Even as humans have exterminated ~130 bird species since the 17th century, around 10,000 are still extant, in most every terrestrial habitat, from the Arctic to the Antarctic.
As the most speciose tetrapod, birds most vividly illustrate adaptive radiation with notably narrow niches. Resource competition and mating preference drive avian sympatric speciation.
The Greenish warblers spread out around the Tibetan plateau are a ring species: reproductively isolated populations, but not by geographic boundary or habitat. Birds in adjacent areas live within flying distance of each other, and are genetically similar enough to interbreed, but don’t.
Over time, populations developed their own distinctive songs. Speciation happened as females preferred males that sang local tunes.