時差N小時:黑頭鶯為何改變遷徙模式?

Blackcaps are small songbirds that breed in CentralEurope. Fifty years ago, most blackcaps went southfor the winter to Spain or Portugal. But since the1960s more and more blackcaps have startedwintering in Britain and Ireland. By 2003, blackcapswere seen perched at one in every three backyardbird feeders in Britain! But why did so manyblackcaps change their winter vacation location?

黑頭鶯是一種體型較小的鳥類，主要生活在歐洲中部。50年前，大多數(shù)黑頭鶯每到冬季就會遷徙到南部的西班牙或者葡萄牙。但是自20世紀(jì)60年代起，越來越多的黑頭鶯開始飛往英國和愛爾蘭過冬。到2003年，有三分之一的英國家庭后院里的喂食器上出現(xiàn)過黑頭鶯。然而是什么促使如此多的黑頭鶯另擇越冬地呢?

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.

鳥類遷徙的方向由基因決定。每個鳥群遷徙時都有個平均方向，但個體可以在偏離這個方向40度范圍內(nèi)活動。盡管歷史上黑頭鶯遷徙的平均方向是朝著葡萄牙，但還是有不少黑頭鶯定位稍有偏差，結(jié)果就來到了英國。

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.

在英國過冬的黑頭鶯數(shù)量激增，至于它們選擇飛往北方而不是南方，其中肯定有一些顯著的優(yōu)勢。也許是因?yàn)橛容^近，不必大費(fèi)周折飛那么遠(yuǎn)，也許是因?yàn)橛性S多鳥類愛好者，在后院到處懸掛喂食器。

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.

但來自德國和英國的研究人員認(rèn)為關(guān)鍵區(qū)別在于英國冬季的白晝較短。白晝短這一點(diǎn)可以影響到鳥類的遷移和繁殖行為，所以，在英國過冬的鳥類會比飛往南方的鳥類提前十天返回夏日的繁殖場所。

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.

開頭的十天會保證這些候鳥們占領(lǐng)最好的領(lǐng)地，迅速開始孕育下一代。并且，與在南方配對的雌鳥相比，與英國的雄鳥配對的雌鳥產(chǎn)蛋更多，孵出的雛鳥也更多。年復(fù)一年，黑頭鶯會把向英國遷徙的基因一代代傳下去。因此，飛往英國過冬的黑頭鶯數(shù)量急劇增長。