Pervasive low-frequency vocal modulation during territorial contests in Eurasian Scops Owls (Otus scops). Grieco, F. 2021 IBIS. DOI: 10.1111/ibi.12989 VIEW
As the fierce sunlight transitions to twilight, when the chorus of cicadas ends, soon begins the monotonous ”tjuut” of the other troubadours of the Mediterranean summer nights, the Eurasian Scops Owls. But if the cicadas engage in serenades for a summer romance, the owls also sing to claim their territories and fend off rivals. Their main call is a hoot, repeated at intervals of 2-3 seconds with clockwork regularity (Cramp 1985), and for long time; an hour without interruption. When more males cluster in the same area, the effect is multiplied as one provokes the other.
When the hoots of the Scops Owls are analysed on a spectrogram, we see scribbles like those in Video 1. Each individual has its acoustic fingerprint (Galeotti and Sacchi 2001, Dragonetti 2007). The apparent simplicity and regularity of these vocalisations may make us think that they are a sort of stereotyped signal, like a dog’s barking. Is that so?
With a few exceptions, there is limited data on the intra-individual variation of the vocalisations of this species. What exactly happens in naturally-occurring male-male encounters is still poorly known. Most of the knowledge comes from playback studies in which the owls responded to a simulated territorial intrusion (Galeotti et al. 1997, Hardouin et al. 2007).
Video 1 Spectrogram of vocalizations of four male Scops Owls singing simultaneously. Note the consistent differences in the shape of the notes, and the rhythm, between individuals.
There are cases when Scops Owls form aggregations or “loose colonies”, where pairs breed near each other (Mebs and Scherzinger 2020). In such situations, rival males often come into close contact and may even be found calling in the same tree (Grieco 2018). And that’s exactly what I was looking for. I was particularly interested in how the individual responds to different territorial challenges, for example when facing a neighbour that sings at different distances. Furthermore, I was intrigued by the dynamics of such interactions – an aspect unexplored in this species. What happens during a spontaneous territorial contest?
The Scops Owl colony of Metaponto, in Southern Italy, offers an ideal scenario for such an investigation. Not only do Scops Owls form a calling group; the structure of the habitat, a dense pine plantation (Figure 1), allows the owls to perch and interact at varying distances from one another. The main assumption here was that a short distance between two opponents meant a higher level of escalation of the territorial dispute. I found that vocal frequency was associated with the distance from the nearest neighbour (Figure 1). This was also apparent within the course of an interaction, providing support for the view that vocal frequency is not a fixed quality of the individual. Furthermore, when a short-distance interaction ended, the owls resumed their typical frequency, even within a matter of a few seconds. This suggests that Scops Owls can control the quality of their vocalisations. When the dispute escalates, they seem to engage in dialogues with their direct rivals, rather than broadcasting a signal to a wide audience.
Figure 1 A simplified visual abstract of the featured study. A: View of the study area. Scops Owls live in relatively simple habitats, basically a monoculture pine plantation with scarce undergrowth. B: Male Scops Owl Red and Blue calling solitarily or far from other males, at its typical vocal frequency. C: At least one of the males lowers the dominant frequency of its hoots during a short-distance interaction with its neighbour.
What does all this mean?
In Scops Owls, hoot frequency correlates with body weight, and heavier males tend to call at a lower frequency when they perceive the hoots of heavy rivals (Hardouin et al. 2007). According to those authors, male Scops Owls communicate their weight, and therefore their strength to their rivals. However, my study showed that hoot frequency is rather variable even during a single interaction, which suggests that frequency as such is unlikely to reliably represent body weight or any relatively constant quality, at least in the short term. Perhaps the hoot frequency also conveys information about qualities that change more often, such as the motivational state of the caller. Hoots may indicate how much the caller is motivated to persist in a contest. The ancient relationship between vocal frequency and body size, found in many taxa, may have been “recycled” to acquire a new, more dynamic meaning in aggressive contexts where the caller adjusts its “message” based on its current state (Morton 2017).
Video 2 Spectrogram of the hoots of two male Scops Owls, Grey and Red, during two interactions recorded in 2020, where the birds were at different distances from each other. These episodes were not included in the featured paper.
Observe, observe, observe
These “most charming owls” (quote Bannerman 1955), have taught me a lesson. First, direct observation cannot be completely replaced by sensor-based studies. A million GPS locations won’t replace the magic of observing fine details of behaviour. Natural variation should always complement the experimental approach, and is the basis of defining hypotheses. Second, measuring the behavior of birds multiple times, and for a prolonged time, is anything but a waste of time. Intra-individual variation in behaviour is not just “noise” that we should get rid of; rather, it is a fundamental property of the individual – its plastic response to variation in environmental and social challenges, and at different time scales. In this ongoing project, even larger datasets are still waiting to be analysed, and more questions are left open. Among others: how does an individual respond to different rivals? Do low-frequency contests occur where territory boundaries are less stable? And does the breeding and/or social status of the birds (for instance, breeder vs non-breeder) influence neighbour relationships? One last note that bears mentioning, is that the Scops Owl is one of the least studied European owl species, and in some regions its populations are declining. Knowledge gaps should be filled, particularly in the Mediterranean region, one of the species’ strongholds. Understanding the social behaviour of the Scops Owls provides insights about how their colonies are formed and maintained, which is crucial for their conservation.
Top right: Eurasian Scops Owl (Otus scops) © Yary Radaelli
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Dragonetti, M. 2007. Individuality in Scops Owl Otus scops vocalisations. Bioacoustics 16: 147-172. VIEW
Galeotti, P., Sacchi, R., Perani, E. 1997. Cooperative defense and intrasexual aggression in Scops Owls (Otus scops): Responses to Playback of Male and Female Calls. Journal of Raptor Research 31: 353-357. VIEW
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Mebs, T. & Scherzinger, W. 2020. Die Eulen Europas, 3rd Ed. Franckh-Kosmos, Stuttgart.
Morton, E.S. 2017. Animal Vocal Communication: Assessment and Management Roles. Cambridge University Press.