<P> 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 . </P> <P> Some birds use pieces of snake slough in their nests . It has been suggested that these may deter some nest predators such as squirrels . </P> <P> 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 utilize food supplies, by following more successful foragers to their foraging sites . </P> <P> Many birds nest close to human habitations and some have been specially encouraged . Nesting white storks have been protected and held in reverence in many cultures . Nest boxes are often used to encourage cavity nesting birds . The nesting of peregrine falcons on tall buildings has captured popular interest . Colonial breeders produce guano which is a valuable fertilizer . 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 . </P>

Who teaches a bird to make a nest