大海上的捕魚高手——小燕鷗

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1990 / 3月

文‧張靜茹 圖‧陳永福


冬季的海岸。

 

沈暗的天幕下只有澎湃的潮音、強烈的風鳴,空曠的大自然舞台上一無生命。但鏡頭拉近,激起波浪的絕崖頂,落單的海鷗,拍起狹長的翅膀飛上天際,忽而急轉直下,順著風速、氣流,雪白的羽翼,展現了完美的滑行……。


天地一沙鷗的場景,常是電影或小說的序幕。海鷗也成為許多人心目中落寞、離群獨行的象徵。

但真實的舞台上,我們所見到的「海鷗」則是許多不同的鳥,牠們也並非那麼「孤獨、寂寞」。

許多種鳥的組合

「海鷗」屬於鷗形目。鷗形目中有鞘鷗科、瓣足鷸科、賊鷗科、鷗科、燕鷗科與海雀六科。這些鳥都有可能是我們口中的「海鷗」,但賞鳥人通常只將鷗科鳥稱為海鷗。

鷗形目鳥中,在台灣沿海地區出現種類和數量最多的則為燕鷗科與鷗科。由於這二科鳥在形態和行為上有較多相同之處,也有在分類上,將二者都歸為鷗科,再區分為燕鷗亞科和鷗亞科。

雖然長相相似,一般人很難一眼將二者分辨出來,但在「內行人」眼中,二者其實「大異其趣」。

鷗科鳥在全世界有四十九種。牠們身體較「豐滿」,頭頸粗短、嘴厚實,上喙較長且呈●狀,尾巴為圓形或方形,蹼大、善於游泳。

燕鷗有四十二種,也遍佈全世界各海域。牠們身體纖長,體型比鷗科鳥小,嘴型較不尖銳,喙前也不具?狀。由於具有和燕子相似的剪刀形尾羽,「燕鷗」名稱便是由此而來。

海鷗飛處,陸地不遠

海鷗喜歡浮在海面上划水前進,潛水取食時,並不全身進入水中;燕鷗則不善於游泳,國外生態學者所著的「以自然為嚮導」書中指出,牠們通常都在魚群上空盤旋,見到獵物再俯衝直下捕捉,且能全身潛入水中捕食。

以活動環境來區分,燕鷗和海鷗都屬於海鳥。全世界海鳥有三百多種,只佔鳥種的百分之三,但牠們出現時的數量往往頗為可觀,曾有上百萬隻聚集一處的記錄。澎湖的貓嶼,在未受到太多人為干擾前,即是成千上萬隻玄燕鷗與白眉燕鷗的家。

大部分海鳥在色澤上也相當一致,羽毛都是黑、白、棕或灰色的組合,只有少數幾種身上有其他顏色點綴。牠們也不似陸鳥,由於求偶需要,雌、雄鳥的羽色非常分明,易於分辨。

根據中央研究院動物研究所副研究員劉小如的解釋,海鳥飛翔在陽光閃爍的海面上,黑白羽色可能較具有保護作用,能增加牠們生存的機會。

但海鳥的嘴在顏色和形狀上則「五花八門」,有的尖銳如匕首,有的彎如利?,或帶有具大喉囊……,成為海鳥身上最醒目的部分。

某些海鳥,如「雷森信天翁」長期生活在海上,到了七、八歲,才飛上陸地繁殖;海鷗則常在近海地區或大型內陸湖泊邊活動。

牠們白天飛到海上捕魚,晚上便回到岸上,最遠不會飛離海岸廿公里,因此水手們在清晨或傍晚看到海上成群的燕鷗或海鷗,往往心中大喜——因為陸地已不遠了。有些海鷗甚至常在港口或海邊公園草地上追逐,搶食遊客留下的殘餘食物。

老祖先的鳥類觀察記錄

曾在台灣出現、有記錄的鷗科鳥類有十八種,燕鷗佔了十二種。其中最常出現與數量最多的應屬「小燕鷗」了。

小燕鷗廣泛分佈於世界各地,在台灣地區為冬候鳥,常成群結伴出現,少部分為終年可見的留鳥。

牠是鷗科鳥中體型最小的一種,身長在廿到廿八公分間,翅膀全長約五十公分。修長的身軀,戴著黑色的「禮帽」,身著白色與青灰色的羽衣,嘴與腳為黃色,冬季時,黑色頭羽會逐漸褪換為白色,嘴、腳則轉為黑色。

牠們多棲於海岸、河口、沼澤中,台灣的一些溪口,如大肚溪口,常能見到牠們的身影。除了繁殖季節,小燕鷗大部分時間都低空飛翔於海面,頭部不時左右擺動,嘴則垂直朝下,搜索海中獵物,有如偵察機,選中目標後,鼓起雙翼、垂直下降捕捉。

東海大學生物系教授張萬福所著的「台灣的水鳥」中指出,小燕鷗亦可潛入水中追捕魚類,獵物到手後,會直接從水中拍翅垂直上升。

終日覓食,卻仍身輕如燕,張萬福認為古人的一首詩最能代表小燕鷗:

「秋江水冷無人渡,群鷗忍饑愁日暮。

白頭來往似漁翁,心思捕魚江水中。

眼明見魚深出水,復恐魚驚隱藏葦。

須臾銜得上平沙,鱗鬣半吞猶見尾。

江魚食盡身不肥……。」

老祖先對小燕鷗的「觀察記錄」,被鳥類學者認為十分生動。

小燕鷗的「天下父母心」

和其他鷗科鳥相同,小燕鷗有成群築巢的習慣,牠們多營巢於海岸砂礫地。燕鷗求偶時,則是由雄鳥獻上好吃的小魚,有時兩隻鳥還會雙雙在空中盤旋、追逐,有如展現飛行技巧。

英國動物學者大衛.艾登堡在「活生生的地球」書中指出,許多鳥類確實會將炫耀飛行技巧,當成求偶表演的一部分。

燕鷗每巢約生二至三個蛋,雌、雄鳥一起輪流孵蛋、育雛。幼鳥大都要靠雙親養育四、五周以上才能夠獨立。

曾有人做過燕鷗的研究發現,在大風暴吹襲過後,沙子掩埋了牠們的蛋,地面上足供辨識的石頭、雜物也被取走,四周環境完全改觀,燕鷗爸、媽仍可很快地找到牠們未出世的小寶寶。

牠們也有很強烈的「護巢行為」,並有不少方法可以對付各種不同的騷擾者。除了「擬傷行為」——假裝自己受傷,引誘敵人追捕、遠離鳥巢;牠們也會從空中猛烈俯衝下來,用堅硬的嘴狠狠地啄對方的頭部。

海闊天空任飛翔

燕鷗科鳥類中,還有許多鳥種本領特殊。被譽為「最偉大的旅行者」的北極燕鷗即是一例。

牠們在北極的夏天享受每天廿四小時的陽光,在此築巢、養育後代。當八、九月北極冬天來臨,才滿數周,羽翼初豐的小鳥就隨隊出發,進行一萬八千公里的長征——經歐洲西岸、非洲、南極海到達南極大陸,隔年五月,再以平均每天二百公里以上的行程回轉故鄉——北極。

因此,牠們年年生活在太陽很少落到地平線下的南北極的夏天,比世界上任何其他生物擁有更多白天。

包括燕鷗在內的許多海鳥,生活環境「浩瀚無邊」,人類若想更深入了解牠們,恐怕得先想法克服一切困難,使自己也能在海上長期生活才行。

〔圖片說明〕

P.28、P.29

生態攝影家陳永福去年夏天在大肚溪口拍攝的小燕鷗家庭。

P.30、P.31

右圖為夏天的小燕鷗,左圖為冬季時,黃色的嘴漸轉為黑色。

P.32、P.33

「須臾銜得上平沙,鱗鬣半吞猶見尾」是小燕鷗的最佳寫照。

P.34

飛累了,停息於海岸邊伸展一下「四肢」,好不舒服。

相關文章

近期文章

EN

The Little Tern, Master Fisherman of the Sea

Chang Chin-ju /photos courtesy of Ch'en Yung-fu /tr. by Andrew Morton

A wintry coastline.

In the gathering dusk the pounding of the breakers and the stiff sea breeze hold sway; no sign of life stirs in nature's vast arena. But look! From a clifftop high above the flying spray a solitary seagull soars aloft beating its long, narrow wings, then abruptly turns and swoops straight down with the wind, its wide extended wings flashing white in a glorious glide . . .


Films and novels often open with a seagull alone amid earth and sky. For many people seagulls are a symbol of loneliness and independence.

In fact, however, there are many different types of seagulls and they are by no means solitary in their habits.

The class of seagulls consists of six different families, of which the tern family and the gull family are the most common in Taiwan. These two families share a number of similarities in morphology and behavior.

Terns and gulls look so alike to most people that they would be hard put to tell them apart, but to the expert eye they are very different.

There are 49 species of gulls in the world. Gulls are plump-bodied with a short thick neck, a stout bill with a long hooked upper beak, a round or squared tail and large webs well adapted for swimming.

The tern family includes 42 different species found all round the world. Terns have a slender body which is smaller than a gull's, and a blunter bill without a hook. Due to their forked tails they are also known as "sea-swallows."

Both gulls and terns are seabirds, of which there are over 300 different species worldwide. These only account for 3% of all bird species, but they often appear in large numbers and colonies of a million birds have been recorded. Before it was disturbed by visitors Cat Islet in the Pescadores was home to vast colonies of sooty terns and little terns.

Most seagulls are fairly uniform in coloring. Their plumage is a combination of black, white, brown or gray with only a small minority displaying other colors.

However seagulls do exhibit great variety in the color and shape of their bills, some being as sharp as a dagger, some curved like a sharp hook, or with a large pouch. This is the most striking part of the seagull's body.

Some seabirds live for long periods at sea and only return to breed on land at the age of seven or eight. Seagulls often live in coastal waters or beside large inland lakes. By day they fly over the sea to catch fish, returning to shore at dusk. They usually fly no more than 20 km. from shore, so mariners rejoice when they see flocks of terns or gulls at sea because land cannot be far off. Some seagulls even live permanently in harbors or in parks near the coast where they scavenge scraps left behind by picnickers.

Eighteen species of the seagull family have been spotted in Taiwan, twelve being terns. The little tern is the most commonly sighted and appears in the largest numbers.

The little tern is widespread throughout the world. Colonies of them winter in Taiwan, with a small minority staying here all year round.

It is the smallest species in the seagull family with a body length of between 20 and 28 cm. and a wingspan of about 50 cm. It has a slender body with a black cap and a white bluish-gray plumage. The bill and legs are yellow, but in winter its black cap turns white and the bill and legs turn black.

Terns mostly nest by the seashore, by estuaries or in marshland. In Taiwan, for example, they are to be seen at the Tatu River estuary. Apart from in the breeding season the little tern spends most of its time hovering low above the sea looking from side to side with its beak pointing down as it searches for its prey. Once it has found its target it plunges vertically down to catch it.

In his book Waterbirds of Taiwan Chang Wan-fu, biology professor at Tunghai University, points out that the little tern can also dive for fish. After catching its prey it soars straight up into the air from the water.

Chang Wan-fu thinks the following old poem perfectly captures the little tern:

On a chill, deserted autumn river

Hungry gulls fret as nighttime falls.

White heads bobbing like old fishermen

They fish intently in the stream.

Boldly advancing to spot a fish

They quickly hide among the reeds again.

In a moment one brings its catch on shore,

The tail still jutting from its bill.

In this ornithologist's opinion the old poet had considerable powers of observation.

The little tern nests in a colony and usually builds its nest on the sandy shore. The male bird offers a small fish to its mate, and sometimes a pair of birds will swoop together in the sky as if to display their flying skill.

Two or three eggs are laid in each nest, and the male and female take turns incubating them and feeding the young. The fledglings stay with their parents for four or five weeks before going their own way.

Studies have shown that even when a storm has covered their eggs in sand and swept away all landmarks, terns can still quickly find their unhatched offspring again.

They will also defend their nest and have several ways of dealing with intruders. Besides simulating injury to draw the intruder away from the nest they will swoop ferociously and peck the intruder's head with their hard bill.

Many seabirds, including the tern, live in the wide open spaces of the ocean. To understand more about them you might have to be prepared to spend long periods at sea yourself.

[Picture Caption]

A family of little terns photographed at the Tatu River estuary by nature photographer Ch'en Yung-fu.

Right pictures show little terns in summer, left picture shows them in winter when their bills turn from yellow to black.

"In a moment one brings its catch to shore, the tail still jutting from its bill" is indeed an apt description of the tern.

Tired of flying, a tern rests on the shore and preens its plumage in comfort.

 

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