Many thanks for all your comments. I guess todays 'German' Blackcaps who now come here for winter are a good example of how it might have worked in some cases.
John.. the book will have to wait for a lottery win!
Yes, good example Mary - a case of 'positive feedback' reinforcing migratory patterns and resulting in a species exploiting a new opportunity which appears to have arisen due to an ongoing climatic trend (in this case generally milder winter weather) coupled with new feeding opportunities
via bird-tables / winter feeding by humans. If these conditions reversed and people no longer put out food, I guess it wouldn't be long before different migratory patterns prevailed whereby wintering further south became more favourable and the Blackcaps wintering in north-west europe became 'deselected'
via increased winter mortality rates.
Examples of where very similar species have evolved very different migratory strategies can be found among the Swallows of the African continent. This family is particularly well represented in Africa and it is probably where they first originated. Being among the most aerial of all birds, they have since radiated out across the globe. Two species in particular strike me as examples of how divergent behaviours in the distant past have led to what we see today:
Our Swallow (
Hirundo rustica) is generally a VERY long distance migrant, as most people appreciate - wintering in South America, Southern Africa and Australia and spending the boreal summer as far north as the arctic circle.
The Red-chested Swallow (
Hirundo lucida) closely resembles
Hirundo rustica (so much so that it was formerly considered a subspecies) - but it is entirely non-migratory and stays in West Africa and Ethiopia throughout the year.
At some point in the past, the original Swallow 'superspecies' probably originated in this part of Africa but, due to competitive pressures, a portion of that original population found advantages / opportunities in moving north during the summer and south during the winter. This emerging behaviour allowed the overall population to increase as it could expand and exploit new niches (
i.e. there was a net increase in the 'biomass' of the population). As generations went by, the pioneering migratory behaviour was subject to positive feedback and the frontiers of this portion of the population were widened further and further. Over time, physiological adaptations to migration took hold and phenotypic and genetic differences arose and the two populations became isolated,
i.e speciation had occurred and this bifurcation was driven by migratory behaviour opening up new horizons.
BTW: The Ian Newton book is a great read but I wouldn't pay that much (£90+) for it, either - glad I got mine when it was only £19.