10 April 2024

Polish scientists looked at the feather and body colours of birds. The study was conducted to see if environmental pollution and the type of pollution affect the birds’ colouration.

Polish scientists conducted a study that showed that environmental pollution affects the feathers and body colour of birds in different ways, depending on the type of colouration. The research aimed to show how significant this effect is. The results of a meta-analysis conducted by Katarzyna Janas, Agnieszka Gudowska and Szymon Drobniak, were published in Biological reviews.

For this meta-analysis, the researchers analysed 59 scientific papers published between 1997 and 2022, which described studies on the colouration of 25 bird species. The aim of these studies was to determine whether ornamental plumage (ornament), which owes its colour to different colour production mechanisms, differs in sensitivity to anthropogenic factors.

The authors of the meta-analysis wanted to find out whether there was an interaction between the mechanism of colour production and the type of anthropogenic factor. For the study, the researchers distinguished four categories of anthropogenic factors: heavy metals, persistent organic pollutants, urbanisation and others, which included radioactive contamination and petroleum pollution. The researchers also identified the type of mechanism responsible for the colouration of each ornament analysed.

In birds, colour is produced both by pigments present in the feathers, the unfeathered parts of the body, and by the interaction of light with regular nanostructures within the feathers – so-called structural colouration. The most common pigments are eumelanin, responsible for black and grey tones, and pheomelanin, creating colours ranging from warm browns to reds to light fawn. In a huge number of species, there is also colouration based on carotenoid pigments, which give colours ranging from yellow and olive to orange and red.

Birds cannot produce these pigments on their own and must take them with food, so the quality of this colouration may depend on the availability of food in the environment. Structural colouration, on the other hand, is responsible for iridescent and metallic colours and a range of blues with a strong admixture of ultraviolet, invisible to humans but seen by birds.

In a meta-analysis, the researchers divided the types of colouration into three categories: carotenoid, melanin and structural colouration. They were able to find significant differences in the sensitivity of ornamentation depending on the colour production mechanism. Carotenoid colouration showed higher sensitivity than that based on melanin or structural colouration.
The study concluded that one of the reasons for this may be the need to extract carotenoids from fruit or invertebrates. In degraded or polluted areas, the availability of this pigment may be reduced. Contrary to expectations, the researchers were not able to detect a relationship between the mechanism of colour production and the type of pollution.

The researchers wrote that although they have been able to demonstrate the sensitivity of ornaments with carotenoid colouration, their use in environmental bioindication seems quite remote for the time being. On the one hand, it requires a better understanding of the natural variation occurring in these traits, including the differences between carotenoids in feathers after previous metabolic modification (e.g. to red keto-carotenoids) and those deposited without previous conversion (e.g. lutein and zeaxanthin in the rich-tailed godwit). On the other hand, unification of methods to objectively measure colour quality is needed.

Scientists are trying to determine how significant this impact is, but they are still lacking some data – for example, the significance of the presence of pharmaceuticals in the environment.

The researchers warned that the easiest way to spot changes in the carotenoid colouration of great tit and eurasian blue tit is in the centres of large cities, where there are individuals with particularly faded colouration. However, it should be remembered that the colour of the feathers is indicative of the condition of the bird and the quality of the environment it was in during the moulting period, and therefore not necessarily where and when we observe it. Nevertheless, it is always worth keeping an eye out for unusually coloured individuals and documenting them – they can provide important alarm signals.