What do you know about the Cory's shearwater? (Part I)

10 September 2025

NOVA FALCONS, A COMPANY SPECIALISING IN ENVIRONMENTAL STUDIES, WILDLIFE MANAGEMENT AND OTHER ENVIRONMENTAL SERVICES

As a result of the participation of our biologist Lara Abascal in part of the study and conservation work carried out by the Societat Ornitológica de Menorca (SOM) on Illa de l'Aire, we bring you this new post, which we hope will serve to raise awareness of this bird that is so familiar to us, yet so unknown.

The Cory's shearwater (Calonectris diomedea)

The Cory's shearwater (Calonectris diomedea) or "virot gros" is a seabird notable for its large size and characteristic high-speed flights over the sea surface, where it searches for fish and cephalopods to feed on (Reyes-González, González-Solís 2016). It belongs to the order Procellariiformes (like albatrosses), which means it has a series of morphological adaptations that allow it to fly at high speeds to find food thanks to its large wingspan (117-135 cm) and the narrow, pointed shape of its wings. In addition, this aspect ratio allows it to migrate long distances very efficiently.

The prominent nostrils of the Cory's shearwater are one of its most characteristic features. They form a thick beak adapted for fishing marine prey, and their length and height also allow us to differentiate between the sexes: the male has a longer and higher beak (Wink and Ristow, 1979; Lo Valvo 2001).

Shearwaters are pelagic birds, meaning they live most of their lives at sea, only coming ashore to breed. During spring, they gather in colonies on islets or cliffs in the Mediterranean Sea, where hundreds of pairs come together to breed. They are a monogamous species, so pairs remain together year after year, and only one of the members is replaced in the event of death or reproductive failure in the previous season.

Pardela cenicienta incubando a finales del mes de junio.

Interestingly, in addition to their partner, they also tend to keep the nest in which they breed each year. They hardly build anything that could be called a structure, laying their only egg in pre-existing hollows between rocks, or even among vegetation, where they feel protected to incubate for more than 50 days. Both sexes take care of the egg and take turns incubating it for between 3 and 7 days: while one member of the pair remains in the nest, the other leaves the colony to feed in the open sea.

Cory's shearwater incubating its egg in a nest among vegetation (above), although they usually nest among rocks (below).

empresa impacto ambiental Mallorca Ibiza Menorca

Detail of the feet of a Cory's shearwater, showing the small claws they have developed to help them dig the entrance to their nests, sometimes with underground tunnels several metres long (Reyes-González and González-Solís 2016).

Birth and development of chicks

Shearwaters lay a single egg per pair, which hatches around July. As mentioned above, the incubation period is more than 50 days; this, combined with one egg per pair in a bird that is extremely well adapted to life on the open sea (they are extremely clumsy on land), gives us an idea of their total dependence on maintaining their breeding colonies stable and undisturbed, in terms of the presence of predators or destruction and alteration of the environment.

This single chick is born and remains in the nest for 90-99 days, leaving it in October. At this time, juvenile mortality is usually highest because the parents gradually stop caring for the chick, leaving it alone for 1 to 3 days (Sánchez 1997, Reyes-González and González-Solís 2016). Once reproduction is complete, postnuptial migration to wintering areas in the Atlantic begins.

Shearwaters that breed in the Balearic Islands usually winter on the west coast of Africa, although due to the influence of prevailing winds and the timing of their departure, in many cases they can reach the Brazilian coast, especially those that breed further south (Felicísimo et al. 2008; González-Solís et al. 2009). And although, as we have indicated, it is a monogamous species, the pairs migrate separately, and once winter is over, they begin their pre-nuptial migration to their breeding colonies, returning to the Mediterranean. The male is usually the first to arrive and occupy the nest, with the arrival of the female marking the beginning of courtship and copulation, which in turn marks the start of a new reproductive cycle on land.

The blue lines correspond to C. borealis from the Canary Islands and the magenta lines to C. borealis from the Azores. Image from González-Solís et al. (2009). Data obtained using light-level geolocators; the points on land correspond to location errors intrinsic to the method.

Each yellow line corresponds to the annual migration of a Cory's shearwater (C. diomedea) nesting in the Balearic Islands. Routes followed by four breeding shearwaters from the Balearic Islands to spend the northern winter in the waters of the Canary Current. Image from González-Solís et al. (2009). Data obtained using light-level geolocators; the points on land correspond to location errors intrinsic to the method.

Yellow: Migration routes followed by two Balearic shearwaters, with their main wintering grounds in the Benguela Current and Agulhas Current (Indian Ocean). The blue lines correspond to C. borealis from the Canary Islands and the magenta lines to C. borealis from the Azores. Image from González-Solís et al. (2009). Data obtained using light-level geolocators; points on land correspond to location errors intrinsic to the method.

Population status

It has an estimated population of between 285,000 and 446,000 adult individuals, with a wide distribution area due to its migration, which is why it is classified internationally as ‘least concern’ according to the IUCN. However, it only breeds in Mediterranean areas of Algeria, Croatia, France, Greece, Italy, Malta, Spain, Tunisia and Turkey (Derhé 2012). Furthermore, the species is suspected to be in slight decline, and in Mediterranean areas its population appears to have fallen by up to 25% in the last three generations, which is why the species is classified as ‘endangered’ in the Red Book of Birds of Spain. (EN) (BirdLife International 2015, Defos du Rau et al. 2012, Derhé 2012, Carboneras et al. 2013, SEO/BirdLife 2021).

World distribution map showing the distribution area of the Cory's shearwater in green (Image from Birdlife International 2018).

Especially in the Mediterranean, the Cory's shearwater is subject to pressures and threats that have caused this population decline. In fact, in the Mediterranean it is one of the species most frequently caught as bycatch or accidentally caught in fishing gear. It has been estimated that the Spanish fleet may capture 4-6% of the breeding population, mainly by longline fishing (Cortés et al. 2017, 2018; Genovart et al. 2017, 2018).

Another significant threat is predation by species such as the black rat (Rattus rattus), which greatly reduces the reproductive success of shearwaters and other seabirds. Therefore, monitoring breeding colonies for early detection of rats and controlling their numbers is key to the conservation of the Cory's shearwater (Igual et al. 2009, Pascal et al. 2008).

Shearwaters look for hard-to-reach places to build their nests, so they feel more protected from potential predators.

For all these reasons, and due to the importance of the Cory's shearwater as an emblematic species in our waters, several breeding colonies located in the Balearic Islands, Columbretes, Murcia, Almería, and the Chafarinas Islands have been monitored for years. The largest Spanish colony is located in the north-west of Menorca, in Cala Morell, where up to 1,000 pairs were estimated in the 1990s and 400 in 2021-2022, as well as some 750 pairs in Menorca as a whole (Catchot 1991, Genovart et al. 2022, SEO/BirdLife 2022b). In Mallorca, there are around 200 pairs on the islet of Pantaleu, apart from a smaller population on Dragonera and the Malgrats islets; and in the Pitiusas, the number is estimated at between 250 and 375, pending a thorough review (SEO/BirdLife 2021, 2022a). In total, there must be around 1,500 pairs or slightly more in the Balearic archipelago, below the previously estimated range of 1,800-7,000 pairs (Madroño et al. 2004, SEO/BirdLife 2022a).

Among the colonies in the Balearic Islands, the one on the islet of l'Aire, in Menorca, stands out. This is one of the most important in the area, which is why for years the SOM (Ornithological Society of Menorca), in collaboration with the CEAB-CSIC, has been carrying out annual monitoring over the last few decades, providing invaluable information on the breeding habits and ecology of Cory's shearwaters in the Mediterranean.

One of the aspects measured during colony studies is egg size, as this could be related to reproductive success.