Animals living in cities not only show adaptations in their behavior, but also their appearance can differ from that of their land-living conspecifics. This is also the case with great tits in European cities – their plumage is paler, a study shows. The biologists attribute this to the fact that the great tits in the city take in fewer carotenoids with their food. This circumstance could also have a negative effect on their immune system.
Urban areas are expanding, forcing wildlife to adapt to urban life. This even has an impact on their appearance: it is repeatedly observed that urban animals have paler fur or plumage compared to their relatives in rural areas. But these sightings are mostly isolated cases that have only been scientifically examined in more detail in locally limited areas.
“Urban pallor” affects all of Europe
Researchers led by Pablo Salmón from the Swedish University of Lund have now investigated the phenomenon of “urban pallor” for the first time in Europe, using great tits. To do this, they analyzed feather samples from over 200 animals from Gothenburg, Malmö, Milan, Madrid and Lisbon, which lived either in the middle of the city or in the wooded countryside. And indeed: In all five cases, the urban great tits wore significantly paler yellow plumage on their breasts than those from the forest.
How big the difference was, however, depended on the respective city. For example, Salmón and his colleagues observed that Lisbon’s urban titmice had paled in comparison to their forest-dwelling counterparts. In Malmö, on the other hand, the difference in plumage color between town and wood tits was not quite as pronounced.
Carotenoid deficiency as a cause
But why are city tits paler in comparison? How intense the yellow color of a great tit is is closely related to its diet, Salmón and his colleagues explain. If the little songbirds eat a lot of carotenoid-containing insects, the nutrient colors their plumage bright yellow. However, if there is a lack of carotenoids in their diet, the feather color will also be paler. Similar to flamingos, which get their intense pink color from the crabs and algae they eat.
“Our results indicate that the birds in the city are not getting the right food,” says Salmón’s colleague Hannah Watson. Either they find too few caterpillars and other insects there or they contain fewer carotenoids. This could be the case, for example, if urban pollutants such as cadmium or lead inhibit carotenoid synthesis in the insects’ food plants, which means that they too can absorb less of the nutrients.
But the lack of carotenoids not only changes the great tits’ appearance, but also probably affects their immune system, as the researchers report. Carotenoids are usually important antioxidants for birds, protecting their bodies from the toxic effects of pollution. Fewer carotenoids therefore correspond to a poorer immune system. But the city tits can possibly be given a helping hand. “By planting more native trees and plants in our gardens and parks, we can provide small birds like great tits with a healthy diet of insects and spiders for themselves and their chicks,” says Watson. The cities from the study in which the color difference was not quite as pronounced, such as Malmö, could serve as a model for such plantings.
Source: Lund University; Article: Journal of Animal Ecology, doi: 10.1111/1365-2656.13982