鉛色水鶇

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1988 / 7月

文‧張靜茹


台灣地區有全世界十二分之一種類的動植物,其中不少為特有或稀有種。本專欄陸續為您介紹這些動植物。本期介紹生活於溪澗,霸氣十足的台灣特有亞種溪鳥——鉛色水鶇。


為護衛自己的領土,鉛色水鶇是具有最多「小動作」的溪鳥。

師大生物系發表的鉛色水鶇生態研究報告中指出,它們生活的領域界線分明,彼此會在互相認定的界線兩邊爭吵。爭吵的產生,往往是由一隻雄鳥在自己的領域內,發現隔鄰的雄鳥出現在邊界附近;它會立刻飛到邊界旁發出急促而高亢的聲音,另一隻聞聲,也會發出相同的聲音飛近前者,爭吵於是展開。

兩鳥交兵,裝模作樣

雙方邊叫邊俯身翹尾地跳向對方,有時一方展示時,另一方則靜止不動呈直立狀,如此相互交替,隨著雙方的接近,動作頻率亦增加,眼看就要引發一場肉搏戰……。

所幸,它們對同胞手足,極少會出現暴力行為,雙方往往保持在廿公分以外,進行不接觸、但也不妥協的戰爭。

如何停止這場沒有肉搏的戰爭?它們互相展示、嚇阻,不久,雙方會漸漸遠離,直到一方自認失敗,離去為止。

沒有如人類戰場上的短兵相接,它們之間又如何判定誰勝誰負?誰得先離去?鳥類觀察者也無法解釋。但可以確定的是,領域的爭吵,都發生在雄鳥身上,家眷僅止於觀望或暗地加油。

兩國交兵,敗者割地,鉛色水鶇亦然。它們的領域以溪流長短為準,勝方擴張領土,敗方領域相對縮短。但它們亦有自知之明,野心不大,領域一般保持在一百九十公尺內。因為領土太大,必須浪費精力從事巡視疆界的工作,易造成體力不支,給對方可乘之機。

溪澗王國的主宰

鉛色水鶇分佈在台灣山腳至海拔二千公尺左右的溪澗河谷。它所生長的環境,在自然界被視為一個封閉的小國度。師大生物系副教授王穎的解釋是,溪澗空間狹窄,加上兩岸受水流侵蝕,土質貧瘠,可提供的養分少,生物的種類、數量自然也不多,形成的食物鍊很簡單。

因此在台灣溪澗常見的溪鳥,屈指可數。除了鉛色水鶇,只有紫嘯鶇、魚狗、河烏、小剪尾、小白鷺五種,再加上只在秋末冬初滯留的候鳥——灰、白鶺鴒,「就組成了溪澗王國最上層的主宰」。「隨鳥走天涯」的作者劉克襄,以其豐富的賞鳥經驗寫道:「溪鳥幾乎就是此一食物鍊的最高點。」

在視野狹隘的王國中,除求偶季節,溪鳥幾乎都獨來獨往;不像水鳥生活在視野廣闊的沼澤、溪口,易暴露自己身體,需要靠團體生活來維護自己的生命安全。在溪澗,我們見不到如關渡平原、大肚溪口成千上百隻水鳥群飛而起的景象,而是分擁據點、各自為王的情形。

小魚吃大魚?

物質不豐的封閉環境中,它們需要佔有固定的面積覓食,也確保供應求偶、繁殖下代的「熱量」不虞匱乏。如此造成了溪鳥的領域性,也「便宜」了賞鳥者。

山鳥、水鳥的生活範圍廣闊,賞鳥者往往要跋涉萬水千山,跟著它們由此山顛到彼山頭,此水涯到彼沼澤。許多水鳥,每年更飛渡重洋,巡迴於南、北半球,不停在各個國度遷移,到了一地只作暫時停歇,賞鳥者想追隨更不可能。

觀察溪鳥,「只要找到適當的地形,坐下來枯坐就成了」,鍾情賞鳥的劉克襄也認為觀賞溪鳥最輕鬆。

在幾種溪鳥中,最具霸氣的鉛色水鶇,被賞鳥者認為最有看頭。

它的領域性最強,除同種間的相互威嚇,對侵入其領域的其他鳥類,更是毫不留情地加以攻擊,一些體型比它大的鳥也望而生畏。「……我經常看見體型大它半倍的小剪尾遭到驅逐,落荒而逃……,卻從未看見小剪尾打贏過鉛色水鶇。」劉克襄形容。

不僅如此,偶爾經過溪畔,還摸不清狀況的山鳥,如繡眼畫眉、山紅頭,就常因誤入領空而遭無妄之災。

晚起的蟲兒被鳥吃

貧瘠的溪澗環境,造就了個性頑強、能力十足的鉛色水鶇。但是,只要不侵入它的領土,倒也能與其他鳥類相安無事,互不侵犯。對食物不同的鳥,如魚狗,還有肚量容納它在自己領土活動、覓食。事實上,它們並不願花那麼多力氣互相殘殺,因為溪澗食物既不似沼澤、森林豐富,浪費體力或四體不勤都等於自尋毀滅。尋求食物才是它們最重要的任務。

鉛色水鶇以在水面飛翔的昆蟲為主食。中國人勸人勤奮總愛說「早起的鳥兒有蟲吃」。從事鉛色水鶇調查的師大生物系研究生卻表示,其實昆蟲的活動在下午較活躍,鉛色水鶇的最高覓食紀錄也在下午,每分鐘高達九十七次,曾有五十分鐘捕食三百四十隻小蟲的戰績,等於一分鐘就吃掉七隻。

難怪賞鳥者形容它們是「鎮日吱叫不停,專挑一粒粒朱紅櫻桃啄食的小雞們」。

為便於捕食飛行中的昆蟲,鉛色水鶇有一雙快速的短翅。它偶爾像直升機的起降,忽然由立足的岩石上拔空飛起,再落回原地,就在一剎那的上下間,小蟲已進入腹中;有時,它會飛到溪澗岩壁,啄食壁上的昆蟲,然後以雙腳彈壁飛回地面。據調查報告指出,鉛色水鶇諸如此類覓食姿勢,共有六種之多。

由雌鳥選擇「金屋」地點

鉛色水鶇屬鶇科鳥類,根據東海大學環境科學中心研究員張萬福所著的「台灣鳥類彩色圖鑑」中記載,鶇科鳥類共同的特性是,鳴聲爽朗,棲息時身體挺直,在地面活動以跳躍方式前進,有一雙大眼睛和細而長的尖嘴。

鉛色水鶇雄鳥身體是鉛藍色,腹部、尾巴為赤栗色;雌鳥顏色較淺,尾部白色,臉頰呈黃白色。成鳥由嘴到尾巴長約十二公分左右,屬中小型。它在體型、體背和腰部的顏色與生長在其他地方的鉛色水鶇有差異,因此被分類為台灣特有亞種。

它們為一夫一妻制,繁殖期在每年二至七月,常利用上午昆蟲還未完全甦醒時築巢,由雌鳥選定造窩地點——多半在溪畔巖隙。人工建築物——橋,侵入溪床後,它們又多了一種選擇——橋墩的縫隙。

築巢期,它們很敏感,發現有蛇、鼠等天敵在附近,或其他人為干擾,就會棄巢而去。雌鳥甚至會先選定好幾個地點築巢,最後才擇吉遷入。「也許是為了分散天敵的注意力」,一位鳥類專家推測。

為了弟弟妹妹,留你不得

雌鳥每次生二至四個蛋,孵蛋的重責仍由雌鳥擔任,父親負責警戒。依氣候不同,母鳥孵卵日期也不同,例如三月氣溫較低,需要八天小鳥才破殼而出;六月氣溫升高,只需五天,小鳥就會提早到人世報到。

雛鳥全身為淡鉛藍色,喉、胸、腹部滿布白色小斑點。二周後,母鳥已開始準備清巢、補巢等繁殖第二窩小鳥的工作,也開始對小鳥產生「排斥行為」。

「排斥行為在自然界經常發生」,發表鉛色水鶇調查報告的王穎解釋,它和領域大小、窩次多寡有關;一般說來,窩次多、領域小的鳥類有其「排斥」的實際需要。

窩次少,弟、妹來得慢,小鳥可在媽媽身邊多待些日子;領域大,要將雛鳥趕出界去,得消耗許多體力,母鳥多半「懶得趕你」——驅趕意願既然不強,小鳥也就可以賴著不走了。

雖然被雙親狠心趕出家門,但小鉛色水鶇離巢的第一星期,還會保持在巢廿公尺內活動,戀戀不捨,大約一個月後才遠離雙親的領域,自謀生活。

鉛色水鶇回到濁水溪

溪澗王國的子民成分雖單純,天敵也較少,卻不表示它就是個不受外界影響的桃花源。反之,因食物鍊單純,一旦外在環境改變,影響了其中一環,整個食物鍊也就相繼消失。

一位走遍台灣大小溪流的賞鳥者舉大甲溪為例。德基水庫築成後,河水流動變緩,喜歡急湍的蚊蚋小蟲絕跡,溪澗沒有其他蟲食可代替,原本在此活動頻繁的鉛色水鶇也頓時消失。

令愛鳥人擔心的另一類似例子是,萬一立霧溪上游興建火力發電廠的提案成為事實,將導致水位落差改變。喜歡在含氧高水域活動的水生昆蟲隨之消失,也間接影響溪鳥的存亡。

近來汙染河川的農藥、重金屬,也可能成為鉛色水鶇的致命殺手。劉克襄曾在一篇題名「鉛色水鶇回到濁水溪」的短文中作出如下的結語:「我知道重金屬會使它斃命;縱然不死,也孵不出蛋;即使孵出蛋,也孵不出幼鳥。」

試想:孵蛋的母鳥已經幾天徹夜不睡,疲累異常,卻仍強打精神,露出期待的神色;它完全不知道,自己辛苦產下的,其實是幾顆永不會孵育小生命的「壞蛋」……。

你願意這樣的景象成真嗎?

〔圖片說明〕

P.75

岩石上「方便」一下,再理理羽毛,真是愉快。圖為雄性鉛色水鶇。(郭智勇攝)

P.76

雌鳥羽色不若雄鳥鮮豔,遠看全身為鉛藍色。(王嘉雄攝)

P.77

圖為胸、腹部滿布白色斑點的雛鳥。(王嘉雄攝)

相關文章

近期文章

EN

The Plumbeous Water Redstart

Chang Chin-ju /tr. by Peter Eberly

Taiwan is home to one-twelfth of the world's species of plants and animals, many of them native to no other place on earth. This month we introduce a rare species of bird that inhabits the island's creeks and ravines and is something of a little bully--the Plumbeous Water Redstart.


The Plumbeous Water Redstart is the most "action-packed" of any aquatic stream bird in defending its territory.

A study of the bird's ecology and living habits published by the Biology Department at National Taiwan Normal University points out that males have a clear sense of territoriality and will quarrel with each other across a commonly recognized border. The squabbles often get started when one male finds that a neighboring male has approached too close to the border. He will fly up to his side of the border at once and let out a series of sharp, rapid cries. The other will respond with a similar cry and fly towards the other, and that's the way the quarrel begins.

Both birds will squawk and hop towards the other with their wings pressed against their bodies. And sometimes one will stand quiet and motionless while the other goes through his routine as though they were taking turns.

Fortunately, it's extremely rare that they exhibit violent behavior toward birds of their own kind. Both sides usually carry out a "no-contact" but also "no-compromise" war across a DMZ of about twenty centimeters.

How does this battle between noncombatants come to a halt? While putting on their intimidating exhibitions, the birds gradually draw apart until one side simply gives up and leaves.

So what determines who wins and who loses? Birdwatchers have no answer. But it's clear that territorial disputes occur only between males. The rest of the family just stands by and watches, providing moral support perhaps.

The birds' territories are normally less than 190 meters in diameter because a larger area would entail the expenditure of too much energy in border patrol work, taxing the bird's strength and giving his antagonist an advantage.

The Plumbeous Water Redstart (Rhyacornis fuliginosus affinis) is found at an altitude of around 2,000 meters above sea level among creeks and ravines on the lower slopes of Taiwan's mountains, in what are more or less closed-off little worlds in the greater natural environment. Because the banks have been eroded and are poor in nutriments, NTNU biology professor Wang Ying says, the variety and quantity of life in the ravines are limited and the food chain is simple.

That limited space has its advantages for birdwatchers. "As long as you find a place with the right kind of topography, you can just sit back and wait," says Liu K'e-hsiang, a birdlover who believes that stream birds are the easiest kind of birds to observe.

And the feisty little Redstart is considered the stream bird most fascinating to watch. Besides threatening and intimidating members of their own species, Redstarts are quite merciless in attacking other birds that infringe on their territory and sometimes scare even the bigger ones off. Birds from up the mountain that pass through without knowing the situation may find themselves short of feathers.

Despite the occasional tussle, Redstarts actually don't like to spend a lot of energy fighting over territory because finding food is their biggest job.

Their main foodstuff is the insects that glide along the surface of the stream. In Chinese the saying goes, "The early bird catches the bug," but in this case it's not true because the insects are most active in the afternoon. That's when the birds do most of their catching--up to one hundred in an hour, although the top mark recorded is 340 in 50 minutes, about seven bugs a minute.

Redstarts are monogamous, and they mate and reproduce each year from February to July. They build their nests in the morning, when the insects are less active. The female picks the spot, usually in a crack or crevice in the creek bank. Each female lays from two to four eggs, which hatch in five to eight days, depending on the temperature.

Although they live in a small world with few natural enemies, their habitat is not a shangri-la unaffected by the outside world. On the contrary, just because the food chain is so simple, a change in the outside environment that disturbs a single link can destroy the whole chain.

Think about it: a mother bird, exhausted from sleepless nights and the travails of labor, forces herself to pick up her spirits and look cheerful. She has no idea that what she's worked so hard to bring into the world are just "rotten little eggs" that won't develop into healthy chicks....

Do you want this kind of a scenario to come true?

[Picture Caption]

Cleaning up after making a "pit stop" on a convenient rock. Shown is a male. (photo by Kuo Chih-yung)

The female's plumage, not as brilliant as the male's, looks leaden-gray from a distance. (photo by Wang Chia-hsiung)

The chicks have speckled bellies and breasts. (photo by Wang Chia-hsiung)

 

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