European Herring Gull Lesser Black-backed Gull Ring species Cline, European Herring Gull, png | PNGWing
File:Rings species example.png - Wikimedia Commons
The herring gull complex is not a ring species
Darren Naish: Tetrapod Zoology: No no no no NO: the Herring gull is NOT a ring species!
There are no ring species – Why Evolution Is True
A novel approach for finding ring species: look for barriers rather than rings | BMC Biology | Full Text
SOLVED: Among arctic seagulls, hybridisation is possible between some species but not others. Heuglin's gull can hybridise with both the Black-backed gull and the Vega gull: The American herring gull can hybridise
Discovering Darwin: Season 1 Episode 3: Chapter II - Variation Under Nature
Similar Species to Ring-billed Gull, All About Birds, Cornell Lab of Ornithology