Caspian Tern

Distribution, movements and phenology

The Caspian Tern has a wide distribution, breeding between April and June on large lakes and coastal areas of North America and, locally, in Europe, Asia, Africa and Australasia (Billerman et al. 2026). Populations in the northern hemisphere are migratory, wintering in tropical regions, whilst those in Africa and Australasia are mostly resident or undertake short-distance dispersals. In Mainland Portugal, it occurs as a winter visitor and a rare passage migrant, being more common between August and March in the Algarve, in estuaries, wetlands and salt pans (Catry et al. 2010a). The birds occurring in this region originate mainly from colonies in the Baltic Sea and winter primarily in West Africa (Rueda‐Uribe et al. 2022). Based on data from four birds fitted with GPS in 2018 and 2019, originating from colonies in Sweden and Finland, the use of the Portuguese EEZ in the southern part of the Mainland sub-area was observed during both migratory periods, but with a more prolonged occurrence during the post-breeding period. These data do not show use of the Azores and Madeira sub-areas, although there have been some, albeit few, observations of the species in both regions (Correia-Fagundes et al. 2021; Birding Azores 2022).

Abundance and population trends

The global population has been estimated at between 250,000 and 470,000 individuals, with approximately 11,800 to 14,800 breeding pairs in Europe (BirdLife International 2025). The global population trend is increasing, although some populations are in decline, as is the case with the Baltic Sea populations (Rueda‐Uribe et al. 2022). Others show some stability or even unknown trends (BirdLife International 2025).

In Mainland Portugal, it is most abundant during migratory passages, with a small wintering population. However, there are no estimates of its abundance or population trends, which has made it impossible to assess its environmental status. The same situation applies to the Azores and Madeira.

Ecology and habitat

The Caspian Tern nests on sandy or stony beaches, dunes, flat rocky surfaces, sheltered reefs or islands with sparse vegetation and shallow, undisturbed waters (Billerman et al. 2026). The species feeds mainly on fish, supplementing its diet with other animal resources, including bird chicks, carrion and invertebrates (Shuford & Craig 2002). In winter, it is mainly concentrated along the coast, using sheltered shores and coastal wetlands, but may also occur in inland wetlands.

Threats and conservation

The main threats to the species include habitat and nesting site loss and climate change, as the coastal and freshwater ecosystems on which they depend have been degraded by human activities, including pollution, acidification, rising water temperatures and hydrological changes (Rueda‐Uribe et al. 2022). It is also particularly vulnerable to human disturbance, as well as to reduced fish availability due to overfishing (Shuford & Craig 2002). In Portugal, no direct threats have been identified, but it is essential to maintain the protection of wetlands and the monitoring of resting and feeding areas (Catry et al. 2010a).