In the temperate zone in spring, temperatures fluctuate greatly from day to day, but day length increases steadily by a predictable amount. 春天的溫帶地區，氣溫每天都變化不定，但是日照長(cháng)度卻以一個(gè)可預知的數量穩步增加。
Thus many temperate-zone birds use the increasing day lengths in spring as a cue to begin the nesting cycle, because this is a point when adequate food resources will be assured. 所以許多生活在溫帶地區的鳥(niǎo)類(lèi)把春天里逐漸變長(cháng)的日照長(cháng)度當作開(kāi)始筑巢的提示，因為這是能保主充足的食物來(lái)源的時(shí)候。
