雀鷹，像雀鷹的大杜鵑，與小型鳥：一種猛禽能帶來多大改變行為的力量？
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| 大杜鵑 (Cuculus canorus) photo from ouse washes |
摘要
Prey typically respond to potential predators by taking flight. This results in an optimal flight initiation distance (FID) at which the risk of remaining and the cost of flight are equal. Thus, FID is strongly negatively correlated with susceptibility to predation by the sparrowhawk Accipiter nisus across species of small European birds. However, not everything that looks like a hawk is in fact a hawk. Aggressive mimicry arises from the resemblance between a dangerous model such as a predatory hawk and an innocuous mimic such as a cuckoo that makes errors made by individuals that encounter models and mimics potentially dangerous and life threatening. A prime example of such aggressive mimicry is the hawk-like appearance of common cuckoos Cuculus canorus and other cuckoos. If mimicry is efficient, we should expect that species of small birds that are prey of hawks and hosts of cuckoos react just as strongly to a cuckoo as to the presence of a dangerous model like a sparrowhawk. We used FID of small birds as a measure of the reaction of prey to models and mimics, predicting that FID would be negatively related to susceptibility to predation and positively to risk of cuckoo parasitism. Both susceptibility to sparrowhawk predation and rate of brood parasitism by the common cuckoo independently explained variation in FID, consistent with the expectation that both hawks and cuckoos have imposed significant selection pressures on FID and that species of small birds perceive cuckoos as true mimics of hawks.
獵物遇到天敵要逃跑的時候，通常會評估「到底要不要跑？」
覺得奇怪嗎？遇到天敵，先跑，就對了啊，難道還等天敵到面前了才傻傻的要跑嗎？
對於野外一天到晚被人盯著瞧，隨時可能被吃掉的物種來說，隨時隨地都可能遇到天敵，但不是隨時隨地都有力氣可以跑，如果太早跑反而提早被發現，因此這些物種會評估到底捕食者離我多近才要跑，這稱為flight initiation distance (FID)，也可以叫做初始逃跑距離，就是捕食者離我到多近的食後才開始逃跑。那麼為什麼會有這個距離呢？因為有些獵物身上有些隱蔽色或有些警戒色之類用來躲避天敵的機制，如果捕食者雖然靠的很近但其實沒發現，那麼就不用浪費力氣逃跑，可是相對的會遭遇非常高的風險：萬一被發現就跑不掉了。
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| 雀鷹 (Accipiter nisus) photo from Eurasian sparrow hawk (wiki) |
那麼有意思的事情就發生了。大杜鵑 (Cuculus canorus)眾所皆知，有把蛋下在別人的巢裡，讓別的鳥幫牠孵蛋的行為。有人覺得大杜鵑長的像雀鷹 (Accipiter nisus)，早期的科學家認為這是一種貝氏擬態，但後來發現有另一種可能：這些大杜鵑長的像雀鷹，把那些要幫牠孵蛋的鳥嚇走，大杜鵑就能順利的去這些鳥的巢裡下蛋了？
作者藉由測量那些幫忙孵蛋的小型鳥的FID來測試這件事。之前有研究顯示，歐洲的小型鳥的FID與感知捕食的壓力的平衡點是負相關（體型越小靠的越近），因此如果小型鳥感應到的是捕食壓力，那麼應該是負相關，如果感知到的是普通的杜鵑偷生蛋行為，則是正相關。大杜鵑看起來就像雀鷹的話，那麼小型鳥的FID應該會與看到雀鷹的FID結果相似。作者觀察了63種鳥類行為的結果顯示，雖然某些小型鳥對大杜鵑的反應就像對雀鷹一樣，但相較於捕食的壓力，對於認知「杜鵑來下蛋」這件事還是比較強烈，因此大杜鵑擬態雀鷹可能有侵略型擬態的作用，但作者還是認為這還是種貝氏擬態，功用可能在於寄生到比較兇的寄主時，不會被這些寄主反過來攻擊甚至喪命。綜合以上顯示兩種行為都帶給這些小型鳥不同的壓力，這使FID的測量與結果可能來自於更為複合的行為觀察才能解釋。
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