鶺鴒的警戒與擺尾行為

Abstract

棲地中的遮蔽物是影響小型動物掠食風險的重要因子之一。有些遮蔽物屬於保護性的(protective cover)，可以為小型動物提供躲藏之處，降低被掠食的風險。但有些遮蔽物並不能提供藏身處，還阻礙了小動物的視野，反而提高了牠們被掠食的風險。因此許多動物會依據棲地中遮蔽物的性質，調整自己的警戒程度。鶺鴒(wagtails)以明顯的擺尾行為(tail-wagging)而聞名，牠們的擺尾行為被認為可能是誠實的警戒信號，且可能具有遏止掠食者追擊的功能(pursuit-deterrent function)。我預期擺尾行為也會受到棲地遮蔽的影響，一方面因為警戒會因棲地遮蔽而改變，另一方面若遮蔽程度較高時，提高擺尾的頻率可能可以傳遞給鶺鴒未察覺到的掠食者一些資訊。我以在台灣渡冬的三種鶺鴒為對象，於台北地區在2007年一月至三月及九月和十月間，研究河堤等遮蔽物對牠們的警戒和擺尾行為的影響。結果發現，河堤可能使黃鶺鴒(Yellow Wagtail, Motacilla flava)感受到較低的被掠食風險，但需要更大的樣本來進一步證實；灰鶺鴒(Grey Wagtail, Motacilla cinerea)和白鶺鴒(White Wagtails, Motacilla alba)的警戒行為並不受到棲地的影響。檢視三種鳥的擺尾行為，發現灰鶺鴒的警戒頻率愈高時，擺尾的頻率也愈高；但在黃鶺鴒和白鶺鴒則沒有這種關係。不過黃鶺鴒共出現在兩個樣區中，同時在兩區內的警戒程度有顯著差異，在警戒程度高的樣區中之擺尾頻率顯著高於在警戒程度低的樣區。白鶺鴒則是在開闊地所佔比例愈高的環境中之擺尾程度愈高。本研究中僅有灰鶺鴒的結果支持前人「擺尾行為可能是警戒信號」的說法，且未能排除擺尾只是鶺鴒移動時的附帶反應之可能性。擺尾在鶺鴒之中應是一個相當原始的性狀，在不同種的鶺鴒間是否可能演化出不同的功能，需要進一步的探究。

Vigilance is a universal antipredator behavior. Animals could adjust their vigilance level according to the predation risk. Some covers in nature could provide hiding places for prey animals (protective cover), whereas some other covers provide nothing for prey animal to hide in but obscure their vision (obstructive cover). Thus the effects of cover on vigilance may be complex. Vigilance could also deter predator’s pursuit, because predators could benefit by attacking non-vigilant prey, and a conspicuous vigilance signal thus could evolve to convey the information of vigilance state to predators. The tail-wagging behavior in wagtails is suggested to be an honest signal of vigilance and serve pursuit-deterrent function. I hypothesized that the habitat could affect tail-wagging behavior through the adjusted vigilance and the purpose to communicate with undetected predators. I study three wintering species of wagtails on the riverine foraging habitats to test my hypotheses. The results show that vigilance in M. flava could be affected by river dike, but the large sample sizes are needed to be conclusive. The tail-wagging frequency in M. cinerea increases as scanning frequency increases, but it is not affected by habitat parameters. M. flava in the site with higher vigilance level show higher tail-wagging frequency, however, the tail-wagging frequency do not significantly associated with scanning frequency. The tail-wagging frequency of M. alba is independent with scanning frequency, but positively associate with the proportion of open area. Thus, the suggestion that tail-wagging signals the vigilance might not be universally applicable. The possibility that tail-wagging is just a passive response to any body movements can not be ruled out. The adaptive functions of tail-wagging behavior remain to be investigated further.