The plumage of birds plays an essential role in thermal insulation and influences
the heat tolerance of birds. These plumage functions are mainly determined by the
number and the density of feathers, but it is unclear how feather density responds
to environmental changes in wild populations. In urban birds, both high temperature
and limited food could generate changes in plumage traits. To investigate the effect
of urbanization, we compared the number of feathers in nestlings between urban and
forest Great Tits Parus major using a novel non-invasive method. We showed that urban
nestlings have fewer feathers than forest nestlings at 6-9 days old. Although the
density of feathers was slightly higher in urban nestlings, this was the result of
the smaller size of their feather tracts. We suggest that the reduced feather number
may be the result of either adaptation to higher urban temperatures, constrained feather
development due to limited optimal nestling-food sources in urban environments, or
both. Concentrating body feathers in a reduced tract area may also help birds to adapt
to higher urban temperatures because this can increase the relative size of bare body
surfaces which may facilitate heat dissipation. We suggest several possibilities for
future studies that would help to disentangle the underlying mechanisms responsible
for the observed patterns.
