The swans keep water repellent by oiling their entire body with the oil from their uropygial oil gland (preen gland) located near their rump. The oil allows the swan to stay afloat even when it is asleep along with the fact that its body is basically hollow (filled with air). Something you do not see is that the birds will use their legs to propel themselves as they are resting their heads on their back and wings. Finally, even though the birds sleep, they are very light sleepers and the least amount of movement or noise will awaken them. Though they may appear to be fully asleep, at least one eye may be open and they are always alert. The Regal Swan