Arctic Tern

Distribution, movements and phenology

During the breeding season, between May and August, the Arctic Tern has a circumpolar Arctic distribution, migrating to sub-Antarctic and Antarctic waters to winter (Billerman et al. 2026). It undertakes one of the longest known migrations, covering over 80,000 km per year (Egevang et al. 2010). It is during these long journeys that the species occurs in Portugal, preferring predominantly pelagic waters (Fijn et al. 2013; Meirinho et al. 2014). Pre-breeding migration takes place between March and May and post-breeding migration between August and October. September is when the species is most abundant along the Mainland coast (Catry et al. 2010a; Elmberg et al. 2020).

Individual tracking of 25 birds from colonies in Greenland, the Netherlands and the United Kingdom, fitted with GLS devices, confirms the extensive use of the Portuguese EEZ during both migratory periods, indicating a higher occurrence in the Azores and Madeira sub-areas during the pre-breeding migration, coinciding with the end of the non-breeding period. The Mainland region was used solely during the final phase of the breeding season, in August, likely by birds that initiated migration early.

Abundance and population trends

The global population has been estimated at over 2,000,000 birds, of which 564,000 to 906,000 pairs breed in Europe, showing a declining trend (BirdLife International 2025).
There is little information regarding the number of Arctic Terns occurring in the Portuguese EEZ, as they are generally observed singly or in small flocks. On the Mainland, land-based observations of the species occur following storms and the passage of cold fronts with strong westerly winds (Moore 2000). During the post-breeding migration in 2015, 1,003 birds were recorded migrating south from Cape Carvoeiro (Elmberg et al. 2020). In the Azores and Madeira, there are occasional records (Meirinho et al. 2014). However, the lack of systematic information regarding its abundance and population trends has prevented an assessment of the species’ environmental status within the Portuguese EEZ.

Ecology and habitat

The Arctic Tern nests on the ground on islands, but also away from the coast, in lakes and other wetlands. It nests in Arctic regions and in the northern temperate regions (Billerman et al. 2026). Colonies can range from a few pairs to hundreds or thousands of birds. In Portugal, it occurs during migration throughout the EEZ, in pelagic areas, mostly far from land. It feeds at the surface and its diet consists of small fish, crustaceans, molluscs and insects.

Threats and conservation

Potential threats to the species are likely related to the effect of climate change on breeding areas and Arctic ecosystems, which could lead to a decrease in food availability (Møller et al. 2006) or a reduction in available nesting habitat. There is a strong relationship between breeding success and prey availability. Some populations have been affected by the collapse of their main prey (e.g. sand eels), reducing or preventing breeding altogether (Billerman et al. 2026). In some colonies, eggs and chicks are subject to predation by invasive predators, such as the American mink Neovison vison. No threats have been identified for Portugal.

Autores

Joanne Morten, Ruben Fijn, Tânia Nascimento

Owners/sources of tracking data

Egevang 2008, Fijn et al. 2013, Morten et al. 2018, Redfern 2018b, Redfern 2018c