Beckers Gabriël J L, Suthers Roderick A, ten Cate Carel
Behavioural Biology, Institute of Evolutionary and Ecological Sciences, Leiden University, PO Box 9516, 2300 RA Leiden, The Netherlands.
J Exp Biol. 2003 Jun;206(Pt 11):1833-43. doi: 10.1242/jeb.00364.
Birdsong assumes its complex and specific forms by the modulation of phonation in frequency and time domains. The organization of control mechanisms and intrinsic properties causing such modulation have been studied in songbirds but much less so in non-songbirds, the songs of which are often regarded as relatively simple. We examined mechanisms of frequency and amplitude modulation of phonation in ring doves Streptopelia risoria, which are non-songbirds. Spontaneous coo vocalizations were recorded together with concurrent pressure patterns in two different air sacs and air flow rate in the trachea. The results show that amplitude modulation is the result of the cyclic opening and closure of a valve instead of fluctuations in driving pressure, as is the current explanation. Frequency modulation is more complex than previously recognized and consists of gradual, continuous time-frequency patterns, punctuated by instantaneous frequency jumps. Gradual frequency modulation patterns correspond to pressure variation in the interclavicular air sac but not to pressure variation in the cranial thoracic air sac or air flow rate variation in the trachea. The cause of abrupt jumps in frequency has not been identified but can be explained on the basis of intrinsic properties of the vocal organ. Air sac pressure variation as a mechanism for frequency modulation contrasts with the specialized syringeal musculature of songbirds and may explain why the fundamental frequency in non-songbird vocalizations is generally modulated within a limited frequency range.
鸟鸣声通过在频域和时域中对发声进行调制而呈现出其复杂且特定的形式。在鸣禽中已经对导致这种调制的控制机制和内在特性的组织进行了研究，但在非鸣禽中研究得要少得多，非鸣禽的歌声通常被认为相对简单。我们研究了非鸣禽环颈鸽（Streptopelia risoria）发声的频率和振幅调制机制。记录了自发的咕咕叫声以及两个不同气囊中的同步压力模式和气管中的气流速率。结果表明，振幅调制是瓣膜周期性开闭的结果，而非如当前解释那样是驱动压力的波动。频率调制比之前认为的更为复杂，由逐渐的、连续的时频模式组成，并穿插着瞬时频率跳跃。逐渐的频率调制模式与锁间气囊中的压力变化相对应，但与颅胸气囊中的压力变化或气管中的气流速率变化无关。频率突然跳跃的原因尚未确定，但可以根据发声器官的内在特性来解释。气囊压力变化作为频率调制的一种机制，与鸣禽特化的鸣管肌肉组织形成对比，这可能解释了为什么非鸣禽发声中的基频通常在有限的频率范围内被调制。
