Bluetit 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.
bluetit
Header picture by DKG; others CC0 from pixabay
Southwick Country Park Nature Reserve 
Thanks, I had never heard of that before, always learning!
I know that most pigments come from plants but so far haven’t been able to find out if the green caterpillars get their pigmentation from the plants they eat. It seems to be a fairly new area of research.