Atlas of Seabirds of Portugal

Distribution, movements and phenology

The Common Tern has a circumpolar distribution, breeding across most of Europe, Asia and North America. It winters along the coasts of South America, Africa, the Arabian Peninsula, India, Southeast Asia and Australia (Billerman et al. 2026).

In Portugal, it breeds between April and August in the Azores and Madeira. It also breeds, albeit rarely, on the Mainland, where it is more commonly seen as a passage migrant (Equipa Atlas 2022). Pre-breeding migration takes place in April and post-breeding migration from August to October, involving mainly birds from the British Isles, the North Sea and Scandinavia (Catry et al. 2010a). The Azorean populations winter mainly in South America (Neves et al. 2002; Neves et al. 2015), whilst populations from central and northern Europe prefer the African coast (Billerman et al. 2026).

Data from 10 birds fitted with GLS in 2009 and 2012 in the Azores, Germany and Sweden show that the birds used Mainland and Madeiran waters mainly during pre- and post-breeding migrations. In the Azores, however, occurrences were concentrated during the breeding season in the area surrounding the breeding colony, located in the central group of the archipelago.

Abundance and population trends

The global population has been estimated at between 1,600,000 and 3,600,000 individuals, of which 316,000 to 605,000 pairs breed in Europe, with an unknown trend (BirdLife International 2025).
The Azores are home to the largest breeding population in Portugal. It breeds on all the islands and has a population of 2,419 to 3,411 pairs, with considerable annual fluctuations. In the Madeira region, estimates do not exceed 1,000 pairs. On the Mainland, it nests sporadically and in small numbers (Equipa Atlas 2022). In this region, the migratory population is more significant, with a total of 700 birds recorded during the post-breeding migration in 2015, observed from Cape Carvoeiro (Elmberg et al. 2020).

The assessment of environmental status shows that the Azorean population has achieved Good Environmental Status for the abundance indicator. Regarding the situation on the Mainland and in Madeira, the existing abundance data do not allow for an assessment of the status of these populations.

Ecology and habitat

The Common Tern feeds mainly on small mesopelagic fish (Granadeiro et al. 2002), preferably in calm waters and relatively sheltered bays (Monticelli et al. 2006). On Mainland Portugal, nesting sites are found in estuaries, salt pans and fish farms (Elias & Leitão 1992), whilst in the Azores and Madeira, colonies are generally located on coastal cliffs and islets (Equipa Atlas 2022).

Threats and conservation

The main threats include human disturbance in the areas surrounding the colonies (Monteiro et al. 1996a) and predation, both by native species such as starlings (Neves et al. 2011b) and by introduced mammals (Amaral et al. 2010). The eradication of introduced mammals is a key conservation priority to ensure the viability of breeding populations (Bried et al. 2009b; Amaral et al. 2010).