 The disturbance of breeding colonies by humans is often a problem as well—visitors, even well-meaning tourists, can flush brooding adults off a colony, leaving chicks and eggs vulnerable to predators.The build-up of toxins and pollutants in seabirds is also a concern.Seabirds, being apex predators, suffered from the ravages of the insecticide DDT until it was banned; DDT was implicated, for example, in embryo development problems and the skewed sex ratio of western gulls in southern California.