In the temperate zone in spring, temperatures fluctuate greatly from day to day, but day length increases steadily by a predictable amount. 春天的温带地区，气温每天都变化不定，但是日照长度却以一个可预知的数量稳步增加。
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. 所以许多生活在温带地区的鸟类把春天里逐渐变长的日照长度当作开始筑巢的提示，因为这是能保主充足的食物来源的时候。
