text: Dietary carotenoids and their metabolic derivatives are responsible for bright yellow to red coloration in birds. Studies estimate that around 2956 modern bird species display carotenoid coloration and that the ability to utilize these pigments for external coloration has evolved independently many times throughout avian evolutionary history. Carotenoid coloration exhibits high levels of sexual dimorphism, with adult male birds generally displaying more vibrant coloration than females of the same species.
  These differences arise due to the selection of yellow and red coloration in males by female preference. In many species of birds, females invest greater time and resources into raising offspring than their male partners. Therefore, it is imperative that female birds carefully select high quality mates. Current literature supports the theory that vibrant carotenoid coloration is correlated with male quality—either though direct effects on immune function and oxidative stress, or through a connection between carotenoid metabolizing pathways and pathways for cellular respiration.
  It is generally considered that sexually selected traits, such as carotenoid-based coloration, evolve because they are honest signals of phenotypic and genetic quality.  For instance, among males of the bird species Parus major, the more colorfully ornamented males produce sperm that is better protected against oxidative stress due to increased presence of carotenoid antioxidants.  However, there is also evidence that attractive male coloration may be a faulty signal of male quality.  Among stickleback fish, males that are more attractive to females due to carotenoid colorants appear to under-allocate carotenoids to their germline cells.   Since carotinoids are beneficial antioxidants, their under-allocation to germline cells can lead to increased oxidative DNA damage to these cells.  Therefore, female sticklebacks may risk fertility and the viability of their offspring by choosing redder, but more deteriorated partners with reduced sperm quality.