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。