Larus argentatus · sidabrinis kiras

https://en.wikipedia.org/wiki/European_herring_gull The taxonomy of the herring gull/lesser black-backed gull complex is very complicated, with different authorities recognising between two and eight species.

This group has a ring distribution around the Northern Hemisphere. Differences between adjacent forms in this ring are fairly small, but by the time the circuit is completed, the end members, herring gull and lesser black-backed gull, are clearly different species. The terminal forms do not interbreed, though they coexist in the same localities.