These are Edible-nest Swiftlets. Their whitish nests, constructed from the bird's saliva, are collected to make soup. As a rule I do not take pictures of birds at their nests. In this case I made an exception because these particular nests are right beside a busy boardwalk through the cave; the regular passage of tourists does not disturb them. More broadly, the site (Gomantong Cave) is subject to industrial scale nest collection (for soup) and in this context the presence of tourists, birders and photographers is irrelevant. The photographs also serve a functional, rather than aesthetic purpose, as three of the swiftlets present in the cave cannot be identified in the field by their plumage and calls but only by the appearance of their nests. This is the only image I have ever sold. The story is here: www.flickr.com/photos/paulbjones/8261881400/in/photostream