Atlas of Seabirds of Portugal

Distribution, movements and phenology

The Balearic Shearwater breeds exclusively on the Balearic Islands (Spain). Some individuals begin arriving at the colonies as early as September, with egg-laying taking place between February and March and the fledging of juveniles in June (Ruíz & Martí 2004). During the non-breeding season, it is mainly concentrated along the Atlantic coasts of South-west Europe, particularly in the waters of Spain, Portugal and France, extending also to the South-west of the United Kingdom, South-east of Ireland, North-west of Morocco and the Western Mediterranean (Arcos et al. 2009; Guilford et al. 2012; De la Cruz et al. 2025). The vast majority of the global population uses Iberian coastal waters as a migratory corridor. During the post-breeding migration, which takes place in summer, a large proportion of breeding adults and juveniles move along the coast of Mainland Portugal (Ruíz & Martí 2004; Guilford et al. 2012). Over 50% of the global population remains in the North-central region of the country to spend the winter there, specifically in the Aveiro-Nazaré SPA (Araújo et al. 2022a). Some birds, likely immatures and non-breeding adults, remain in the region throughout the year. In the Azores and Madeira, its occurrence is quite rare (Meirinho et al. 2014).
Data from 10 birds fitted with GPS between 2000 and 2023, in the Balearic Islands and at sea off France, confirm this distribution.

Abundance and population trends

Estimates from colonies indicate a population of fewer than 3,000 pairs (Arcos et al. 2017), but censuses from vessels and coastal counts suggest that the global population may be around 25,000 individuals (Arcos et al. 2012; Arroyo et al. 2016). The species presents a sharp reduction in its breeding population, with a projected annual decline of about 14% (Genovart et al. 2016; Genovart et al. 2019). In Mainland Portugal, the population was estimated at 9,093 to 20,436 individuals between 2010 and 2015 (Araújo et al. 2022a). There are regular concentrations of several hundred birds in Portuguese waters (Catry et al. 2010a). The negative trend of this population, combined with the fact that it has not achieved Good Environmental Status for the abundance indicator on the Mainland, appears to be in line with the global trend. There is no up-to-date information on abundance or population trends for the Azores and Madeira.

Ecology and habitat

The Balearic Shearwater is a coastal bird that does not occur beyond the continental shelf (Ruíz & Martí 2004). It generally breeds in cavities on small islets and on cliffs. It feeds on small pelagic fish (e.g. anchovies and sardines) and cephalopods, also making use of fishing discards (Navarro et al. 2009). In Portugal, it has been observed feeding in large groups, including on schools of sand eels (Catry et al. 2010a).

Threats and conservation

It is the most threatened seabird species in Europe. The main threats identified are predation of adults by introduced mammals (e.g. cats and rodents), disturbance from human activities, and bycatch in fishing gear, which accounts for almost 50% of mortality (Arcos 2011; Genovart et al. 2016). Although bycatch appears to be irregular, Balearic Shearwater bycatch events have been recorded, mainly in longlines (Arcos & Oro 2004; Cortés et al. 2017) and purse seines, but also in trawls and gillnets (Abelló & Esteban 2012; Oliveira et al. 2015), with some incidents involving over a hundred birds being caught.