The genus "Aerodramus" is of special interest due to its use of echolocation and its intricately constructed saliva nests which in some species contain no other material such as feathers, moss or twigs and are collected, selling at extremely high prices (see Bird's nest soup). It has been argued that the high demand for these nests could have had an adverse effect on their populations, but other authorities have shown that modern techniques of nest farming have increased the bird population.