of studies have attempted to determine whether secondary cavity nesters preferentially choose cavities with entrance holes facing certain directions, the results remain inconclusive. While some species appear to preferentially choose holes with certain orientations, studies (to date) have not shown consistent differences in fledging rates between nests oriented in different directions. Cavity-dwelling species have to contend with the danger of predators accessing their nest, catching them and their young inside and unable to get out. They have a variety of methods for decreasing the likelihood of this happening. Red-cockaded woodpeckers peel bark around the entrance, and drill wells above and below the hole; since they nest in live trees, the resulting flow of resin forms a barrier that prevents snakes from reaching the nests. Red-breasted nuthatches smear sap around the entrance holes to their nests, while white-breasted nuthatches rub foul-smelling insects around theirs. Eurasian nuthatches wall up part of their entrance holes with mud, decreasing the size and sometimes extending the tunnel part of the chamber. Most female hornbills seal themselves into their cavity nests, using a combination of mud (in some species brought by their mates), food remains and their own droppings to reduce the entrance hole to a narrow slit. A few birds are known to use the nests of insects within which they create a cavity in which they lay their eggs. These include the rufous woodpecker which nests in the arboreal nests of Crematogaster ants and the collared kingfisher which uses termite nests. Cup The cup nest is smoothly hemispherical inside, with a deep depression to house the eggs. Most are made of pliable materials—including grasses—though a small number are made of mud or saliva. Many passerines and a few non-passerines, including some hummingbirds and some swifts, build this type of nest. Small bird species in more than 20 passerine families, and a few non-passerines—including most hummingbirds, kinglets and crests in the genus Regulus, some tyrant flycatchers and several New World warblers—use considerable amounts of spider silk in the construction of their nests. The lightweight material is strong and extremely flexible, allowing the nest to mold to the adult during incubation (reducing heat loss), then to stretch to accommodate the growing nestlings; as it is sticky, it also helps to bind the nest to the branch or leaf to which it is attached. Many swifts and some hummingbirds use thick, quick-drying saliva to anchor their nests.