Isaksson C. (2009) The chemical pathway of carotenoids: From plants to birds. ARDEA 97 (1): 125-128

Carotenoids are interesting in avian ecology due their dietary origin and the resulting environmental dependence of certain functions and traits such as plumage pigmentation and the immune system. Carotenoids are frequently assumed to be limited for birds, but data on availability is (are??) rarely collected. Thus, to understand the role of the carotenoids as mediators of trade-offs, additional information on the whole chemical pathway of carotenoids is needed. In conjunction with previous studies on carotenoid ecology and physiology of the Great Tit Parus major living in Swedish urban vs. rural environments, I here present data on 1) carotenoid concentration in two primary carotenoid producers, birch Betula verrucosa and oak Quercus robur and 2) vegetation cover on Great Tit territories. Previous studies found lower carotenoid concentrations in caterpillars and plumage pigmentation in urban compared to rural areas. Correspondingly, urban trees had significantly lower total carotenoid concentration in comparison to rural trees, but there was no environmental difference in the relative amount of lutein and zeaxanthin (Lut:Zx ratio) in sun-exposed leaves. Second, urban Great Tits had significantly more shade in their territories than rural birds, which may affect the leaves Lut:Zx composition. The results, taken together, show that the environmental difference in carotenoid concentration of caterpillars and Great Tit yellow breast coloration, may occur because urban trees synthesise less or have used up more carotenoids in response to environmental stress compared to rural trees. Thus, tracking carotenoid content in different steps of the food chain raises questions about how fine-scale environmental heterogeneity can affect optimal foraging behaviour and individual variation in carotenoid-dependent traits.