The incorporation of aromatic plants into nests by birds is suspected to constitute an example of preventative medicine use, whereby the phytochemical compounds within plants reduce infestation by parasites and increase offspring condition 1,2. In China, russet sparrows (Passer cinnamomeus) incorporate wormwood (Artemisia verlotorum) leaves into their nests around the same time that local people hang wormwood from their doors as a traditional custom during the Dragon Boat Festival. The belief that this behaviour confers protection against ill health 3 is supported by the description of anti-parasite compounds in wormwood 4. It has been suggested that the incorporation of fresh wormwood leaves into nests may serve a similar function for sparrows. Here we show that sparrows choose nest location and resupply established nests with fresh wormwood leaves using olfactory cues, that nests containing wormwood leaves have lower ectoparasite loads, and that nests with more wormwood leaves produce heavier chicks. Our results indicate that sparrows use wormwood as a preventative medicine to control ectoparasites and improve the body mass of their offspring. Several bird species incorporate aromatic plant material into their nests, but why they do this is a subject of ongoing debate. A new study by Yang et al. indicates that birds use wormwood to decrease parasite loads within nests and increase offspring condition.