Vocal signaling and multimodal cues in aggressive behavior in owls: Insights from Tropical Screech-Owl, Megascops choliba (Strigiformes: Strigidae)

ABSTRACT

Fighting is costly, and vocalizations are often used to mediate aggressive interactions. While aggressive vocal signals are well-documented in songbirds, no study has empirically identified such signals in owls or evaluated the use of visual cues in aggressive contexts. During a long-term study of the Tropical Screech-Owl, Megascops choliba (Vieillot, 1817), in southeastern Brazil, a rare outbreak of aggression in a single territorial pair in 2024 led us to conduct targeted experiments testing the effects of visual stimuli and assessing whether vocalizations previously hypothesized to be aggressive actually function as such. We documented nine attacks across three trials, all but one involving the combination of conspecific playback and a conspecific visual stimulus (taxidermic mount). Vocalization Type II (sensu Peixoto et al. 2021) showed a significant correlation with attack frequency, supporting its role as an aggressive signal under the context criterion. However, no vocalization reliably preceded aggression, failing to meet the predictive criterion, suggesting that escalation may occur without vocal mediation. A taxidermic mount of a larger owl (a potential predator) elicited only an aborted attack, which, combined with the fact that attacks occurred after playback had ceased, indicates that M. choliba integrates acoustic and visual cues in target recognition. Given the limited territorial replication due to the rarity of aggression in this species (attacks recorded in 3.6% of 84 trials), our findings highlight the need for further research on owl species with higher baseline aggression levels.

KEYWORDS:
Acoustic communication; agonistic interactions; playback experiments; predator recognition

Fighting is costly, and not only for the losers (Lane and Briffa 2017). Engaging in combat increases the risks of injury (Enquist and Leimar 1990), predation (Jakobsson et al. 1995), and loss of access to resources (Briffa 2015). Consequently, direct physical fights are not the most common way conflicts are resolved. Instead, conflicts typically involve hierarchical, ritualized, repetitive, and often non-injurious behaviors that ultimately limit damage (Enquist and Leimar 1990, Briffa and Hardy 2013). Moreover, ‘weapons’, once considered to impose restraint, are now seen as features that can increase an individual’s chances of winning contests (Palaoro and Peixoto 2022). Owls are particularly interesting models for the study of agonistic interactions, as they possess sharp talons and strong beaks and exhibit cryptic, almost silent behavior (König et al. 2008, Mikkola 2017) - traits that may be crucial in this context (Palaoro and Peixoto 2022). Adding further complexity, as top predators, owls engage in intraguild predation (e.g., Schubart et al. 1965, Sergio et al. 2007, Zuberogoitia et al. 2008), where eliminating a competitor can also represent a foraging opportunity (e.g., Mikkola 1976, Rohner and Doyle 1992). This dual role of removing competitors and gaining resources is likely to influence the balance of risks and rewards in agonistic encounters (Sergio and Hiraldo 2008).

However, even prior to conflict resolution - and the conflict itself - an individual must recognize potential competitors, which ensures that energy is directed toward appropriate targets and minimizes unnecessary risks. In birds, species recognition primarily relies on visual or acoustic cues (e.g., Hauber et al. 2001), though it can also involve multimodal signals (Campbell and Hauber 2009). This integration of sensory modalities is particularly intriguing in owls, as most are nocturnal and inhabit wooded areas where the effectiveness of visual cues is often limited by low light levels, amplifying reliance on acoustic signals for hunting and communication (Marks et al. 1999, König et al. 2008). However, vision cannot be ruled out in owl biology, as nocturnal owls do engage in visual communication (Penteriani and Delgado 2017) and possess some night vision capabilities (Mikkola 2017, Sieradzki 2022).

Despite the promising set of characteristics in owls, studies on their aggressive behavior remain scarce. This gap is likely attributable to the rarity of such behaviors, as illustrated by Zuberogoitia et al. (2005), who documented attacks in less than 1% of interspecific aggression trials with Little Owl, Athene noctua (Scopoli, 1769); Long-eared Owl, Asio otus (Linnaeus, 1758); Barn Owl, Tyto alba (Scopoli, 1769); and Tawny Owl, Strix aluco Linnaeus, 1758. The scientific literature on aggression in owls is mostly broad in scope, covering attacks on potential predators (including humans) (e.g., Coulombe 1971, Thomsen 1971, Galeotti et al. 2000, Fisher et al. 2004, Carrete and Tella 2017, Cavalli et al. 2018), combats with other species (Brazil 1986, Zuberogoitia et al. 2005, Baucells 2011, Van Lanen et al. 2011), aggression among siblings (Allen 1924, Gehlbach 1994), and between members of the same pair (Mikusek 2019). Regarding conspecific interactions, most existing studies focus on non-aggressive agonistic behaviors (e.g., Herting and Belthoff 1997, Grieco 2022), while fewer than a handful consider aggressive acts (Finck 1990, Galeotti et al. 1997, Severinghaus 2000, Grzywaczewski et al. 2024). Of these few studies, none empirically identified vocalizations as aggressive signals (see Searcy and Beecher 2009 for criteria in determining aggressive signals) or tested the limits of target recognition and the role of visual cues in aggressive contexts.

Here, we report on aggressive behavior observed in the Tropical Screech-Owl, Megascops choliba (Vieillot, 1817), and describe experiments designed to address these gaps. During a study on the vocal behavior of this species initiated in 2019, playback trials apparently elicited occasional attacks on the loudspeaker, accompanied by a specific vocalization type, leading to the hypothesis that this vocalization was an aggressive signal (Vocalization Type II; Peixoto et al. 2021). In 2024, however, we recorded recurring attacks by a pair in a single territory monitored since 2019 (hereafter, the ‘focal pair’). The onset of this aggressive behavior was sudden and its cause remains unclear. Given the rarity of such behavior, we conducted targeted experiments in this territory specifically designed to assess: (a) whether any vocalization functions as an aggressive signal, focusing especially on the hypothesis that Vocalization Type II is aggressive; and (b) which stimuli the owls use as cues in aggressive behavior, both in species recognition and target selection.

This study was conducted at the Poço das Antas Biological Reserve in southeastern Brazil (22°32’S, 42°17’W), where we used playback in 14 territories of M. choliba to attract individuals for further experimentation, following the protocol of Peixoto et al. (2021). This protocol consisted of broadcasting three repetitions of a two-minute conspecific song stimulus, with five-minute intervals between repetitions, using a pool of nine recordings with one recording randomly assigned per sampling point, and no taxidermic mount. From 2023 onwards (84 trials), we adjusted this protocol to increase its effectiveness following evaluation of the original protocol (see Peixoto et al. 2021). Specifically, we increased stimulus duration from two to three minutes and reduced repetitions from three to two, using a subset of five from the original nine recordings, with one random recording drawn per sampling point per night. Additionally, in this set of 84 trials, a taxidermic mount of M. choliba was present in half of the trials (randomly assigned), drawn from a pool of five mounts. The loudspeaker (JBL Flip Essential, 2 × 8W RMS, 80-20,000 Hz) and taxidermic mount were placed on a 3-meter tripod, and a dim diffuse light source was placed on the ground below the tripod to improve our ability to observe the trial area and minimize the effect of moonlight. Playback amplitude was adjusted at each sampling point to approximately 76-83 dB SPL (measured at 1 m from the speaker) to ensure clear sound rendition and compensate for variation in ambient noise levels, following Peixoto et al. (2021).

The targeted experiments conducted with the focal pair consisted of: (a) two trials with conspecific playback only (no taxidermic mount), (b) two trials with conspecific playback and a taxidermic mount of M. choliba (conspecific visual stimulus), and (c) two trials with conspecific playback and a taxidermic mount of Pulsatrix koeniswaldiana (Bertoni, 1901) (Tawny-browed Owl) (heterospecific visual stimulus of a larger owl). No more than one trial was performed per night. The start time and visual stimulus condition for each trial were assigned randomly. All taxidermic mounts were prepared in an upright perched position. Artificial eyes matching the color and size of each species’ iris were used to ensure a more realistic visual stimulus (see Supplementary Fig. S1). At least two observers, positioned ≥5 m from the tripod and partially concealed in vegetation, recorded the experiments from multiple angles to minimize observer bias. Vocalizations uttered by target owls during each experiment were recorded using a Sennheiser ME62 microphone coupled to a Marantz PMD661 digital recorder; audio recordings were archived in the Arquivo Sonoro Elias Coelho (UFRJ). A Sony ICD-PX240 handheld recorder was used for redundancy and observer notes. Video was occasionally recorded using a Nikon D610 with a Nikkor 200-500 mm f/5.6E ED VR lens and an iPhone 13.

To assess whether vocalizations met the criteria for aggressive signals, we followed the framework of Searcy and Beecher (2009), which outlines two criteria focusing on the emitter’s perspective: the context criterion, which evaluates whether a signal is used more frequently in aggressive contexts, and the predictive criterion, which examines whether a signal reliably predicts escalation to physical aggression. To assess whether Vocalization Type II (sensu Peixoto et al. 2021) met the context criterion, we used a generalized linear model (GLM) with Poisson error distribution to test whether the number of phrases of this vocalization (see Fig. 1) was significantly correlated with the number of attacks. As overdispersion was not detected (dispersion index = 0.12), a Poisson distribution was retained. The model was fitted using the R glm() function, and model significance was evaluated through the z-value and associated p-value of the predictor. We also evaluated qualitatively whether any vocalization type reliably predicted attack, i.e., whether any vocalization was consistently uttered prior to attack. Statistical analyses were performed in R 4.0.0 (R Core Team 2020) using the following packages: vegan for data transformation, MASS for stepwise model selection, and jtools for effect visualization.

We documented nine attacks in three of the six targeted experiments (Table 1). Three attacks were preceded by vocalizations, while the remaining six were silent (Fig. 1). One attack was preceded by A-song (sensu König et al. 2008) and two by low-amplitude song (sensu Peixoto et al. 2025), both uttered minutes before the attacks. These two vocalization types are the most common response to playback in this species, even in non-escalated situations (Peixoto et al. 2021), and were recorded in all trials; therefore, there is no evidence linking them specifically to attacks. These findings support the recent understanding that low-amplitude songs in owls are not aggressive signals (Peixoto et al. 2025), despite being often uncritically labeled as such, including in M. choliba (e.g., Krabbe 2017, Lane in Schulenberg et al. 2007).

Figure 1
Spectrogram of sounds recorded during one of the attacks (Supplementary Audio S1), illustrating the most common pattern - silence preceding the strike followed by Vocalization Type II (sensu Peixoto et al. 2021): (A) active playback (dotted vertical boxes); (B) noise caused by the physical impact of the attack (horizontal bracket); (C) two phrases of Vocalization Type II (dashed horizontal boxes). This pattern contrasts with the expected vocal mediation prior to escalation.

Table 1
Summary of behavioral responses and associated vocalizations across trials with different visual stimuli. Attacks occurred much more frequently when a conspecific taxidermic mount was present; no attacks were observed in trials using a heterospecific predator model. Vocalization Type II was frequently emitted during or after attacks, and no vocalization consistently predicted aggression.

Visual stimulus	Trials	Attacks	Behavior before attack	Behavior after or during attack
Conspecific model	2	8 (88.9% of all attacks)	Silence (75%) or low-amplitude song (25%)	Vocalization Type II (75%), Silence (12.5%) or low-amplitude song (12.5%)
P. koeniswaldiana model	2	0	-	-
None (loudspeaker only)	2	1 (11.1% of all attacks)	A-song	Silence
Total	6	9	Silence (66.7%), low-amplitude song (22.2%) or A-song (11.1%)	Vocalization Type II (66.7%), silence (22.2%) and low-amplitude song (11.1%)

Our results partially support the hypothesis that Vocalization Type II is an aggressive signal. It was significantly associated with the number of attacks per trial (Z = 2.194, p = 0.0282), suggesting its frequent use in aggressive interactions and thus meeting the context criterion (Searcy and Beecher 2009). However, contrary to what would be expected of an aggressive signal under the predictive criterion, Vocalization Type II was not uttered before attacks, but only during or immediately after most of them (Table 1, Fig. 1, Supplementary Video S1). The owls attacked without any prior vocal mediation, which contradicts the expectation that animals would strongly avoid escalated aggression due to the high risk of injury (Enquist and Leimar 1990, Briffa and Hardy 2013). This possibly indicates that Vocalization Type II occupies a lower position in the escalation hierarchy, making it a poorer predictor of escalation (Searcy and Beecher 2009). Alternatively, the focal pair may represent under-signalers, i.e., individuals that escalate without consistent warning signals (Akçay et al. 2015), or these attacks may be ritualized forms of lower-cost aggression, given that talons were apparently not used (Supplementary Video S2) and the mounts suffered no damage.

Regarding the cues used in aggressive interactions, if visual stimuli were irrelevant and interactions were modulated only by acoustic cues, all trials using conspecific playback should have elicited equal responses regardless of the taxidermic mount used. However, this was not the case. Eight of the nine attacks occurred during the two trials with a taxidermic mount of M. choliba (Table 1), whereas only one attack was recorded without any taxidermic mount (when the loudspeaker was targeted) and no attack occurred in the two trials with the P. koeniswaldiana mount (see below). These results indicate that aggressive interactions were not exclusively acoustic but multimodal. In the attack without a taxidermic mount, the owl initially sang its A-song near the loudspeaker and then flew away into the vegetation, apparently retreating or moving on. From afar, it then launched a long flight that culminated in an attack on the loudspeaker, which was inactive at that moment. This suggests that when close to the loudspeaker the owl identified it as a target - whether aurally, visually, or both. At the time of the attack, however, when the loudspeaker was silent, the attack was accurate - an observation also made during trials with the M. choliba mount - indicating that the target was tracked visually even if initial recognition was acoustic. One of the trials with a taxidermic mount of P. koeniswaldiana prompted an aborted attack, during which the owl veered away from the target shortly before contact. This suggests that the owl aimed at an auditorily acquired target (as owls are known to do; e.g., Konishi 1973) but, after visual assessment of the larger, differently colored owl, changed direction mid-flight.

Regarding the absence of attacks on P. koeniswaldiana mounts, two explanations can be considered. First, by identifying the target as a heterospecific, the owl may have judged aggression unnecessary; however, this seems unlikely, as several owl species exhibit heterospecific aggression (e.g., Zuberogoitia et al. 2005, Baucells 2011). Alternatively, the owl may have identified P. koeniswaldiana as a potential predator and avoided confrontation to reduce risk (e.g., Gehlbach and Leverett 1995). The latter interpretation is more likely, especially given that M. choliba has been documented as a victim of intraguild predation by the Spectacled Owl, Pulsatrix perspicillata (Latham, 1790) (Schubart et al. 1965).

Taken together, these findings suggest that M. choliba uses acoustic and visual cues in both target identification and tracking, consistent with multimodal recognition documented in other birds (Gill and Murray 1972, Irwin and Price 1999). Future studies could experimentally vary decoy size, color pattern, and species identity to isolate recognition cues (e.g., conspecific vs. predator, sympatric vs. allopatric), which would help disentangle recognition of conspecifics, threat sensitivity, and size-based risk assessment. Furthermore - although sexual dimorphism in M. choliba is not detectable in the field, preventing identification of which individual in the pair was responsible for the observed vocalizations and attacks - the sex of both the focal individual and the stimuli used may affect responses to playback in other owl species (Galeotti et al. 1997), warranting further investigation.

Overall, M. choliba exhibited very low levels of physical aggression, with attacks recorded in only 3.6% of 84 trials, consistent with reports on other owl species (e.g., Zuberogoitia et al. 2005). The exception represented by the focal pair (attacks recorded in 50% of six trials) warrants further discussion. One potential explanation is individual personality differences (Sih et al. 2004, Groothuis and Carere 2005, Réale and Dingemanse 2010); however, personality alone cannot explain the results, as we observed interannual variation in the pair’s aggression. It is plausible that aggression increased in 2024 due to proximity - temporal or spatial - to key reproductive resources (Enquist and Leimar 1990). The timing of the attacks (April-June 2024) aligns with the pre-reproductive period in southeastern Brazil, preceding the typical hatching season of M. choliba (August-October; Marini et al. 2007, Dias and Lima 2015, Motta-Junior et al. 2017), and may represent a particularly sensitive time window during which territorial defense becomes more intense. The absence of aggression in June 2025, however, suggests that resource timing or location may have changed, highlighting the complexity and plasticity of such behaviors. Importantly, the observed attacks were unsignaled - not preceded by any observable vocal change - defying the common expectation that overt aggression is mediated and avoided through ritualized displays (Enquist and Leimar 1990). This calls for further testing, as variation in owl aggression has been explored only in relation to nest defense and melanin-based color polymorphism (Da Silva et al. 2013). Whether such silent attacks reflect under-signaling phenotypes (Akçay et al. 2015), or whether the recorded attacks represent low-cost ritualized aggression or less injury-prone combats, remains to be tested.

Although our findings are based on a limited sample from a single pair, they open new perspectives on aggressive signaling and target discrimination in owls and other nocturnal birds. The rarity of escalated conflict in M. choliba highlights the challenges of studying these behaviors in a low-aggression species. Future work should therefore focus on species with higher baseline aggression levels - such as appears to be the case for Athene noctua (Finck 1990) - which may provide more tractable systems for experimental manipulation.

ACKNOWLEDGMENTS

This paper is part of the Ph.D. requirements of Luis F. Peixoto at the Biodiversity and Evolutionary Biology Graduate Program, Universidade Federal do Rio de Janeiro. We thank the staff of Reserva Biológica Poço das Antas, especially Gustavo L. Peixoto, for their consistent support; Bárbara A. de Carvalho and Filipe B. de Sousa for field assistance; Ana Galvão for preparing the taxidermic mounts; and Marcos A. Raposo (Museu Nacional, Rio de Janeiro) for donating specimens for taxidermic preparation. We also thank two anonymous reviewers for their comments on a previous version of the manuscript.

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• ZooBank register
  https://zoobank.org/86D91FCA-0922-4299-86BD-857BACBFA832
• Data Availability Statement
  Datasets generated or analyzed in this study are available from the corresponding author on reasonable request.
• Funding Statement
  This study was supported by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES; Finance Code 001), the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq; grant 306788/2021-7), and the Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ; grant E-26/200.375/2023) through fellowships and grants to LFP and PCP.
• Ethical Statement
  All procedures involving live vertebrate animals were approved by the Animal Ethics Committee of UFRJ (protocol 087/19). Field activities were conducted under collection permits issued by Instituto Chico Mendes de Conservação da Biodiversidade (ICMBio/SISBIO, permit 78588-4).
• AI Statement
  Artificial intelligence tools (Claude Opus 4.5) were used solely to assist with language editing and grammar.
• How to cite this article
  Peixoto LF, Paiva PC, Gonzaga LP (2026) Vocal signaling and multimodal cues in aggressive behavior in owls: Insights from Tropical Screech-Owl, Megascops choliba (Strigiformes: Strigidae). Zoologia 43: e25121. https://doi.org/10.1590/S1984-4689.v43.e25121
• Published by
  Sociedade Brasileira de Zoologia at Scientific Electronic Library Online - https://www.scielo.br/zool

Supplementary Material

Supplementary Figure S1

Taxidermic mounts used in playback experiments, shown in upright perched position. Megascops choliba mounts were used in conspecific trials and Pulsatrix koeniswaldiana mounts in heterospecific trials. All mounts were fitted with artificial eyes matching the color and size of each species’ iris.

Data type: Morphological species data.

Supplementary Audio S1

Field recording (LFPSD5047) corresponding to the spectrogram in Figure 1, comprising active playback, the noise of the physical impact of the attack, and two subsequent phrases of Vocalization Type II.

Data type: Behavioral species audio.

Supplementary Video S1

Agonistic attack by Megascops choliba during an experiment with a conspecific taxidermic mount, illustrating the typical behavioral sequence: absence of vocalizations immediately before the attack, followed by emission of Vocalization Type II (sensu Peixoto et al. 2021) during or immediately after contact.

Data type: Behavioral species video.

Supplementary Video S2

Close-up footage of an attack on a Megascops choliba taxidermic mount, showing that contact apparently did not involve the use of talons and caused no significant damage to the mount.

Data type: Behavioral species video.

Author: Peixoto LF

Copyright notice: This dataset is made available under the Open Database License - ODBbL (https://opendatacommons.org/licenses/odbl/1.0/). The ODbL is a license agreement intended to allow users to freely share, modify, and use this Dataset while maintaining this same freedom for others, provided that the original source and author(s) are credited.

Link: https://doi.org/10.1590/S1984-4689.v43.e25121

Edited by

• Editorial responsibility
  Vinicius Tonetti

Publication Dates

• Publication in this collection
  07 Sept 2026
• Date of issue
  2026

History

• Received
  29 Nov 2025
• Accepted
  01 Apr 2026