My dissertation contains four chapters in which I describe, evaluate and apply two different on-animal microphone methods as well as appropriate analytic tools for studying call communication in group-living songbirds, using zebra finches and jackdaws as model organisms. In the Appendix, I included the abstracts of two further manuscripts which I was involved in during the course of my PhD studies. They provide more detail on specific aspects – timing of vocal events (Appendix 1) and individuality of calls (Appendix 2) – that are related to the topic, but slightly beyond the main scope of my dissertation.
Chapter 1
In the first chapter of my dissertation (Gill et al., 2016), I describe a tool that allows studying vocal communication with individual-level resolution in small, group-living birds, and discuss its potential and limitations, both for the animals and for the scientific questions that can be addressed. In this joint authorship study, my co-authors and I examined the performance and the effects of 0.6g microphone tele-metric backpacks on zebra finches under controlled conditions. By placing the backpacks directly on the animal, it was possible to record and reliably assign the vocalisations of specific individuals, even in different noisy environments. Trans-mitting the audio signal to a receiving base station and performing multi-channel recordings instead of recording directly on the animal (see Table 1) not only res-ulted in strongly reduced device weight, but also allowed long-term and perfectly synchronous recordings of multiple individuals behaving freely in structured en-vironments and socially complex settings, without post-hoc synchronisation. In this manuscript, we also explored the backpacks’ effects on animal locomotor activity, as well as on the target behaviour of such methods: vocalisations. Com-pared to previous studies, we found that this methodology resulted in shorter ha-bituation times (less than 3 days). However, we also highlight the importance of extended recordings that exceed potential initial alterations in behaviour, which was not possible in a previous study using a bio-logging approach in this species.
Chapter 2
In chapter 2 (Gill et al., 2015), I apply this technical approach (using an older, slightly heavier version) to gain an understanding of call communication in a small, group-living bird (zebra finch) over a changing environmental and socio-sexual context – which required a lightweight device allowing long-term individual-level recordings of loud and soft vocalisations from freely behaving birds. As argued above, in species with male-exclusive song, calls have the potential to be used by both sexes to form interactive social behaviours that may facilitate important life-history stages. Using microphone telemetry (after an appropriate habituation period), we continuously and simultaneously recorded all individuals present in-side a closed-system group setting. In this way, I followed the individual vocal-isations of male and female zebra finches previously unknown to each other, and analysed their vocal behaviour while the social group formed, pairs emerged and the birds passed through different breeding stages. We found that calling be-haviour not only changed with respect to call-type usage on the individual level, but also during naturally occurring vocal interactions in the group. To objectively define signallers and responders in vocal interactions, we used the temporal in-formation encoded in between-individual call timings1. Investigating the timing and call types involved in such vocal exchanges between mates and other mem-bers of the group suggested that temporally fine-tuned calling interactions were associated with successful breeding of a pair. This means that call communica-tion may not only play an important role during pair formacommunica-tion and early phases of the breeding cycle, but might be more directly linked to reproductive success than previously assumed, thus highlighting its evolutionary importance in com-munication systems.
Chapter 3
In chapter 3, I bring up the notion that microphone backpacks have the potential of providing us with more than vocalisation recordings, namely acoustic context.
As described above, studying the vocal behaviour of individual animals moving naturally in their habitat has become more feasible through the use of on-board microphones2. However, because sound recordings have high energy and stor-age demands, field studies using such technological approaches for investigating vocal behaviour of small animals are as yet extremely rare. Also, as discussed
1Appendix 1 provides a more thorough discourse on the analytic tools for investigating the timing of calls in vocal interactions, by analysing call data from zebra finches under standardised conditions as well as reanalysing the more complex dataset described in chapter 2.
2But not all topics are equally well suited for this technical approach. For example, confounding factors call for caution, making external microphones and standardised acoustic backgrounds at-tractive for studies on individual identity and vocal recognition (see Discussion, Appendix 2).
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above, it is necessary to set the individual vocal behaviour in relation to other con-textual data. But simultaneous observations can be limited in the field, and, espe-cially in birds, severe weight limitations restrict the amount of additional data that can be collected on-board (such as accelerometry or GPS). Because backpack microphones designed for continuous vocalisation recordings move through the environment with their carrier animal, their recorded sounds often contain dif-ferent acoustic events that occurred in the background, such as anthropogenic sounds or the vocalisations of conspecifics or heterospecifics. Additionally, some of the animal's movements picked up by the microphones are reflected by char-acteristic sound patterns. Making this additional source of information available could thus be highly valuable for studying the vocal behaviour of animals, embed-ded in different contexts, in the field.
To investigate this, I applied audio loggers on freely behaving captive and on free-living wild jackdaws (Corvus monedula), and conducted a video-validation study. I started a collaboration with a machine-learning expert who used my video-validated and human-coded annotations to test different machine learning approaches on the data containing bird movements and background sounds. Although fully auto-mated event detection and classification require further efforts, the soundscape analysis approach did succeed in identifying focal and non-focal vocalisations, and in extracting behavioural and contextual information that other technical ap-proaches cannot provide on their own (GPS, accelerometry, etc.). Given the fact that on-animal sound recordings are likely to become more efficient and available with advancing technological developments, analysing the full information spec-trum of these streams of sounds seems promising and an effort worth taking to understand when, how and why animals produce specific vocalisations.
Chapter 4
In this chapter, I applied some of the methods explained in chapter 3 to invest-igate a specific call type associated with a specific context: copulatory calls in jackdaws. These calls are produced during copulation, are very loud and con-spicuous, and can be heard from dozens of metres away. But because copulations take place inside the dark nest cavities in this species, it was not yet possible to identify which sex produced them, and thus, their function was completely un-known. Although not all animals produce such sounds, in some vertebrate spe-cies, including humans, such vocalisations have been subject to intense study, and have revealed very different functions, ranging from mate attraction to the source of complex social information (Hauser, 1998; Bradbury and Vehrencamp, 2011).
Using the abovementioned audio loggers on free-living wild jackdaws, as well as nest-box video footage and sound analyses, I found that males were the source of these conspicuous vocalisations. In addition, I discovered not unsubstantial amounts of extra-pair copulations which, too, were supposedly accompanied by the loud copulatory calls. This information contradicts the commonly accepted notion of jackdaws being exclusively monogamous, and calls for modern, state-of-the-art genetic and behavioural investigations. Although I can only speculate on the function of these vocalisations so far, this study demonstrates that on-animal sound recordings may help describing and revealing unknown vocalisations and exciting non-vocal behaviours, to facilitate future detailed investigations of vocal communication in natural contexts.
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