With its black silhouette, the great cormorant(1) has a wingspan of approximately 1.5 meters and weighs 2 to 3.5 kg. But its wettable plumage and very limited body insulation was what intrigued David Grémillet, particularly due to the fact that the bird is widely found from Equatorial Africa to the Arctic, where it lives along the west coast of Greenland (60-80°N).
In Greenland(2) , during the last leg of a project begun ten years ago, he worked with Sarah Wanless, a researcher at the Natural Environment Research Council (NERC, GB), to study the feeding strategy of this big bird, accused by fishermen of pillaging fish reserves. Cormorants actually use technical prowess (localization, flight and diving) to feed themselves in these hostile environments without having undergoing a specific physiological or morphological adaptation (insulating fat or waterproof plumage).
David Grémillet recorded the diving efforts and food intake of wild great cormorants with miniaturized electronic sensors. He showed that their strategy was based on an extraordinary predatory performance, much more effective than that of other species of diving birds, catching fish 30 times more quickly. Cormorants can therefore catch a large number of fish in a short period of time.
The comparison between the cormorants of Europe and those of Greenland shows that they adapt the quantity of food they eat to the temperature of their environment. At European latitudes, they need two hours per day to find enough fish. In Greenland, in the winter (water: -1° C, air: - 25° C), they are able to reduce their daily diving time to less than 10 minutes by taking advantage of the high density of prey.
At the deepest level of ecology, these discoveries strongly suggest that diving birds were capable of colonizing high latitudes well before they could adopt specific body insulation. From the point of view of sea resource management, the scientist shows that the quantity of fish caught by the cormorant is normal for a bird of this size (it can be calculated in hundreds of grams per day and not in kilos!). In other words, cormorants do not need more fish than other species of diving birds.
But how do cormorants localize areas without ice and rich in fish during the Arctic winter? A new study by the CNRS and the Institut Polaire would make it possible to continue this research. As for now, David Grémillet is involved in an international project, east of the Scottish coast, the site of industrial fisheries that process a million tons of fish per year, possibly endangering the highest concentration of sea birds in the British Isles. An international project sponsored by the European Union was set up to evaluate the ecological impact of this activity.
Finally, sea birds can help oceanographers – thanks to the miniaturized sensors they wear all year around and to the precise and continuous data they register. For example, David Grémillet has participated in a NERC study that shows that even ephemeral ocean structures such as internal waves can be registered by diving birds equipped with electronic sensors. While generating essential data for modeling the climate, David Grémillet will be able to study sea birds to enhance our understanding of marine physics.
(1) The great cormorant can be found just about everywhere in France, where it arrives by the tens of thousands each winter from Scandinavia.
(2) This research has been financed by the Institut Polaire Français Paul-Emile Victor since 2001.