Hi Harry,
One of the interesting things about bird migration is that whilst they could theoretically go anywhere, most birds end up exactly where they came from. Some birds will fly from the UK, across Southern Europe, Northern Africa and the Sahara desert, only to return and breed in the exact same tree they nested in the year before.
Even more interestingly, this does not preclude diversification. Since the breeding group is always (more or less) in the same place each year (barring a small number of vagrants) their genetic diversification can progress unimpaired.
A good example of this is the white wagtail
(Motacilla alba), a pretty little black and white bird, which displays a great deal of variation across Eurasia (Added by Edit - it's not a ring species, but I mention it as an example of a highly variable migratory bird). Some are migratory, others (in the milder parts of their range) are resident. The Wiki page has some photo's to illustrate the wide variation between sub-species/clades.
Basically, it doesn't matter so much where they are during the non-breeding season. What matters is where they are when they breed. Obviously though, the more geographically isolated a population, the stronger the tendency to diverge.
Greenish warblers do migrate by the way. They winter in India.
Mutate and Survive
Edited by Granny Magda, : No reason given.
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