Nuthatches Vary Their Alarm Calls Based Upon the Source of the Eavesdropped Signals

Predator avoidance is crucial for survival. Animals often utilize alarm calls, including mobbing calls (typically given to non-immediately-threatening predators), to warn conspecifics and sometimes heterospecifics about potential dangers. The acoustic structure of these calls often encodes details about the predator, such as its threat level, impacting receiver behavior. Many species eavesdrop on heterospecific alarm calls, gaining indirect information about predators. However, indirect information's reliability is often lower than direct observation. Red-breasted nuthatches frequently interact with black-capped chickadees, eavesdropping on their mobbing calls. While nuthatches' behavioral responses to chickadee calls are known, it remains unclear how they integrate this indirect information into their own alarm calls. This study investigates whether nuthatch alarm call structure differs when responding to direct predator vocalizations versus indirect information from chickadee mobbing calls. Understanding how nuthatches process and transmit information about varying predator threats from different sources is important for comprehending the complexity of animal communication networks and decision-making in the face of predation risk. The authors hypothesize that nuthatches will produce more nuanced alarm calls in response to direct predator information, accurately reflecting the threat level, compared to a more generalized response to indirect information from chickadee mobbing calls.

Extensive research highlights the significance of alarm calls in predator avoidance. Studies have shown that the acoustic features of alarm calls can convey detailed information about the predator's identity, size, distance, and behavior (Templeton et al., 2005; Carlson et al., 2017). The use of heterospecific eavesdropping is also well documented, with evidence suggesting that receivers benefit from obtaining information about predators from other species (Templeton & Greene, 2007; Raghavan et al., 2014). However, the reliability of indirect information obtained through eavesdropping can be lower than direct observation. Previous research has established that nuthatches respond to chickadee mobbing calls indicating varying levels of predator threat (Templeton & Greene, 2007). This study builds on this foundation by investigating how nuthatches encode this indirect information in their own alarm calls, compared to their responses to direct predator vocalizations. The authors discuss various factors influencing the reliability of alarm calls, such as the sender's experience, the predator's life history, and the context of the encounter.

The researchers conducted a playback experiment using wild red-breasted nuthatches. Nuthatches were exposed to playbacks of either direct information (great horned owl and northern pygmy owl vocalizations representing low and high threats respectively) or indirect information (chickadee mobbing calls in response to the same owl species). The experiment was designed with a control group which played only chickadee calls. The acoustic features of the nuthatches' resulting mobbing calls were recorded and analyzed using Raven Pro acoustic software. Specifically, call rate, peak frequency, and call length were quantified. Linear mixed-effects models were used to test for significant interactions between predator threat level (low/high), information source (direct/indirect), and the acoustic parameters of the nuthatch mobbing calls. The model included random effects to account for spatial variability in mobbing behavior. The authors carefully considered potential confounding factors, such as the number of nuthatches present, call presentation order, and potential variations in individual bird responses.

The study revealed a significant interaction between predator threat level, information source, and nuthatch mobbing call acoustic parameters. In response to direct predator vocalizations, nuthatches exhibited clear acoustic differentiation between high-threat (northern pygmy owl) and low-threat (great horned owl) stimuli. High-threat stimuli elicited higher call rates, higher peak frequencies, and shorter call lengths compared to low-threat stimuli. However, when exposed to indirect information (chickadee mobbing calls), nuthatch call parameters were intermediate between the responses to high- and low-threat direct information. This suggests that nuthatches produce a more generalized alarm signal when relaying indirect information. Statistical analyses supported these findings, showing significant differences in call rate (χ²= 6.43, P = 0.011), peak frequency interaction (χ² = 12.78, P < 0.001), and call length interaction (χ² = 5.57, P = 0.018) across experimental conditions. No significant overemphasized effects were found in the analyses.

The results demonstrate that nuthatches selectively propagate information based on its reliability. Direct information from predator vocalizations allows for a fine-tuned alarm call response, accurately reflecting the threat level. However, the less reliable indirect information from chickadee mobbing calls leads to a more generalized alarm. This nuanced response to information source aligns with optimal information use strategies, where individuals prioritize reliable information when making crucial decisions. The study's findings contribute significantly to the understanding of information transmission in mixed-species communities and the complexities of animal communication. It also emphasizes that the structure of alarm calls does not always directly reflect the sender's arousal level but can instead reflect the quality and reliability of the information being conveyed.

This study provides compelling evidence that red-breasted nuthatches differentiate between direct and indirect information sources when producing alarm calls. They accurately reflect high vs. low threat levels when directly exposed to predators but produce a more generalized response to indirect information from chickadees. This demonstrates the importance of information reliability in shaping communication strategies. Future research could explore the underlying mechanisms of this information discrimination and assess how other eavesdropping species process and propagate indirect information. Further investigations could also focus on the effectiveness of these generalized alarm calls in alerting other nuthatches to danger and consider the factors that influence the reliability of information transmitted by chickadees.

The study's geographic scope is limited to a specific region. The use of playback experiments may not perfectly replicate natural predator encounters. The sample size, while sufficient for statistical analysis, could be increased to enhance the generalizability of the findings. While controls were included, subtle variations in ambient noise or other environmental factors could have influenced the responses observed. It is also worth considering the potential for individual variation in nuthatch responses that is not fully accounted for within the current model.