Brood parasitism risk drives birds to breed near humans.

Urbanization is transforming ecosystems at a global scale and at an increasing rate, and its profound consequences for wildlife have been well documented. Understanding how animals thrive in the urban environment and how this environment affects (co-)evolutionary processes remains an important challenge. Urban environments can provide resources such as food or nest sites (e.g., cavities) and also reduce exposure to predators. For some species, urban environments may also affect susceptibility to brood parasitism, but this has never been tested experimentally. Here, we use a combination of field observations and experimental manipulations to show that Daurian redstarts, Phoenicurus auroreus, a common host of the common cuckoo, Cuculus canorus, nest in proximity to humans to avoid brood parasitism. First, redstarts were more likely to be parasitized with increasing distance to the nearest building. Second, redstarts adjusted their nesting location in response to a seasonally predictable change in the risk of brood parasitism. Third, experimentally simulating the presence of cuckoos during a period when they are naturally absent increased the likelihood that redstarts nested indoors or closer to human settlements. These findings suggest that redstarts actively choose to place their nest in the vicinity of a human residence as a defense against cuckoos. Our study exemplifies how animals take advantage of the urban environment by using it as a novel line of defense against detrimental interspecific interactions.