After studying breeding Great Cormorants at wetlands where the two European subspecies breed side by side, ornithologists unpicked differences in their breeding success.
Since the 1970s, Great Cormorant populations in Europe have generally increased in response to protective legislation, limiting the lethal measures anglers and fish farmers can take against the species. This has resulted in growing numbers of cormorants wintering in south-western Europe, with France in particular seeing winter numbers of the Continental form sinensis soar.
This inland-breeding subspecies has subsequently become re-established as a nesting bird in France, where it has come into contact with breeding populations of the nominate form carbo, which mainly breeds along the Atlantic coast. By 2021, the sinensis breeding population in France had swollen to more than 9,000 pairs, compared to fewer than 2,000 pairs of carbo.
Great Cormorants of the Continental form sinensis have enjoyed a resurgence in Western Europe over the last few decades (Rob Cockbain).
Since the 1980s, both subspecies have bred alongside one another (and sometimes hybridised) at Grand-Lieu, a wetland some 20 km inland in Loire-Atlantique. It is now the largest French cormorant colony, with a peak of 1,379 pairs recorded in 2007.
Loïc Marion and Jérôme Le Gentil studied the ecological specialisation of the two cormorants forms nesting at Grand-Lieu, determining subspecific identity of nestlings by genetic analysis.
They found that both sinensis and carbo laid at the same time each year, with no difference in clutch size between the forms. However, average fledging success was at least 30% lower for carbo pairs, with intermediate results for mixed pairs.
Marion and Gentil concluded that the largely coastal-nesting carbo was not as well adapted as sinensis to inland wetlands during the breeding season, and they suggested that by faring better in this habitat, the recently re-established sinensis population might contribute to carbo colonies remaining restricted to coastal areas.
Reference
Marion, L, and Le Gentil, J. 2022. Habitat Specialisation Affects Fitness of the Marine and Continental Great Cormorant Subspecies in a Recently Evolved Sympatric Area. Ardea, 109(3), 471-480. DOI: https://doi.org/10.5253/arde.v109i2.a17
