Blue tit factoid
Most birds can’t create pigments, other than melanin, on their own. This bluetit, high in the park’s canopy, can’t produce the pigment, carotene, that makes his tummy yellow; it comes from pigments in the green caterpillars he eats. The more caterpillars he eats and the brighter his tummy, the more likely he is to attract a mate.
Header picture by DKG; others CC0 from pixabay