The sympatric breeding of Larus fuscus, L. cachinnans and L. argentatus in western France

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Abstract
An opportunity to study the validity of the species Larus cachinnans, considered by some authors as merely a group of races related to either L. fuscus or L. argentatus, is given by its sympatric breeding with these species. Both of these latter species have recently extended their breeding range southwards, while Mediterranean representatives of the cachinnans group have spread northwards, and the three forms now breed side by side over c. 350 km along the Atlantic coast of France, where their breeding biology was studied from 1983 to 1990. L. cachinnans is very territorial, some pairs holding territories almost all year round; argentatus pairs may defend territories from mid‐winter; and the supposedly migratory fuse us are on their territories for only a short period before laying. L. cachinnans is an early breeder, laying from late March to early May, while fuscus lay from May into June, and argentatus has a protracted breeding season overlapping the other two. Interbreeding occasionally occurs; it can be productive in the case of fuscus x argentatus whereas fuscus x cachinnans attempts did not produce any young. No cachinnans x argentatus pair has been observed. This indicates strong reproductive isolation, evidence that L. cachinnans must be considered a good species.