The most famous examples of avian brood parasites are obligate, laying only in the nests of other species. Much less is known about facultative conspecific brood parasites, where females have their own clutches but can also parasitize others. We will study barn swallows (Hirundo rustica) as a model species to further our understanding of this evolutionary strategy. Using an established long-term study population where individuals can be reliably identified, we will investigate what factors lead certain birds to adopt a parasitic strategy and how they choose their hosts. We will also consider whether parasites have evolved adaptations that facilitate their lifestyle, and whether there is any evidence for the development of an ‘arms race’, with hosts evolving defences. Finally, we will study the relative costs and benefits of parasitism for both hosts and parasites. Our findings will provide a uniquely detailed description of a conspecific brood parasite system and will contribute more generally to our understanding of the ecology and evolution of reproductive cheats.