Two or three gull species (Laridae) commonly breed together in one colony, often caused by a shortage of optimal breeding sites or as an anti-predator behaviour. Observers usually assume that the benefits of colonial breeding outweigh potential costs. Still, our study shows that the costs can be substantial. We show that short distances between nests of Black-headed Gulls and Common Terns breeding in a colony often lead to aggressive interactions between these birds in which terns lose eggs during laying and incubation. We created a buffer zone between the nests of terns and gulls to reduce losses in the terns' clutches. All nests of Black-headed Gulls closer than 120 cm to nests of Common Terns were gradually moved away over 1–4 consecutive days, reducing the terns' egg losses by almost six times. Our results show that our method was safe for both species, and is non-invasive, cheap and easy. This conservation measure could be used in those regions of Europe and North America where breeding Common Terns are declining and perhaps globally in other mixed Larid colonies where species are under threat.