舊金山附近三種值得認識的海鳥 Three kinds of pelagic bird near San Francisco that are worthwhile knowing about

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       許多海鳥一生都在海上渡過，不會靠近海岸，頂多只有在繁殖期間會登上偏遠的海中小島或岩礁，所以這些海鳥對多數人來說，會相當陌生。這裡，我想和大家分享三種值得認識的海鳥，它們都是我在舊金山附近出海時遇到的。

       當我第一次見到 Sooty Shearwater （灰水薙鳥）時，看到它們很笨拙地跑步起飛的滑稽模樣，所以並沒有把它們特別放在心上。後來，才曉得它們榮登鳥類每年遷徙距離的冠軍。有科學家在它們身上放置了電子追蹤器，結果發現它們每年的遷徙距離，長達 6.4 萬公里，比之前的遷徙冠軍 Arctic Tern （極地燕鷗）的 3.3 萬公里距離，幾乎多出了一倍。不過到目前為止，Arctic Tern 的遷徙距離，還只是跟據它不同季節的棲地位置去估計出來的。科學家還沒有找到夠輕、夠小的電子追蹤器，可以用在Arctic Tern身上，所以Arctic Tern還有翻身的機會。更有趣的是，從北極直線飛到南極，再直線飛返北極，還不到4 萬公里。這顯示出Sooty Shearwater 在遷徙的過程，會順便到處趴趴走呢！

       我有一次出海時，遇到一群數千隻的Sooty Shearwater ，貼近海面飛行。當它們經過我們的船隻時，就好像一朵雲快速地穿透過我們。那是一次既壯觀，又空靈的經驗。

          Many pelagic birds spend their entire life on sea, and will not even approach sea shore, barring from landing on small remote islets or rocks for breeding. Hence, these birds often appear unfamiliar to most people. In this article, I would like to share about two kinds of pelagic bird that are worthwhile knowing of, both of which I came across on boat trips near San Francisco.

          When I first encountered Sooty Shearwaters, the clumsiness in their taking off from sea surface made me not to think much of them. Only later did I realize that they have become the champion of migration in avian world. From the radio transmitters that scientists put on them, the annual migration length sums up to be 64,000 Km, almost twice as much as the 33,000 Km scientists estimated for the previous champion, the Arctic Tern. The only caviar is that Arctic Tern’s annual migration length is estimated from its seasonal geological distribution. There has not been radio transmitter small and light enough for Arctic Tern to carry. Hence, Arctic Tern still has a chance to overthrow Sooty Shearwater. An even more interesting fact is that the distance of straight-line circumventing the earth in north-south direction only takes less than 40,000 Km, much less than the 64,000 Km that Sooty Shearwater logs from the radio monitoring. That shows that during migration, Sooty Shearwater often strays away from a straight path.

In one of my boat rides, I had encountered a flock of thousands of Sooty Shearwater gliding near sea surface. When they flew passing our boat, it seemed that a large patch of cloud swiftly passing through us. It is an extraordinary experience, both grandeur and surreal.

       另外一種海鳥，是 Common Murre（崖海鴉）。Common Murre的孵卵及育雛的工作，是由雙親共同分擔的。但是雛鳥出生二十幾天後就會離巢，入海去見見世面。這時候，父鳥會跟隨護航，長達數十天之久。這期間在茫茫大海裡，父子(父女) 主要依靠鳴聲來互相辨認及保持連絡。其實在雛鳥尚未離巢之前，親子之間就已經發展出靠鳴聲來互相辨識的能力了。否則Common Murre的育雛地，經常是數千隻的擁擠社區。如果缺乏有效辨識方式的話，親子之間很容易就會失散了。

       在Common Murre 雛鳥初航的季節裡，我曾經在海面上看到過一對接一對的父子(父女) Common Murre，隨波逐浪，同時不斷地互相呼叫，來保持連繫。那種情景，讓人覺既可愛、又溫馨。

      即使在鳥類的國度裡，像Common Murre 那樣的育幼模式，也算是蠻特別的。

          The birds in the following two photos are Common Murres. Both male and female Common Murres share the task of incubating the egg and rearing the chick, who would wander away from the breeding ground into the sea at about 20-days old. The father bird will serve as chick’s chaperon for up to 2 months during such maiden voyage. In the immense sea, the father-chick pair mainly relies on calls to identify each other and keep connected. In fact, they have already mastered such an identification skill long before the chick leaves breeding ground, for there are usually thousands of individuals cramping in a colony. Hence, without an effective method of identification, it is quite easy for parent-chick to lose each other.
 

          During the season of Common Murre chick’s maiden voyage, I had come across pairs after pairs of father-chick, riding along the wave while keeping calling each other to stay connected. That scene has left a quite lovely and heart-warming mark in my memory.

           Even in avian world, Common Murre’s mode of rearing young is still unusual.

 

從我小的時候開始，〝信天翁〞這個中文名字，一直帶著一種遙遠的神秘。直到最近幾年，我才對這個謎樣名字背後所代表的鳥類，開始有了一點瞭解。

          我不確定中文裡的〝信天翁〞，原來是不是專指一種鳥類？可是英文裡的〝Albatross〞是一種泛稱，它包括了將近二十個鳥種，大部分都分佈在南半球，只有三到四種是分佈在北部球。而且由於信天翁特別的飛行模式，它們通常無法飛越赤道區，所以南、北半球的信天翁，大致來說是互相隔離的。

           信天翁是鳥類中的滑翔專家，它們可以持續長距離滑行上幾十公里，而很少須要振翅。其秘訣是很精巧地利用空氣的動態，來維持飛行速度。 其中一種方式，被稱為〝dynamic soaring〞，解釋起來有點複雜，不過基本上，是利用海面風速常常隨著高度而變化的現象，而在不同的風速層之間來回穿梭來獲得加速。 所以信天翁可以長時間保持滑行，而無須振翅。

          最有利於滑翔的翼形，是窄而長，而信天翁的翼形，正是如此。其中最大的信天翁鳥種，翼展可達3.4公尺，是鳥類裡的冠軍。不過這樣的翼形，也有它的弱點，那就是振翅飛行的效果不佳。也因此，它們通常無法穿越赤道附近的無風帶。

           在舊金山附近海域比較常見到的信天翁鳥種，是〝Black-footed Albatross〞（黑足信天翁）。它們的個頭在信天翁理只能算是中型的，可是翼展也有兩公尺。它們絕大多數會定期回到夏威夷群島繁殖。其它的時間就四散各處，而且不會飛臨陸地。 不過極少數黑足信天翁，會在加州及墨西哥以西的海上小島繁殖，所以我在舊金山附近出海的時候，才得以邂逅到這個記憶中的傳奇鳥類。