Although these species dwell in close geographic proximity in the wild, they remain genetically distinct from one another. The natural barriers in Africa, such as jungles, mountains, and lakes, have kept them separate so that hybridization does not happen in their natural setting. This group of lovebirds includes the masked, Fischer's, Nyasa, and black-cheecked species. They are called the eye-ring species because of a pronounced white periophthalmic ring around the eyes. These birds are not sexually dimorphic, meaning distinguishing the sexes simply by visual apperance is generally not easy. The behavior of these four species is somewhat similiar. All collect nesting material with the beak rathr than tucking it into the feathers the way peachfaced and other lovebird species do. Most of the eye-ring species are threatened in their natural habitat, particularly the blackcheeked and Nyasa lovebirds. For this reason, maintaining pure species lines and not inbreeding the different eye-ring species of lovebirds is essensial, Notes on the Eye-Ring Lovebirds. In fact, finding pure blackcheeked and Nyasa lovebirds can be difficult. Some attempts have been made, particularly in Germany, to establish pure lines of these birds. In Belgium, top breeders have begun to maintain studbooks for Nyasa lovebirds in an attempt to maintain the purest lines possible.