geographic Distribution and Habitat –
Thalasseus sandvicensis is a medium-large sea tern belonging to the order Charadriiformes and the family Laridae, according to the taxonomic classification currently widely adopted. The genus Thalasseus includes some of the largest terns most closely associated with coastal marine environments. The species is also known by the English name Sandwich Tern. The genus derives from the Greek thalassa, “sea,” while the specific epithet sandvicensis refers to Sandwich, Kent, where the typical material used to describe the species was collected.
The species has a wide Holarctic range, with populations distributed primarily along the coasts of Europe, Africa, and western Asia, while related populations occupy the Atlantic coasts of the American continent. Within the Palearctic population, large geographic clusters are traditionally distinguished, including the eastern North Atlantic and the Baltic Sea, the northern Black Sea, and the Caspian Sea. The European population therefore constitutes a complex biogeographic complex, in which breeding colonies may be separated by considerable distances yet still maintain a certain demographic connectivity.
In Europe, the breeding distribution mainly involves the Atlantic and northwestern coasts, the Baltic basin, and some Mediterranean coastal areas. The species is a long-distance migrant: populations breeding in northern Europe may reach the western coasts of Africa during the non-breeding season, while populations in the Caspian area migrate to the Indian subcontinent and Sri Lanka. In Italy, the Sandwich Tern is present both as a migrant and winter visitor and, to a more localized extent, as a breeder.
The breeding habitat consists primarily of low, open, and sparsely vegetated coastal environments, such as sandy islands, salt marshes, lagoon islets, sandbanks, salt marshes, embankments, and artificial islands. The species generally avoids tall, compact vegetation, favoring bare or sparsely vegetated surfaces, often located immediately adjacent to water. Proximity to water is particularly important because it allows adults to quickly reach feeding areas and, at the same time, provides colonies relatively isolated from terrestrial predators.
In Italy, the environments used for nesting are primarily the lagoons and coastal wetlands of the Northern Adriatic, particularly the Venice Lagoon, the Po Delta, and the coastal wetlands of Emilia-Romagna. The national monitoring methodology sheet highlights that colonies can establish themselves on sparsely vegetated sandy outcrops within lagoons or salt marshes, and that the location of these sites can change rapidly depending on environmental and hydrological conditions.
Breeding in Italy is relatively recent. The first documented regular reproduction dates back to 1979 in the Comacchio Valleys; the species subsequently colonized the Venice Lagoon, where the first colony was established in 1995, and other sites in the Northern Adriatic. Breeding has also been reported irregularly in Puglia and, more recently, in other Italian coastal sites.
The Venice Lagoon today represents one of the most interesting cases of the expansion and consolidation of the species in Italy. The study by Scarton and Valle, which analyzed the period 1995-2023, documents an increase in breeding pairs from 202 in 1995 to 3,503 in 2023, with an estimated average annual increase of 5.7%. A total of 73 colonies were identified during the period studied, distributed across 21 sites used at least once. The species has utilized both natural salt marshes and, especially in more recent years, artificial islands for depositing dredged material, which can offer open and relatively protected surfaces.
This case demonstrates a fundamental characteristic of the Sandwich Tern’s ecology: its highly dynamic breeding site selection. Colonies are not necessarily permanent; they can move from island to island in response to flooding, erosion, vegetation changes, predation, disturbance, and the availability of suitable surfaces. In the Venice Lagoon, over a 29-year period, the overall turnover rate of colonial sites was 41.4%.
This same characteristic, however, constitutes a vulnerability: the loss or alteration of a single islet can cause the colony to move to a site that is not necessarily as safe. In lagoon environments subject to rising sea levels, storm surges, and changes in water regimes, the availability of suitable nesting surfaces can therefore become a limiting factor.
During migration and wintering, the Sandwich Tern remains predominantly tied to coastal environments and wetlands adjacent to the sea, frequenting beaches, estuaries, river mouths, lagoons, salt marshes, inlets, and relatively shallow seawater. However, feeding areas can also be located at considerable distances from colonies: Italian monitoring indicates trophic movements of more than 30 km from the coast under certain circumstances.

Description –
The Sand Tern (Thalasseus sandvicensis) is an elegant, robust, and relatively large tern, approximately 36–41 cm long, with a wingspan of approximately 95–105 cm and a body mass generally between 200 and 300 g. Compared to smaller terns, it has a more robust body, but retains the characteristic aerodynamic morphology of larids specialized in rapid flight and sea fishing.
Adults are predominantly white on the underparts and silvery-gray on the upperparts. The wings appear very pale in flight, creating a particularly striking contrast with the darker tips of the flight feathers. The legs are short and black.
The most obvious diagnostic feature is the long, thin, black bill with a distinctive yellow tip. The combination of a black-yellow bill and black crest makes the species easily recognizable throughout most of its range. The head features a striking black skullcap with a fringed nuchal crest, particularly noticeable during the breeding season.
Head coloration is seasonal. After the post-breeding molt, which generally occurs between July and August, the forehead gradually turns white and the black area of ​​the head diminishes. The black pattern is reacquired with the spring molt, approximately between February and March. This seasonal variation is important for identifying adults and reflects the normal plumage dynamics.
Juveniles are markedly different from adults. The back and wings have a characteristic scaly appearance, determined by alternating gray, black, and lighter shades of the feathers. The bill is generally completely dark, without the characteristic yellow tip of the adult, or has only a yellowish tinge at the base.
The species is difficult to confuse with most other European terns, but some species of the genus Thalasseus can pose identification problems. The Cabot’s Sandwich Tern (Thalasseus acuflavidus), in particular, also has a black bill with a yellow tip, but its bill is generally more robust and has differences in the structure of its feathers and in the phenology of its molt. The elegant sandwich tern (Thalasseus elegans) and the lesser sandwich tern (Thalasseus bengalensis) generally have orange bills, while the Chinese sandwich tern (Thalasseus bernsteini) has an inverted bill color scheme, with a yellow base and black tip.
Taxonomically, the species was described by John Latham in 1787 as Sterna sandvicensis. It was later transferred to the genus Thalasseus, separated from Sterna on the basis of morphological and behavioral characteristics, and, above all, modern molecular analyses. Phylogenetic classification has highlighted the sandwich tern’s close relationship with other members of the genus Thalasseus and has helped redefine some forms previously considered subspecies.
In particular, the American populations once included in the sandwich tern complex are now largely treated as Cabot’s sandwich tern (Thalasseus acuflavidus), as they are genetically closer to the elegant sandwich tern than to the true T. sandvicensis. The taxonomic situation of the Thalasseus complex is therefore a good example of how genetic analyses have changed species delimitations compared to classifications based solely on morphology.
The Sandwich Tern is also an extremely vocal species. Its repertoire includes harsh, metallic, and piercing calls, often transcribed as kear-ik or kerr-ink. The noise level of colonies can be considerable, especially during the arrival of adults, social interactions, nest defense, and the feeding of the young.

Biology –
The Sandwich Tern is a highly specialized fish-eating predator. Its diet consists primarily of small, gregarious fish, caught primarily in coastal marine waters, but also includes, in varying quantities, cephalopods, small crustaceans, and insects.
Its characteristic hunting technique is plunge-diving. The bird locates its prey from above, briefly holds its position or slows its speed, and then rapidly drops to the surface, penetrating it with its body and, especially, its beak. This technique differs from that of terns, which perform a more prolonged and controlled hovering phase above the water’s surface.
The depth and method of diving depend on water clarity, the presence and depth of fish schools, and sea conditions. Prey is often concentrated by currents, hydrodynamic fronts, tides, or the activity of larger predatory fish.
Feeding behavior therefore directly links the Sandwich Tern to the productivity of coastal waters. The presence of small pelagic fish in sufficient numbers is essential for the reproductive success of colonies.
During the breeding season, adults can make considerable movements between the colony and feeding grounds. Under favorable conditions, several individuals may temporarily congregate near fish aggregations and, sometimes, associate with other seabird species.
The Sandwich Tern is a highly gregarious and colonial species. Breeding colonies can comprise hundreds or thousands of pairs, with nests very close together. Under normal conditions, nests can be located within a few dozen centimeters of each other.
The nest is extremely simple: it generally consists of a small depression or scrape in the ground, made of sand, gravel, shells, bare soil, or sparsely vegetated substrates. Adults may add small fragments of shells or other material available in the immediate vicinity.
Spawning generally occurs between late April and May, with hatching and fledging concentrated between June and July; However, phenology may vary depending on latitude, weather conditions, and the local pattern of the breeding season. National monitoring indicates that egg-laying begins in early May and fledging occurs in the second half of June, with fledglings also observed later in the summer.
The clutch normally comprises one or two eggs, while three eggs are rare. Eggs range in color from whitish to pink or beige, with brown spots. Both parents participate in incubating and raising the chicks.
Data collected in the Venice Lagoon from 1995 to 2023 indicate an average clutch size of 1.7 ± 0.5 eggs out of 1,338 analyzed clutches.
After hatching, chicks quickly become relatively mobile and may gather in crèches or juvenile groups, within which adults are still able to recognize and feed their own chick. This social organization allows the young to be concentrated and reduces, at least partially, the cost of individual surveillance.
Reproductive behavior is particularly interesting. A key component of courtship is the male’s presentation of fish to the female.
The male may arrive at the colony carrying a fish in his beak, vocalizing and attracting the female’s attention. The female may adopt a posture begging for food. The presentation and subsequent delivery of prey are an important component of the pair bond and may also serve as a means of assessing the partner’s abilities.
Courtship also includes terrestrial displays and nuptial flights, in which the two individuals may rapidly ascend and then perform a coordinated descent, accompanied by vocalizations and specific wing and head postures.
Unlike some particularly aggressive terns, the Sandwich Tern is not generally characterized by intense individual aggression toward predators. Its defense strategy is partly based on colony density and the choice of sites difficult for terrestrial predators to access.
A particularly interesting ecological element is its association with more aggressive species. Sandwich Tern colonies may, in fact, be associated with Black-headed Gull (Chroicocephalus ridibundus), Common Tern (Sterna hirundo), and other colonial species. The aggressive behavior of these species may provide a form of indirect protection against some predators. In the Venice Lagoon, a strong association between Sandwich Terns and Black-headed Gulls and a moderate association with Common Terns has been documented.
The Sandwich Tern is a long-distance migrant. Migration routes vary significantly depending on the geographic origin of the population.
Individuals breeding in the British Isles and other regions of northwestern Europe can reach the western coasts of Africa, reaching as far as southern Africa. Populations in the Caspian Sea, however, migrate to the Indian subcontinent and Sri Lanka.
The Italian population presents a particularly interesting situation. National monitoring indicates that wintering birds in Italy originate primarily from the Black Sea, while individuals breeding in Italy may migrate to the Atlantic coasts of Africa, even as far as South Africa.
The species also exhibits remarkable social fidelity to colonial systems, but not necessarily to a single site. Individuals may move collectively between neighboring breeding sites or even between different regions, causing sudden increases or decreases in the numbers of individual colonies. Ring readings in the Venice Lagoon have revealed the presence of individuals from, among others, Scotland, Denmark, the Netherlands, and Spain, as well as colonies in the Black Sea and the Sea of ​​Azov.
Historical data for Italy reported approximately 798-821 pairs in 2004, with a trend considered to be increasing. These data underpinned the previous national assessment, which highlighted the extremely localized population and the presence of fewer than five IUCN locations. These factors led to an Italian assessment of Vulnerable (VU), criterion D2, despite the overall positive trend.
More recent findings, however, show a much more dynamic demographic situation. Between 1995 and 2023, the Venice Lagoon alone increased from 202 to 3,503 breeding pairs, while the 2020-2023 national marine monitoring survey recorded an overall Italian population of just over 2,000 pairs in the monitored areas.
The apparent discrepancies between the various estimates should not be automatically interpreted as contradictions: they reflect different reference years, different census methodologies, the highly mobile nature of the colonies, and the movement of pairs between sites. In particular, a national estimate based on a specific period may not reflect the situation in subsequent years characterized by strong increases in a single large colony.
The Venetian case is emblematic: in 2023, 3,503 pairs were counted in the Venice Lagoon, an average annual increase of 5.7% since 1995. This increase was also accompanied by significant arrivals of immigrants, particularly marked in 2014 and 2023.

Ecological Role –
Thalasseus sandvicensis plays an important ecological role in coastal marine ecosystems and brackish wetlands, occupying a relatively high position in the food web as a predator of small fish and other aquatic organisms.
Its feeding activity contributes to the transfer of energy from aquatic ecosystems to the terrestrial environments of its breeding colonies. During the breeding season, in fact, large quantities of fish biomass are transported by adults from the sea to islets and salt-marsh habitats, where they are used to feed the chicks.
The species can therefore also be considered a biological indicator of the functionality of coastal environments. Its breeding presence simultaneously requires:
– availability of suitable nesting sites;
– relative tranquillity during the breeding period;
– open, sparsely vegetated surfaces;
– protection from terrestrial predators;
– availability of small fish in the surrounding marine and lagoon environments;
– hydrological conditions compatible with nest persistence.
The Sandwich Tern is therefore particularly sensitive to the fragmentation and transformation of coastal wetlands, because the loss of a single ecosystem component can compromise a specific stage of its biological cycle.
Sandwich Tern colonies constitute complex ecological communities. Association with gulls and other terns may provide mutual, or at least indirect, benefits. More aggressive species may respond to aerial or terrestrial predators, thereby providing a form of protection for less aggressive species as well.
In the Venice Lagoon, a strong positive association has been documented with the Black-headed Gull (Chroicocephalus ridibundus) and a moderate relationship with the Common Tern (Sterna hirundo); by contrast, a moderate negative relationship has been recorded with the Little Tern (Sternula albifrons). These patterns may reflect differences in microhabitat selection, colony structure, competition for space, and reproductive strategies.
One of the main conservation challenges facing the Sandwich Tern is that its apparent global abundance conceals a high degree of local vulnerability.
At both global and European levels, the species is currently classified as Least Concern (LC). The 2021 European assessment indicates an estimated EU28 population of approximately 118,000–140,000 mature individuals, with a central estimate of 128,000, and an overall increasing trend.
This global status does not mean, however, that individual populations or colonies are free from problems. The Sandwich Tern concentrates a significant proportion of its breeding activity in a relatively limited number of colonies. The destruction or temporary unsuitability of just a few sites can therefore have disproportionately large effects on local productivity.
In Italy, the main threats include:
1. Nest flooding.
This is probably one of the most important factors in lagoon environments. Nests are built on extremely low-lying surfaces, often only slightly above the mean water level. Exceptional tides, storm surges, intense rainfall, and fluctuations in water levels can therefore inundate entire colonies.
In the Venice Lagoon, an analysis covering the period 1995–2023 showed that extreme high tides were responsible for more than 90% of brood losses up to 2022.
A particularly significant event occurred in 2022: three Venetian colonies contained a total of 1,619 nests, of which 1,111, or 68.6%, were inundated at the beginning of the season. Productivity differed greatly among sites, highlighting the importance of elevation characteristics and the availability of higher artificial surfaces.
2. Sea-level rise and climate change.
Climate change may act simultaneously through sea-level rise, an increased frequency of extreme weather events, changes in sediment dynamics, and modifications in prey availability. For a species that nests only a few decimetres above the water level, even relatively small changes in elevation can have highly significant effects.
3. Habitat transformation.
The construction of infrastructure, alteration of salt marshes, transformation of salt pans and wetlands, erosion, and loss of open surfaces can reduce the availability of breeding sites.
4. Human disturbance.
Close human presence during the breeding season may cause temporary nest abandonment, increased exposure of eggs to predation or heat, and reduced parental surveillance of chicks. This problem is particularly relevant for colonies located near urbanised areas, industrial activities, navigation routes, and tourist areas.
5. Predation.
Rats, large gulls, and other predators may affect breeding success, especially when colonies are established at poorly protected sites.
6. Trampling and physical alteration of colonies.
In some wetlands, the movement of large animals, including flamingos, may damage nests. The inherent fragility of the substrates used by Sandwich Terns also makes colonies sensitive to physical changes in the environment.
7. Highly pathogenic avian influenza.
The highly colonial nature of the species represents an additional health risk: the concentration of thousands of individuals in very restricted areas facilitates the transmission of pathogens. HPAI outbreaks caused significant mortality in European colonies in 2022.
The problem has also been documented in Italy. In 2023, H5N1 cases were recorded in Sandwich Terns in Veneto and Emilia-Romagna; the study conducted in the Venice Lagoon also found that the outbreak essentially reduced reproductive productivity for the season to zero.
Regarding conservation status, Thalasseus sandvicensis is listed in Annex I of the European Union Birds Directive, which requires the adoption of special habitat conservation measures and the designation of suitable areas for the protection of the species. It is also included in Annex II of the Bern Convention, Annex II of the SPA/BD Protocol to the Barcelona Convention, and within the framework of AEWA, the international agreement for the conservation of African-Eurasian migratory waterbirds.

Effective conservation therefore cannot be limited to protecting adults or individual colonies. It must include integrated management of the entire coastal system, including:
– protection of breeding sites;
– maintenance of salt marshes, salt pans, islets, and sandbanks;
– control of disturbance during the breeding period;
– management of predator populations at the most vulnerable sites;
– annual monitoring of colonies;
– maintenance of a mosaic of alternative sites;
– creation or maintenance of elevated artificial islands where necessary;
– conservation of marine feeding areas;
– monitoring of fish populations;
– epidemiological surveillance for avian influenza;
– integration of wetland planning with sea-level-rise scenarios.
Artificial dredging islands, although apparently anthropogenic habitats, may in some contexts play an unexpected role in conservation. In the Venice Lagoon, some of these islands have provided higher surfaces less exposed to tides than natural salt marshes, resulting in much greater breeding productivity. In 2022, for example, a colony located on a dredging island did not experience the flooding losses that severely affected other sites. The authors therefore emphasise the need for long-term management of these structures, particularly in light of rising sea levels.
Thalasseus sandvicensis can be described as a coastal marine, migratory, colonial species highly specialised in catching small fish, whose breeding biology is closely dependent on the availability of small, open, sparsely vegetated surfaces immediately adjacent to water.
Its ecological role derives primarily from its position as a predator of small fish and from the transfer of marine biomass to terrestrial colonies. At the same time, the species represents an important component of the biodiversity of Mediterranean lagoons and coastal wetlands and can be used as an indicator of the quality and functionality of coastal ecosystems.
The Italian situation is particularly significant: from a recently colonising breeding species, first recorded regularly in 1979 in the Comacchio Valleys, the Sandwich Tern has progressively expanded and consolidated its presence in the northern Adriatic, reaching 3,503 breeding pairs in the Venice Lagoon alone in 2023.
The conservation paradox of the species therefore lies in the coexistence of a recent positive demographic trend at the Italian scale and a high vulnerability of individual breeding sites. A colony can grow rapidly when it finds a suitable, undisturbed, and sufficiently elevated island, yet at the same time it can suffer catastrophic losses as a result of a single storm surge, an extreme high-tide event, an epidemic, or intense human disturbance.
For this reason, the primary conservation objective should not simply be to maintain a given number of breeding pairs, but to ensure the continuous availability of a network of alternative, safe, and functionally connected breeding sites, together with the conservation of the marine areas on which the species depends for feeding.
Ultimately, the Sandwich Tern represents an excellent example of a species in which colony dynamics, wetland conservation, climate change, marine resource availability, and land management are closely interconnected. Its protection therefore requires an ecosystem-based strategy that simultaneously considers the lagoon, salt marshes and nesting islands, coastal feeding waters, and international migratory routes.

Guido Bissanti

Sources
– Wikipedia, the free encyclopedia.
– GBIF, the Global Biodiversity Information Facility.
– C. Battisti, D. Taffon, F. Giucca, 2008. Atlante degli uccelli nidificanti, Gangemi Editore, Rome.
– L. Svensson, K. Mullarney, D. Zetterstrom, 1999. Bird Guide: The Birds of Europe, North Africa and the Middle East, HarperCollins Publishers, United Kingdom.

Photo source:
– https://www.gbif.org/occurrence/search?view=GALLERY&taxonKey=7C5FT&entity=o_6130978153