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From flapping to fear: the production and function of the nonvocal acoustic alarm of the crested pigeon, Ocyphaps lophotes




Murray, Trevor

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While the study of nonvocal acoustic signals–or sonations–is enjoying a resurgence, very little research has united the production and the function of these signals. This thesis examines both the mechanisms of production and function of the flight sonations produced by crested pigeons, Ocyphaps lophotes. Studying both mechanism and function is important because a signal’s function can be affected by its mode of production. I show that, as previously suggested, the crested pigeon’s nonvocal acoustic alarm is an inherently reliable “index” signal of flight performance that reliably signals escape from danger. This information about danger is used by conspecifics, and also by heterospecifics, although to a lesser extent. After a brief introductory chapter, the thesis contains four data chapters, formatted as research papers, and a concluding chapter that summarises general conclusions and suggestions for future research. CHAPTER 2: The crested pigeon produces a whistle-like sound during any flapping flight, but most notably when taking off from the ground. Previous research showed that the sounds produced during routine take-offs varied predicably from those produced when fleeing, with the alarmed sounds having both a high amplitude and fast tempo. Using high-speed video recording, I showed that the production of each acoustic element of the alarm sonation is tied to a particular wing motion, with the upstroke producing the low note and the downstroke producing the high note. Furthermore, I used 3D-reconstruction of flight paths to show that the increased wing beat rate of escape flights produces both the rapid tempo of alarm sonations and the increased speed of escape. If these birds use fast tempo to distinguish alarms from routine wing sounds, this means that this alarm sonation is an inherently reliable signal of flight performance that acts as an index signal of danger. CHAPTER 3: All birds produce at least some sound during flapping flight, so I addressed the issue of whether the crested pigeon’s wing sound is indeed a signal rather than merely a non-selected cue of danger. Previous work showed that the 8th primary feather of the crested pigeon is unusually narrow, implying adaptation to produce the notes of their wing sound. I therefore used a feather-removal experiment to show that, unexpectedly, the 8th primary feather produces the high note but not the low note. Instead, the 9th primary feather produces the majority of the low note. I then used a playback experiment to show that the fleeing response of conspecifics to alarm sonations is dependent on the signaller’s possession of their 8th primary and its high note, but not the 9th primary and its low note. The 8th primary feather appears therefore to have been structurally modified to signal about danger. CHAPTER 4: Crested pigeons produce wing sonations during all flapping flight, which raises the question of how these sounds could be used to signal alarm. Previous work found that sonations produced in alarmed flight were louder than routine flight. This high amplitude is necessary to prompt flight by listeners, but a playback experiment showed that amplified routine flight sounds did not prompt flight by listeners, so that amplitude itself was not sufficient alone to signal alarm. Given that the tempo of a crested pigeon wing sound is a reliable indicator of danger, I therefore used a playback experiment to test its role in warning conspecifics to flee. I found that, similarly to amplitude differences, although a fast overall tempo is necessary to prompt conspecifics to flee, it is not alone sufficient. This finding means that additional fine scale features are also required and I identify the duration of the low note and the amplitude of individual notes as potential candidates. CHAPTER 5: Although alarm calls are primarily used to warn kin or flock mates of danger, heterospecifics often use the information conveyed in these signals, whether the signaller intends it or not. I tested whether sympatric heterospecifics were able to eavesdrop on this alarm and whether they used the same acoustic features to identify alarms as conspecifics. I found that magpie larks, Grallina cyanoleuca, and Australian magpies, Cracticus tibicen, responded to the majority of alarms by either fleeing or increasing their vigilance. However, unlike crested pigeons, both species responded to high amplitude routine wing sounds as if they were alarmed sonations. These heterospecifics did use the information on danger in pigeon sonations, but used less a reliable feature than the pigeons themselves. As a whole this thesis advances our understanding of this newly discovered type of alarm signal. It establishes that it is indeed produced non-vocally and that it is constrained to be reliable. It also suggests this signal has evolved specifically for communication: the 8th primary feather shows evidence of adaptation, and is necessary for both signal production and function. This work confirms that heterospecifics, without non-vocal signals of their own, are able to utilise the sonations of other species, while also showing that they may attend to different features than conspecifics. Overall, this thesis lays the ground-work for future studies on this and similar systems, such as the consequences of a reliable alarm and the evolutionary origins of non-vocal alarm signalling.



Index signal, sonation, nonvocal, acoustic, signal, alarm, crested pigeon, 3D reconstruction




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