In this thesis I will investigate the factors determining the mating decisions of two hybridizing bird species: the collared flycatcher (Ficedula albicollis) and the pied flycatcher (F. hypoleuca). These two species hybridize despite the fact that their ability to recognize a mate of their own species appears to be well developed and hybrid offspring represent a fitness loss compared to pure-species offspring. In order to understand why individuals still engage in a mixed–species pairing, I will try to quantify the most important costs and benefits of hybridization. For a proper understanding of this thesis it is important to first get an idea of the general context in which my study fits. Wherever possible, avian examples will be given to clarify general mechanisms, but examples of other taxa will be provided as well. After this broad introduction, I will focus on the study system, giving general information on the morphology, ecology and behaviour of the two flycatcher species and motivate the research compiled in this thesis.