Short answers, pulled from the story.
Carotenoids are yellow, orange, and red organic pigments produced by plants, algae, bacteria, archaea, and fungi. Their color comes from a long chain of conjugated double bonds that absorbs light wavelengths between 400 and 550 nanometers, causing the compounds to reflect yellow, orange, or red light. The longer the conjugated chain, the further the color shifts toward red.
Over 1,100 distinct carotenoids have been identified. They fall into two main classes: carotenes, which contain only carbon and hydrogen, and xanthophylls, which also contain oxygen.
Flamingo feather color comes entirely from carotenoids in their diet. The birds cannot synthesize carotenoids themselves and must obtain them through food. Without a carotenoid-rich diet, the pink and orange coloration would not develop.
Dried carrots contain the highest amount of carotene of any food per 100-gram serving, measured in retinol activity equivalents. Vietnamese gac fruit contains the highest known concentration of the carotenoid lycopene. Beta-carotene is also found in pumpkins, sweet potato, and winter squash, while kale, spinach, and collard greens contain substantial amounts despite being green.
Carotenoids that contain unsubstituted beta-ionone rings, including beta-carotene, alpha-carotene, beta-cryptoxanthin, and gamma-carotene, can be converted into retinol, which is vitamin A. Humans cannot synthesize carotenoids and must obtain them through diet, and consuming them with dietary fat improves absorption.
Studies estimate around 2,956 modern bird species display carotenoid-based coloration, with males typically showing more vibrant color than females. Female birds prefer brighter males because carotenoid coloration is generally correlated with immune function and oxidative stress resistance. Among Parus major males, more colorfully ornamented individuals produce sperm better protected against oxidative stress.
Carotenoids perform two protective functions in photosynthetic organisms. They transfer absorbed light energy to chlorophyll for use in photosynthesis, and they neutralize the reactive oxygen species that light generates as a damaging byproduct. They quench triplet chlorophyll and singlet oxygen through both energy transfer and chemical reactions, protecting lipid membranes from free-radical damage.