藪鳥二重唱行為功能

Abstract

二重唱是指配對的個體將各自的鳴唱聲以特定形式組合在一起的展示行為。越來越多證據顯示，這種看起來是合作的行為展示也可能是兩性衝突的結果。要瞭解二重唱行為的演化，研究參與二重唱的兩造是否具有共同目的是個關鍵。我結合了描述性以及實驗性的研究來探究藪鳥二重唱的行為功能。實驗從2005年到2006年的繁殖季間，於台灣中部山區的梅峰進行。在描述性的研究中，我將藪鳥的二重唱結構以及其行為量化描述以瞭解雄鳥與雌鳥參與二重唱的策略有何異同。我分析在自然狀況下錄到的15對藪鳥的鳴聲。雌鳥極少自行發起鳴唱，藪鳥的二重唱行為幾乎是由雄鳥引發，雌鳥跟隨而形成(96.43%)。雄鳥發出哨音鳴唱聲，而有9.71％的雄鳥鳴聲後是由雌鳥跟隨發出一到九個音節不等的鳴唱聲形成二重唱。雌鳥音節涵蓋的頻率範圍較雄鳥音節窄，但單位時間內的頻率變動較大。每隻鳥的聲音都有獨特的頻率分佈可作為個體辨識標準，但雄鳥鳴唱聲的個體辨識度較雌鳥大，且雄鳥擁有較多鳴聲曲目。雄鳥鳴唱頻度在雛鳥孵出後有下降的趨勢，但雌鳥回應比例不受繁殖階段影響。雄鳥鳴唱次數隨雌鳥回應比例增加與雌鳥回應速度增加而減少。在實驗性研究中，我對十對藪鳥進行雙聲道回播實驗測試解釋二重唱功能的假說。回播處理包括藪鳥陌生的雄鳥單獨鳴唱聲、陌生雌鳥單獨鳴唱聲、陌生配對二重唱，以及鄰居配對二重唱。雄鳥與雌鳥的聲音分別由不同的聲道播放一回，並且再交換聲道播放第二回。播放第一回時，雄鳥引發鳴唱的傾向及反應遲疑時間在前三個回播處理中並無顯著差異，唯獨對鄰居二重唱反應較低落。播放第二回時，雄鳥的鳴唱次數在雌鳥獨唱及鄰居二重唱的處理較低。但雌鳥回應的機率在兩回回播中都是在雌鳥獨唱及陌生配對二重唱處理中較高。在大部分的回播試驗中（10/14），雄鳥與雌鳥都是一起接近同一個喇叭，但在其餘雄鳥跟雌鳥分開行動的回播試驗中，個體趨近播放與自身性別相同的聲音的喇叭。綜合來說，描述性研究與實驗性研究獲得一致的結果。藪鳥的雄鳥與雌鳥鳴聲結構及參與二重唱的策略有顯著的差異。由回播結果推論，除了共同防衛領域，雄鳥與雌鳥的反應差異顯示雌鳥也有防衛配偶的意圖。這兩項假說並非完全互斥，而藪鳥的二重唱行為應同時具有兩種功能。由在鳴聲結構及不同背景下參與二重唱所呈現的性別差異推論，雖然藪鳥的二重唱看起來是種合作的行為展示，其演化可能亦受到兩性衝突的作用。

Duets are formed when pair members coordinate their songs with temporal precision. More and more evidence showed that these apparently cooperative displays could also be driven by sexual conflict. I combined a descriptive study and an experimental study to investigate the functions of duetting in Steere’s Liocichla. Experiments were carried out at Meifeng, central Taiwan during breeding season from 2005 to 2006. In the descriptive study, detailed quantification of duet structure and duetting behavior is provided to understand the vocal strategies of both sexes. I recorded vocalizations under natural condition from 15 breeding pairs. Females rarely sang spontaneously, and almost all duets were led by males (96.43%). Male Steer’s Liocichlas sang whistle songs and 9.71% of which were followed by females with buzz notes to form duets. Female notes extend a narrower frequency range, but modulate more rapidly than male notes. Songs of both sexes are highly individualistic; nevertheless, male songs contain higher individuality than female songs. Each male have a repertoire of 2-4 types, but all female songs are of the same type. While male song rate decreased once offspring hatched, female responsiveness remained equally low irrespective of breeding status. Males sang shorter song bouts if their mates were more willing to join in and joined more quickly. In the experimental study, I performed stereo duet playbacks to ten pairs to test hypotheses of functions of duetting. Treatments included playback of female solo, male solo, pair duet of strangers and pair duet of neighbors. Treatments were played through opposite channels in two rounds. In the first round, male song initiation rates were not significantly different among the three stranger treatments but lower in neighbor duet playback. In the second round, males initiated fewer songs in the female solo and neighbor duet playback. In both rounds, females answered their partner’s song more frequently in response to female solo and stranger duet playbacks. In most cases (10/14), members of a pair approached speakers together. In cases which pair members approached different speakers, birds spent the majority of time on the side of the speaker broadcasting elements of the same sex with them. Taken together, the descriptive study and experimental study showed consistent results. Male and female Steere’s Liocichlas have pronounced differences in vocal structure and duetting strategies. Responses to simulated intrusions indicate that in addition to joint territory defense, duetting in this species may also serve a mate guarding function. Sexual differences in vocal structure comprising duets and the way how duets are delivered suggest that under the apparently cooperative act, sexual conflict may operate in the evolution of duetting in Steere’s Liocichla.