In winter they congregate in large coveys and roost communally. In these areas they congregate in large flocks which may exceed a thousand individuals. For protection, flocks often congregate in large colonies of several thousand individuals. Bed bugs can exist singly, but tend to congregate once established. Areas where mantas congregate are popular with tourists. Natural caves beneath urban centers allow for places where people can congregate. Females congregate in spring to dig nests. At large carcasses or feeding sites, small groups may congregate. Such groups congregate at wallows and grazing areas. Variations in elevation often represent places where fish congregate. Unlike other bears, they do not congregate in feeding groups. Such groups often congregate at migration and winter sites, sometimes in the thousands. Despite being mostly solitary, sirenians congregate in groups while females are in estrus. They are designed to scare animals, such as birds that congregate on airport runways. They often congregate near water, and are good swimmers.