Season-, sex-, and age-specific accumulation of plasma carotenoid pigments in free-ranging white-winged crossbills Loxia leucoptera

Many birds acquire carotenoid pigments from foods and deposit these pigments into feathers and bare-parts to become sexually attractive, but little work has been done on the interindividual and temporal variability in the types and amounts of carotenoids that free-ranging individuals have available for use in coloration or other functions (e.g., in immunomodulation). To address this issue, we studied intra-annual variation in plasma carotenoid profiles of juvenile and adult white-winged crossbills Loxia leucoptera of both sexes. Adult male crossbills exhibit bright red carotenoid-based plumage pigmentation, whereas females uniformly display drab yellow feather coloration and juvenile males only occasionally display some orange or pink color. Yellow xanthophylls (e.g., lutein, zeaxanthin) were predominant in plasma of birds from both sexes and age classes throughout the year. Plasma xanthophylls levels tended to be highest in the summer, when crossbills increase seed consumption for breeding as well as supplement their diet with insects. Blood accumulation of three other, less common plasma carotenoids-β-cryptoxanthin, rubixanthin, and gazaniaxanthin-varied in a highly season-, sex-, and age-dependent fashion. These carotenoids were virtually absent in juvenile or adult female plasma at all times of year and were only present in male plasma, at higher concentrations in adults than juveniles, during the period of feather growth (Sept.-Nov.). These pigments have been reported as valuable precursors of the metabolically derived red pigments (e.g., 3-hydroxy-echinenone, 4-oxo-rubixanthin, and 4-oxo-gazaniaxanthin, respectively) that appear in the plumage of male crossbills. These findings suggest that male crossbills either adopt a season-specific foraging strategy to acquire foods rich in these pigments at the time they are needed to develop red coloration, or have a unique physiological ability to metabolically produce these pigments or absorb them from food during molt, in order to maximize color production.