The colonisation of European towns by Turdus merula began in the early nineteenth century and spread outward city by city, giving researchers an unusually well-documented natural experiment. Urban blackbirds are more often resident where woodland birds of the same region migrate, they begin breeding earlier in the year, and they sing at higher minimum frequencies where low-frequency traffic noise would mask them.
Some of the differences are physiological rather than behavioural. Urban birds show altered stress-hormone responses to handling compared with forest birds, and comparisons of city and rural populations have found differences at genes associated with behavioural variation. Artificial light shifts the timing of dawn song and of reproductive development.
What remains genuinely unclear is how much of this is evolutionary change and how much is flexible response within a bird's lifetime. Birds raised in common conditions retain some differences, which points to a heritable component, but disentangling that from developmental effects of an urban upbringing is still an active problem.