Many birds use carotenoids to colour their plumage in order to attract mates. Carotenoids are also of importance for the immune system by acting as free radical scavengers. Thus, in polluted areas where levels of oxidative stress are high, carotenoids must be used for body maintenance rather than plumage colouration. As a consequence, great tits from urban areas are paler than tits from rural areas. Increased pollution thus not only render the spring quieter, it may also make it less colourful.