Problèmes biogéographiques, écologiques et taxonomiques posés par le Grand Cormoran Phalacrocorax carbo
Authors: Marion Loïc 
Published: November 2022

The taxonomic status of the French Cormorant population which lies on the borderline between the breeding range of the two races, Phalacrocorax carbo carbo whose nearest breeding site is in the British Isles and Ph. c. sinensis which breeds in the Netherlands, is still controversial. Observations of the plumage characteristics of breeding birds in Baie du Mont-Saint-Michel and consideration of the origin of seven breeding pairs at the Lac de Grand-Lieu (Departement de Loire-Atlantique) in 1981 provide an opportunity to re-examine the matter. The results of our investigations can be summarized as follows :
1) The identification of the two races on the basis of differences in adult plumage was not possible. At the beginning of the breeding season in March 67 to 70 % of the cormorants observed at the Ile des Landes (Baie du Mont-Saint-Michel) had a carbo phenotype , 20 to 25 % a sinensis phenotype and 4 to 13 % were intermediates between the two. Moreover the white head and neck feathers did not last as long as the white thigh patches, and this may lead to confusion between the races during most of the breeding season. Indeed it is likely that the proportion of white headed birds (sinensis) in this breeding population was much higher (53 to 59 %) ; asynchronous breeding resulting in only 25 % of the population having a white headed plumage at any one moment. These observations indicate that if carbo and sinensis do have different plumage characteristics (and this still needs to be proved), the French situation can only be explained by one of the following alternative hypotheses : Either French cormorants are a mixture of carbo and sinensis, or the two phenotypes can only be distinguished by the duration of the white head and neck plumage (sinensis keeping the white head for a longer time than carbo).
2) Several facts suggest the possibility that the Grand-Lieu immigrants might come from the Netherlands and not from the British Isles or the nearby French coast. They would therefore belong to the race sinensis, and not to carbo. First -The habitat provided by the Lac de Grand-Lieu closely ressembles that of the Dutch cormorants. This is particularly true of the feeding habitat, a fresh water marsh covered with floating vegetation. Birds of the carbo race breed on cliffs along the shore and feed at sea.
Second -A clear inter-race habitat preference is demonstrated on both the breeding and wintering grounds. Thus although the five departements surrounding the Grand-Lieu lake are wintering grounds both for British carbo and Dutch, Danish, Swedish and East German sinensis, an analysis of the recoveries shows that 87 % of the carbo are found close to the sea shore and 54 % at least of the sinensis in inland fresh waters. Both races are present in estuaries. In contrast banding recoveries have shown that cormorants breeding on the French coast only winter close to the sea.
Third -Since 1967, large tree-roosts of wintering cormorants have been observed along the Loire River, from Nantes to Moulins, 1,000 to 1,500 birds being spread along 500 km of river. This influx of wintering cormorants in French inland waters corresponds to the marked increase of the Dutch and Danish breeding populations of sinensis which took place since 1964. In the meantime, the carbo population of Great Britain remained stable.
In conclusion we argue that the available data support the hypothesis of a difference in racial preference for a given habitat between carbo and sinensis. This does not forcibly imply a genetic determinism of such a habitat preference. The same result could well be achieved by a learned process, such as a traditional preference for the habitat where the adult birds have been born.