Bird nest 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 also 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 Victorian era, naturalists often collected bird's eggs and their nests. The study of bird nests is
The Life of Birds coast favoured by parrots, an ants' nest occupied by a woodpecker, and a tree hole inside which a female hornbill seals itself. Eggs require warmth, and some nests are insulated by the owners' feathers, others from ones found elsewhere. External temperatures dictate how the eggs are incubated. The snowy owl has to do so itself, because of its habitat; however, the maleo is able to take advantage of solar heating. The amount of eggs laid also varies: for example, the kiwi lays just one, whereas the blue tit will deposit many. Their mottled surface serves to camouflage them. Birds that