Colonial nesting Though most birds nest individually, some species—including seabirds, penguins, flamingos, many herons, gulls, terns, weaver, some corvids and some sparrows—gather together in sizeable colonies. Birds that nest colonially may benefit from increased protection against predation. They may also be able to better use food supplies, by following more successful foragers to their foraging sites. Ecological importance In constructing nests, birds act as ecosystem engineers by providing a sheltered microclimate and concentrated food sources for invertebrates. A global checklist lists eighteen invertebrate orders that occur in bird nests. In human culture Many birds may nest close to human habitations. In addition to nest boxes which are often used to encourage cavity nesting birds (see below), other species have been specially encouraged : for example nesting white storks have been protected and held in reverence in many cultures, and the nesting of peregrine falcons on tall modern or historical buildings has captured popular interest. Colonial breeders produce guano in and around their nesting sites, which is a valuable fertilizer from the Andean Pacific coast and other areas. The saliva nest of the edible-nest swiftlet is used to make bird's nest soup, long considered a delicacy in China. Collection of the swiftlet nests is big business: in one year, more than 3.5 million nests were exported from Borneo to China, and the industry was estimated at $1 billion US per year (and increasing) in 2008. While the collection is regulated in some areas (at the Gomantong Caves, for example, where nests can be collected only from February to April or July to September), it is not in others, and the swiftlets are declining in areas where the harvest reaches unsustainable levels. Some species of birds are considered nuisances when they nest in the proximity of human habitations. Feral pigeons are often unwelcome and sometimes also considered as a health risk. The Beijing National Stadium, principal venue of the 2008 Summer Olympics, has been nicknamed "The Bird Nest" because of its architectural design, which its designers likened to a bird's woven nest. In the 19th and early 20th centuries, naturalists often collected bird's eggs and their nests. The practice of egg-collecting or oology is now illegal in many jurisdictions worldwide; the study of bird nests is called caliology. Artificial bird nests Bird nests are also built by humans to help in the conservation of certain birds (such as swallows). Swallow nests are