Long-term decrease in colouration: A consequence of climate change?

Climate change has been shown to affect fitness-related traits in a wide
range of taxa; for instance, warming leads to phenological advancements in
many plant and animal species. The influence of climate change on social
and secondary sexual traits, that are associated with fitness due to their
role as quality signals, is however unknown. Here, we use more than 5800
observations collected on two Mediterranean blue tit subspecies (Cyanistes
caeruleus caeruleus and C. c. ogliastrae) to explore whether blue crown
and yellow breast patch colourations have changed over the past 15 years.
Our data suggests that colouration has become duller and less chromatic in
both sexes. In addition, in the Corsican C.c.ogliastrae, but not in the
mainland C.c.caeruleus, the decrease is associated with an increase in
temperature at moult. Quantitative genetic analyses do not reveal any
microevolutionary change in the colour traits along the study period,
strongly suggesting that the observed change over time was caused by a
plastic response to the environmental conditions. Overall, this study
suggests that ornamental colourations could become less conspicuous due to
warming, revealing climate change effects on sexual and social ornaments
and calling for further research on the proximate mechanisms behind these
effects.