During winter time, even though we have great whale watching tours and encounters we can’t forget a very good friend of us, the Cory’s shearwater or the ‘cagarro’ as we call them in Portuguese. This beautiful marine birds start accompanying our tours since middle spring to middle autumn, their fly is very special as they enjoy to touch with their wings the surface of the ocean and they lead us very often to different species of dolphins (mainly common and also a summer time species the spotted dolphin).
Let’s now learn a bit more about this very special species: who they are, which their main characteristics are, how their migration routes are and what the threats they face nowadays.
Who are these Cory’s shearwaters?
The Cory’s (Calonectris borealis) is a large shearwater in the seabird family Procellariidae. Named Cory’s in English because of the American ornithologist Charles B. Cory.
It is a very symbolic migratory seabird of the Azores, as it covers the world’s largest breeding population of the species (Fig 1). They live in the open ocean, ranging from neritic zones (areas with water less than 200 meters deep) to continental shelves, and only come ashore during the breeding season.
Fig1: Cory’s shearwater abundance. Graphic by Victor Martin
Their open ocean habitat covers warm waters of that are connected with high productive ecosystems, such as ocean fronts, deep-water springs and continental shelves. This creates a food source for many marine animals, like the Cory’s shearwater.
This species has a complex system of sounds. The sounds are only produced during the breeding season, in their breeding colonies. Males only produce high-pitched tones while the females produce more deep tones. This sounds have a very particular tone that can be heard specially at night time close to the cliffs were they nest.
Foto by Mariana Silva
Foto by Mariana Silva
The genus name Calonectris comes from the classical Greek words kalos, meaning "Good" and nectris meaning "swimmer". And, as might be expected, the Cory’s shearwater is a great swimmer! The species has webbed feet as well as long and straight wings that are adapted to allow the birds to use the updrafts of wind on the waves to “shear” over the water’s surface (hence the name “shearwater”).
But, not only the wings are perfectly adapted to an oceanic lifestyle, the species is also adapted to drink seawater due to the existence of highly developed nasal tubes. They have desalination glands that extract and dispose of the salt. This is reason why this birds can spend months in the ocean without coming back to land. They also have an unusually strong sense of smell. This is very handy, since they can find their food over large ocean surfaces and also relocate their colonies.
Cory’s shearwaters and common or spotted dolphins or even other species like tunas are very often seen around the Azores, and also on our whale watching tours, all feeding together in a ‘feeding area’ were the dolphins surround the school of fish forcing them to ‘make’ a ball and other species including Cory’s profit of this feast diving directly into the ball of fish.
When they are not feeding this birds rest on the surface of the ocean, creating big island of birds that are just ‘floating', it is something very beautiful just to observe them taking off using the water as a runway with their legs !!
This marine bird has a “Mediterranean cousin”, the Scopoli’s shearwater (Calonectris diomedea).
For a long time researchers thought that the Cory’s shearwater (Calonectris diomedea borealis) was a subspecies from Scopoli’s (Calonectris diomedea). But now, they are considered by researchers as two different species.
Although their appearance is very similar, they still have important differences that result in them being distinguished as a separate species:
The most important difference is their different and very marked breeding distribution:
• Scopoli’s shearwater à Mediterranean distribution: Algeria, Croatia, France, Greece, Italy, Malta, Spain (excluding the Canary Islands), Tunisia and Turkey.
• Cory’s shearwater à Atlantic distribution: Portugal (including the Azores, Madeira and Berlengas) and the Canary Islands.
Fig 2: Geographical breading area of Scorpoli’s and Cory’s shearwater.
Also their size is quite different: the Cory’s shearwater has a bigger body-mass than the Scopoli’s shearwater, with the different being about 46 percent.
Fig 3. Population genetic structure of Cory’s shearwater. Munilla I. et. al. (2016)
However, genetic material gets mixed between both species if one individual travels from one population to another (e.g. from Baleares Islands to Azores Islands). This event is called gene flow, but in this case the gene flow is low, allowing us to think that there is not a big hibridation between the two species.
There is another characteristic that the researchers use to differentiate the two different species, the bird song. Even though they use their special song for the same purposes: simple vocalization for calls and complex for mating, the sound between the two species is different, being the one of the Cory’s with three syllables and the Scopoli’s using two:
Don’t miss theirs incredible singing! Can you notice the difference?
Written by: Cristina Montoya