may be stuck for camouflage. The cape penduline tit incorporates false entrances, the parent bird carefully making sure to close the actual entrance when leaving the nest. The entrances are lined with spider webs which help seal the openings. Nest protection and sanitation Many species of bird conceal their nests to protect them from predators. Some species may choose nest sites that are inaccessible or build the nest so as to deter predators. Bird nests can also act as habitats for other inquiline species which may not affect the bird directly. Birds have also evolved nest sanitation measures to reduce the effects of parasites and pathogens on nestlings. Some aquatic species such as grebes are very careful when approaching and leaving the nest so as not to reveal the location. Some species will use leaves to cover up the nest prior to leaving. Ground birds such as plovers may use broken wing or rodent run displays to distract predators from nests. Many species attack predators or apparent predators near their nests. Kingbirds attack other birds that come too close. In North America, northern mockingbirds, blue jays, and Arctic terns can peck hard enough to draw blood. In Australia, a bird attacking a person near its nest is said to swoop the person. The Australian magpie is particularly well known for this behavior. Nests can become home to many other organisms including parasites and pathogens. The excreta of the fledglings also pose a problem. In most passerines, the adults actively dispose the fecal sacs of young at a distance or consume them. This is believed to help prevent ground predators from detecting nests. Young birds of prey however usually void their excreta beyond the rims of their nests. Blowflies of the genus Protocalliphora have specialized to become obligate nest parasites with the maggots feeding on the blood of nestlings. Some birds have been shown to choose aromatic green plant material for constructing nests that may have insecticidal properties, while others may use materials such as carnivore scat to repel smaller predators. Some urban birds, house sparrows and house finches in Mexico, have been adopted the use of cellulose from cigarette butts which may be adaptive since these fibres contain nicotine and other toxic substances that repel ectoparasites. Some birds use pieces of snake slough in their nests. It has been suggested that these may deter some nest predators such as squirrels.