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

Predators are a major source of mortality, and many species use alarm and mobbing calls to communicate about danger and recruit others. The acoustic structure of these calls can encode predator features such as type, size, distance, behaviour, and threat level, and these signals are used by both conspecifics and heterospecifics through eavesdropping. Although indirect, public information can aid survival and learning, it may be less reliable than direct, personal information obtained from encountering a predator. Because inappropriate threat responses are costly, selection should favour systems in which signals reflect true threat and receivers assess information quality. The study asks whether red-breasted nuthatches encode and propagate predator threat differently depending on whether they acquire information directly (hearing predator vocalizations) or indirectly (hearing chickadee mobbing calls that themselves encode predator threat). The prediction is that direct information will elicit finely tuned, threat-reflective nuthatch calls, whereas indirect information may elicit more generic signaling due to lower reliability.

Prior work shows many birds produce mobbing calls that encode detailed predator information, and heterospecifics often eavesdrop on these signals to guide antipredator behaviour. Black-capped chickadees, in particular, vary their calls with predator size and type, and nuthatches respond adaptively to these chickadee calls. However, indirect alarm information may be less reliable due to species-specific perception, life history, detection differences, caller state, and context, potentially limiting accuracy when transferred across species. Some species learn heterospecific alarms via associative learning, while others differ in encoding schemes, complicating cross-species interpretation. Thus, how animals use and propagate public versus personal information remains an open question, motivating tests of whether nuthatches selectively transmit threat information based on source reliability.

Field playback experiments exposed wild red-breasted nuthatches to acoustic stimuli indicating nearby predators at two threat levels and from two information sources. Direct information treatments consisted of vocalizations of a low-threat predator (great horned owl) and a high-threat predator (northern pygmy-owl). Indirect information treatments consisted of black-capped chickadee mobbing calls recorded in response to the same owls (low- and high-threat conditions). Control stimuli included non-threatening bird calls (e.g., house sparrow, Townsend’s solitaire), which were pooled after confirming similar responses. Stimuli were broadcast in the field to free-living nuthatches; order of presentation and exemplar usage were balanced across sites to control for order and exemplar effects. Nuthatch vocal responses were recorded using a Sennheiser ME 30 stereo microphone to Sony TCM-150 tape or Marantz PMD661 digital recorders (48 kHz, 24-bit). Due to variation in sampling effort, treatment sample sizes varied: control (n=38), low-threat direct (n=19), low-threat indirect (n=35), high-threat direct (n=21). Acoustic analyses were performed in RavenPro (v1.5/1.6). For each trial, analyses focused on a 60 s window during the highest call rate to standardize measurements. Three response variables were extracted: call rate (calls per minute per number of individuals present), peak frequency (kHz), and call length (s). Linear mixed models with Gaussian errors tested effects of predator threat (control, low threat, high threat), information type (control, direct, indirect), and their interaction. Fixed effects included number of nuthatches present, call exemplar, presentation order, and other relevant covariates; site/location was a random effect to account for spatial non-independence. Model comparisons used likelihood-ratio tests. Ethical approvals were obtained and experiments were limited to daylight foraging windows.

• Nuthatches modulated mobbing call structure based on both predator threat level and information source. In response to direct predator calls, high-threat (northern pygmy-owl) elicited higher call rates, higher peak frequencies, and shorter call lengths compared to low-threat (great horned owl). In contrast, in response to indirect (chickadee) mobbing calls, nuthatch call parameters were intermediate and similar across low- and high-threat chickadee stimuli, indicating a generalized alarm rather than precise threat encoding. - Statistical support: Linear mixed models showed significant effects/interactions: call rate χ²=6.43, P=0.011; peak frequency interaction χ²=12.78, P<0.001; call length interaction χ²=5.57, P=0.018. No overemphasized effects were detected in additional LMM checks (call rate χ²=4.12, P=0.846; peak frequency χ²=3.30, P=0.914; call length χ²=10.42, P=0.237). - Sample sizes and effort: control and threat treatments spanned multiple sites and thousands of calls (e.g., control and threat treatments included on the order of 19–35 sites and 1,204–5,205 calls across categories; see Supplementary Data 1). Overall, direct information elicited finely graded threat-specific mobbing calls; indirect, public information elicited more generic, intermediate signals.

Findings indicate that red-breasted nuthatches discriminate between personal (direct) and public (indirect) information sources when signaling about predators. When they directly hear predators, their mobbing calls encode fine-scale threat differences (faster, higher-pitched, shorter calls to the more dangerous northern pygmy-owl vs. less dangerous great horned owl), paralleling patterns in other species. However, when hearing chickadee mobbing calls, nuthatches do not propagate precise threat levels vocally; instead, they produce intermediate, generalized alarm calls despite adjusting their investigatory/approach behaviour to threat. This pattern suggests sensitivity to information reliability: public information, although useful for behavioural decisions, may be less dependable for transmission to others due to species-specific perception, caller state, context, and encoding differences. Consequently, nuthatches may strategically avoid transmitting potentially unreliable heterospecific threat gradations in their own calls, thereby reducing the risk of eliciting maladaptive responses in receivers.

This study shows that nuthatches encode predator threat levels in their own mobbing calls when information is obtained directly from predators but default to generalized alarm signaling when relying on heterospecific, public information. These results demonstrate selective propagation of information based on perceived reliability of the source, adding nuance to theories of graded signaling and public information use in mixed-species communities. Future research should test receiver responses to these nuthatch call variants via playback to confirm decoding of threat-related variation, examine whether nuthatches treat indirect information from conspecifics differently from heterospecifics, and explore how additional factors (e.g., predator distance/behaviour, caller identity, group composition) shape both encoding and use of direct vs. indirect information.

• Predator presence was simulated via playback rather than direct predator encounters, which may not capture all cues (e.g., visual, distance, behaviour) influencing signaling. - Only two raptor species were used to represent low vs. high threat, which may limit generalizability across predator types. - Sampling effort varied among treatments, resulting in unequal sample sizes. - Acoustic analyses focused on a 60 s window of highest call rate, which could emphasize peak response periods and may not represent entire trial dynamics. - Indirect information was limited to chickadee mobbing calls recorded to specific predators; heterospecific call variability and contextual factors could influence reliability and transferability of information.