Perfluoroalkyl acids (PFAAs) are substances which have been produced for more than five decades. Their unique properties of repelling both water and oil, make them suitable for many industrial and consumer applications such as water and dirt repellents for cloths and carpets, active components in firefighting foams or precursors in Teflon® production. Its extended use, together with their high persistence, resulted in a global contamination of the environment, wildlife and even humans. This ubiquity contrasts sharply with the limited information about their effects on organisms. With this study I will contribute to finding answers to fill some of the most important knowledge gaps in the toxicity mechanisms of these compounds. For this purpose, I will use two model bird species: great tits (Parus major) and canaries (Serinus canaria). Firstly, I will study the PFAAs exposure levels and the fitness consequences in free-living great tits along a PFAAs gradient from a fluorichemical plant in Antwerp, Belgium. Secondly, I will reproduce the exposure levels in captive canaries in order to replicate the results, found in the field, in a more controlled environment. The studied biomarkers will cover several levels of biological organization, from molecular to individual responses. As a result of my study, the PFAAs toxicity mechanisms will be better understood and their impact at an individual and population level can be more accurately forecasted.