What happened to the greenish warbler birds as they migrated from the southern portion of their range up around the Tibetan Plateau? What are two main distinctions between the East Siberian greenish warbler and the West Siberian greenish warbler

The greenish warbler migrates from the Tibetan Plateau to India every year. During their migration, two species of greenish warblers met up, but didn't interbreed, therefore these birds are still two separate species. Because these birds are so closely related, it is rare that interbreeding didn't occur.