Boobies nest in colonies but have a highly developed territorial sense. Many ritualized displays (e.g., head nodding and jabbing) are used to defend the individual's territory within the large breeding colony. Courtship also involves display—an elaborate dance by the male in which the feet are raised alternately several times, followed by a gesture known to ornithologists as sky-pointing (the birds extend their wings horizontally and toward the tail, raise their heads, and emit a long, continuous whistle). The eggs, usually two in number, are laid on the ground in a rudimentary nest. Boobies get their name from their tameness and lack of fear of humans; they were easily killed by early mariners, who named them boobies to denote their presumed lack of intelligence.