黑头莺是一种体型较小的鸟类，主要生活在欧洲中部。50年前，大多数黑头莺每到冬季就会迁徙到南部的西班牙或者葡萄牙。但是自20世纪60年代起，越来越多的黑头莺开始飞往英国和爱尔兰过冬。到2003年，有三分之一的英国家庭后院里的喂食器上出现过黑头莺。然而是什么促使如此多的黑头莺另择越冬地呢?
he direction birds migrate is coded intheir genes. Each population of birdshas an average direction they will go,but individuals can vary as much asforty degrees from the average.Although historically, the averagedirection for blackcaps was towardsPortugal, a few birds' genes pointedthem in a slightly different directionand they ended up in Britain.
鸟类迁徙的方向由基因决定。每个鸟群迁徙时都有个平均方向，但个体可以在偏离这个方向40度范围内活动。尽管历史上黑头莺迁徙的平均方向是朝着葡萄牙，但还是有不少黑头莺定位稍有偏差，结果就来到了英国。
But for the numbers of blackcaps wintering in Britain to change so dramatically, there must besome significant advantage to fly north rather than south. Perhaps the benefit is simply fromthe shorter, less taxing flight to Britain. Or maybe it’s because Britain has many bird loversthat stock backyard birdfeeders.
在英国过冬的黑头莺数量激增，至于它们选择飞往北方而不是南方，其中肯定有一些显著的优势。也许是因为英国比较近，不必大费周折飞那么远，也许是因为英国有许多鸟类爱好者，在后院到处悬挂喂食器。
But researchers from Germany and the UK believe the key difference is the shorter winter daysin Britain. Shorter days affect migration and breeding behavior, so British birds return tosummer breeding grounds about ten days earlier than birds that went south.
但来自德国和英国的研究人员认为关键区别在于英国冬季的白昼较短。白昼短这一点可以影响到鸟类的迁移和繁殖行为，所以，在英国过冬的鸟类会比飞往南方的鸟类提前十天返回夏日的繁殖场所。
This ten-day head start lets British migrants claim the best territories and start breedingsooner. Females paired with British males laid more eggs and hatched more chicks than thosemated to southern birds. Each year, blackcaps with genes orienting them towards Britain passon those genes to more chicks. And so the number of blackcaps with the disposition to fly toBritain steadily increases.
开头的十天会保证这些候鸟们占领最好的领地，迅速开始孕育下一代。并且，与在南方配对的雌鸟相比，与英国的雄鸟配对的雌鸟产蛋更多，孵出的雏鸟也更多。年复一年，黑头莺会把向英国迁徙的基因一代代传下去。因此，飞往英国过冬的黑头莺数量急剧增长。
