It's a well-worn cliche that moving to the big city after a lifetime of country-living can change you forever. That might actually be literally true for blackbirds, as moving to the city has begun to split their species in two.
Blackbirds are found throughout northern Europe, and they began moving into cities in the 1930s. Researchers at the University of Sheffield recently analyzed 168 blackbirds found in cities from Spain to Estonia covering a stretch of Europe over 1,700 miles long. By studying the hydrogen isotopes found on the birds' beak and feathers, the researchers can deduce where in the world the different birds have eaten.
Figuring that last part out is crucial, because then the researchers can determine whether the birds have migrated. As a general rule, blackbirds will fly south to the winter, leaving the cold of northern Europe for the Mediterranean, with some flying as far as northern Africa. But the urban birds consistently flew shorter distances than their rural counterparts, and many of the northern city birds didn't migrate at all.
It's not too hard to figure out why this might be. Cities tend to be warmer in the winter than the country, and there's way more food for the blackbirds to scavenge in the cities than there are in rural climates. In just a few generations, these birds have gone from migratory to sedentary, and that has already had a quantifiable difference on the genetics of the rural and urban populations.
The non-migratory birds might also start breeding sooner, and given enough time the two populations could become so different that they split into different species. All things considered, moving to the city is way more serious than I thought.