Swiftlets in different places have their reproductive seasons at different times, but all of them, before giving birth to their young, will move their heads back and forth rapidly and expectorate stream after stream of sticky, rubbery secretion on the rocks to build their nests. In this process, the adult birds neither sleep nor eat, and they only complete their work after moving their heads continuously tens of thousands of times.
分佈各處的金絲燕，每年繁殖季不同，但在育雛之前，成鳥都會如春蠶吐絲般來回不停擺動頭顱，吐出一道道如絲的膠性唾液黏在石壁上制巢，過程中，成燕不眠不食，頭顱連續擺動數萬次才完成。