Atlas of Seabirds of Portugal

Distribution, movements and phenology

The Common Scoter breeds between May and September in the north of the United Kingdom, eastern Greenland, Iceland, Scandinavia and northern Russia (Billerman et al. 2026). In Mainland Portugal, it has a strictly marine distribution and can be found practically all year round. However, it is during winter and the migratory periods that the population is most abundant. In winter, it is mainly concentrated between Cape Mondego and Espinho (Meirinho et al. 2014; Jesus 2018). Pre-breeding migration mainly takes place from March to April, whilst post-breeding migration is observed from at least August to October (Catry et al. 2010a). During migration, it occurs practically along the entire coast (Meirinho et al. 2014), possibly due to the movements of some birds to other wintering areas located further south, notably the Atlantic coast of Morocco (Aðalsteinsson et al. 2025). This pattern is also suggested by the individual tracking data presented in this work, although it is limited to just two birds, fitted in 2009–2013 with GLS devices in Iceland. In the Azores and Madeira, it occurs occasionally in the coastal waters of most of the islands, between autumn and spring (Correia-Fagundes et al. 2021; Birding Azores 2022).

Abundance and population trends

The global population has been estimated at around 1,600,000 individuals, with an unknown trend (BirdLife International 2025). In Mainland Portugal, it is more abundant in the north, with a population of 20,000 to 21,000 individuals recently estimated between the mouth of the Douro and Nazaré (Jesus 2018). It is frequently observed in large flocks ranging from tens to many hundreds of birds (Rufino & Neves 1990; Petronilho et al. 2004). Despite the lack of recent estimates for the southern half of the country, historically this population would not have exceeded three hundred birds (Rufino & Neves 2004). In general terms, the population appears to have undergone several fluctuations over the last few decades, maintaining a stable trend, which coincides with the Good Environmental Status resulting from the assessment in this work. In the Azores and Madeira, however, the population size is very low, both in winter and during migration (Equipa Atlas 2018), and there is no information regarding the trends of these populations.

Ecology and habitat

The Common Scoter is the only sea duck that regularly winters in Portugal, generally frequenting shallow coastal waters with sandy bottoms. It usually feeds just beyond the surf zone and up to two or three kilometres from the coastline. Molluscs, mainly bivalves, are its primary prey, which it catches by diving to depths of up to 10–20 metres (Billerman et al. 2026).

Threats and conservation

Globally, the Common Scoter appears to be particularly vulnerable to oil spills and chronic hydrocarbon pollution in its wintering grounds. Climate change also appears to have a negative impact, causing habitat loss and the degradation of the ecosystems it inhabits. In Portugal, the overexploitation of bivalves is considered its main threat (Cabral et al. 2005). However, there are no concrete data on its diet in our country (Catry et al. 2010a). Bycatch in fishing gear, particularly in beach seines and gillnets, is also a documented pressure on Portuguese waters (Oliveira et al. 2015). All these threats coincide with the causes cited for the admission of Common Scoters to rehabilitation centres, including poor plumage condition, emaciation, entanglement in fishing equipment, physical injuries and toxicity (Costa et al. 2021).