Differences in feather structure between urban and forest great tits – constraint or adaptation?

Urbanization is one of the strongest habitat transforming processes today
that has resulted in changes in the ecological conditions for wild
populations. In birds, the limitation of natural food sources and a warmer
microclimate in cities can potentially influence the development and
functioning of the plumage that may have important fitness consequences.
Despite its potential significance, the plumage structure of urban birds
are largely unexplored and it is unclear whether and how they respond to
urban ecological processes such as different constraints and selection
pressures. In this study, we compared several structural properties of
contour, primary and tail feathers between two forest and two urban great
tit (Parus major) populations. Our results show that the urban environment
affects only a few structural properties of feathers and only in the
plumage of first-year birds. We found that both the plumulaceous and the
pennaceous parts of their contour feathers are longer and have lower barb
density in the urban than in the forest habitat. We also found that the
primaries of first-year birds have narrower rachis and higher barbule
density in the cities than in the forests, but there were no differences
in other wing feather traits and in any tail feather traits between
habitats. We did not find differences in the feather structure of urban
versus forest adult birds. The habitat differences in first-year birds may
indicate nutritional constraints or the effects of the warmer microclimate
of the urban environment. These differences seem to disappear completely
in adulthood that can be explained by the selective mortality of
first-year birds, or by adults being less sensitive than first-year birds
to environmental effects during their molt.