Many organisms have invaded urban habitats,
although the underlying factors initially promoting urbanization
remain poorly understood. Partial migration may
facilitate urbanization because such populations benefit
from surplus food in urban environments during winter,
and hence enjoy reduced fitness costs of migratory deaths.
We tested this hypothesis in the European blackbird Turdus
merula, which has been urbanized since the 19th century,
by compiling information on timing of urbanization, migratory
status, and population density for 99 cities across the
continent. Timing of urbanization was spatially auto-correlated
at scales up to 600 km. Analyses of timing of urbanization
revealed that urbanization occurred earlier in partially
migratory and resident populations than in migratory
populations of blackbirds. Independently, this effect was
most pronounced in the range of the distribution that currently
has the highest population density, suggesting that
urbanization facilitated population growth. These findingsare consistent with the hypothesis that timing of urbanization
is facilitated by partial migration, resulting in subsequent
residency and population growth