鸟的羽毛不会永远光洁如新，鸟类必须周期性的进行换羽，长出新的羽毛。
Feathers don't last forever. So birds periodically shed their plumage and then sprout replacements.
一些大型鸟类，像是大雁或是体重能够达到30磅的天鹅，是以放弃飞行能力来解决换羽的难题的。
Some large birds, like honking big geese and 30-pound swans, get around the problem by forgoing flight while they get all their molting over with in one fell swoop.
小型鸟类一年进行一到两次换羽，在换羽期间，它们一次会替换掉9到10根左右的飞羽，而这个过程将持续几周时间。
Small birds molt once or twice a year, and they replace their 9 or 10 primary flight feathers a few at a time over the course of a few weeks.
所以，如果大型鸟类要以小型鸟类那样的方式换羽的话，它们就不得不花上整年的时间来做这件事，也就没有什么时间来觅食或繁殖了。
So if big birds tried to do like little birds, they'd spend the whole year molting-leaving little time for finding food or mates.
因为，在此期间它们不能到海里去，这群鸟儿们就只能靠换羽期前5周的大量进食所积累的脂肪过活。
Because the birds cannot go to sea during this time, they are forced to rely on fat reserves built up during five weeks of binge feeding beforehand.
这意味着体积大的鸟为了应对自己老化的羽毛，必须进化出不同的换羽方式。
That means big birds have to evolve different molting patterns to deal with their slowpoke feathers .
来自华盛顿大学的科学家们说，这取决于在换羽的时候，你更换飞行羽毛的速度。
University of Washington scientists say it depends on how fast you can replace your flight feathers when you molt.
在秋季后期，我们如果主要是利用头部全白与否来分辨这两种，那就很难排除那些换羽较迟的织女银鸥个体。
In the late autumn period, it may be more difficult to rule out late - moulting Vega Gulls if we are to use "white heads" as our main method of separating the two.
在秋季后期，我们如果主要是利用头部全白与否来分辨这两种，那就很难排除那些换羽较迟的织女银鸥个体。
In the late autumn period, it may be more difficult to rule out late - moulting Vega Gulls if we are to use "white heads" as our main method of separating the two.
应用推荐