Climate change not only bleaches meadows and withers gardens. The colors of birds are also apparently fading as a result of increasing global warming. This is the conclusion reached by biologists from the University of the Basque Country and the French Center for Evolutionary Research (CEFE) in a long-term study with blue tits in southern France, which is now being carried out in the trade journal The American Naturalist has appeared.
The scientists tracked down the phenomenon through long-term observations of two tit populations in France. For 15 years, PhD students monitored hundreds of breeding birds in a forest near Montpellier and on the island of Corsica. In order to be able to compare the development of plumage coloration over the years, the researchers always captured all adult birds at the same time in spring, when they were nursing young in their nest boxes. By doing this, the biologists ensured that the natural bleaching of the feathers they wanted to study was the same every year. Each animal had some of the distinctive blue feathers on its head and yellow feathers on its chest plucked out and their color intensity measured in the laboratory.
The analysis of the almost 6000 plumage samples gave a clear result: the blue crests and yellowish breasts of the blue tits are now much duller and less colorful than at the beginning of the study. On average, the luminosity of the Corsican tits in particular decreased by more than 15 percent. “The change in plumage color appears to be the result of a combination of a 1.2 degree rise in temperature and a decrease in precipitation, so we see climate change as a likely cause,” says study leader David López-Idiáquez.
Given the French researchers’ findings, it may also be worth taking a closer look at tits at bird feeders and in gardens elsewhere in Europe. The temperatures there have risen by a similar magnitude. In Germany, for example, it was 1.1 degrees warmer in the 30-year reference period that ended in 2020 than in the previous 30-year period, according to figures from the German Weather Service. However, the amount of precipitation remained almost constant over the long period.
When male plumage fades, females may not be able to assess reproductive success
Even if the uniformly paler plumage of the titmouse is hardly perceptible to humans at a cursory glance, it could represent more than a visual petitesse for the birds. Because individual color features in the plumage – referred to as ornaments in ornithological jargon – have an important biological signaling function. A magnificently shimmering male blue tit signals a female that she is willing to accept a healthy, assertive partner who promises the greatest possible success in rearing the young. And the prospect of reproductive success is the most important criterion for partnership decisions in nature.
“Our results may indicate a decrease in the signaling potential of the color ornaments,” says López-Idiáquez. However, if the variance and thus the signal strength among the males is not large enough, the females cannot distinguish between “high-quality” males and those that only promise low reproductive success.
The consequences of this “signal failure” could be higher brood failures of entire populations. For rarer species that may also be affected by the fading, this could prove to be a further threat to the survival of entire populations.
It also shows how important ornaments are for the visual communication of birds Research results from the USA. There, scientists discovered that when feeding their offspring, coots preferred the young birds that had particularly strong red plumage. Within a clutch, these were chicks that hatched later and were more red in color than their siblings that hatched earlier. In a direct comparison, the redder chicks received preferential treatment and more feed portions from their parents. The researchers take this as an indication that the ornamentation of the fledglings indicates to the parents which chicks would benefit most from additional care and nutrition.
It has not yet been clarified which physiological processes climate change influences the ornamentation of the plumage of birds. Poorer food supply during hot and dry summers is suspected to be a possible mechanism. With one record summer after another, that could mean the trend is here to stay. In their work, the authors also prepare for a less colorful future overall. There is some evidence “that warming may make ornamental coloring less noticeable,” they write.