Hissing Predicts Lower Tonic Immobility and Higher Nest Success in Female Great Tits (Parus major)

Simple Summary

Animals use different strategies to avoid predators. In Great Tits, incubating females often hiss like a snake to potentially scare intruders. In Latvia (2023–2024), we asked whether this bold, proactive defense is linked to how fearful the same birds are when restrained during a standard test called tonic immobility (TI), where a more fearful bird stays still for longer. We inserted the head of a taxidermic Great Spotted Woodpecker, a nest predator, into nest boxes and recorded whether female Great Tits hissed and how many times they did so. When their nestlings were 3–4 days old, we caught and briefly tested the same females for TI. Our results show that most females (about 75%) hissed at the intruder. Females that hissed showed much shorter TI and their nests were less likely to fail than those of females that did not hiss. These results suggest that Great Tit females differ consistently in antipredator behaviors: bolder birds defend their nest more actively and appear less fearful when restrained. Because Woodpeckers dominated predation at our sites, such bold defense may bring real breeding benefits. Changes in local predator communities could therefore influence which defensive strategies are favored in the wild.

Abstract

Antipredator behavior can vary consistently among individuals, yet links between proactive nest defense and passive fear strategies are rarely quantified in the wild. We tested whether hissing, a conspicuous, snake-like display at the nest, predicts tonic immobility (TI) and breeding success in female Great Tits (Parus major). In pine forests in southeastern Latvia (2023–2024), we presented a taxidermic Great Spotted Woodpecker (Dendrocopos major) at nest-box entrances during incubation and scored whether females hissed and how many calls they produced. The same females were later assayed for TI by brief supine restraint when nestlings were 3–4 days old. Of 141 incubating females, 105 (74.5%) hissed. TI duration differed sharply between groups: non-hissing females showed significantly longer TI than hissing females. Nest failure was significantly lower in hissing than non-hissing female nests. These results reveal a strong negative association between proactive defense and passive fearfulness, and they show that hissing can translate into higher reproductive success in a Woodpecker-dominated predator environment. We conclude that defense strategies covary within individuals along a personality axis and that predator community composition may shape selection on these strategies.

1. Introduction

Antipredator behavior is ubiquitous across animals and exerts strong effects on survival and reproductive success. Specific strategies to avoid predation are shaped by natural selection to reduce predation risk. Examples range from protean escapes such as abrupt/unpredictable trajectories in fishes and insects to startle/deimatic displays in amphibians to mobbing/alarm calling in birds and mammals [1,2]. These defenses span active strategies (e.g., aggression, alarm calls, and bluffing) and passive strategies (e.g., freezing and tonic immobility), and many show consistent among-individual differences that are now recognized as facets of animal personality [3,4].

Cavity-nesting Tits (Parus spp.) provide a classic model of specialized nest defense. Incubating females often produce a conspicuous snake-like hiss at the nest entrance that can deter intruders. In some cases, Tits escalate to pecking attacks from within the cavity as part of a broader proactive nest-defense repertoire [5,6,7]. Hissing is context-dependent, being more common where and when snakes are prevalent, and it can enhance nest success [5,6,7,8]. While hissing intensity is individually repeatable [9,10], evidence that it covaries with other behavioral traits is mixed: no consistent association with female–female aggression has been found [10,11], and some studies did not assess covariation with other parental ‘risk-taking’ behaviors [9]. We therefore treat the relationship between hissing and other defensive tendencies as unresolved and test it here by relating hissing to tonic immobility.

In contrast to this proactive defense, many birds exhibit tonic immobility (TI) when restrained by a predator or handler. TI is defined as a transient, reversible state of immobile posture interpreted as an antipredator adaptation that may reduce attack intensity or facilitate release [12,13,14,15]. TI duration is widely used as an index of fearfulness and reactivity, with longer bouts of TI suggesting higher fearfulness. It is repeatable across tests in several avian taxa, including poultry, quail, and pheasants [12,13,16]. Experimental selection lines in insects and birds further support TI as a robust behavioral trait indicative of fitness correlates [13,14,17].

These two strategies, hissing at the nest and TI under restraint, represent divergent antipredator behaviors: one is proactive and conspicuous, while the other is passive and immobile. If both reflect a shared personality axis (e.g., proactive–reactive or bold–shy), we might predict a negative correlation: females that hiss intensely (proactive and bold) should show shorter TI durations [9,10]. Alternatively, if both reflect a general arousal and reactivity dimension, a positive correlation is plausible. In this instance, we might expect highly reactive individuals to express strong responses in both contexts. Ecological contingency may also maintain variation if different predator communities or disturbance regimes favor different defense strategy combinations [5,6,7,8].

Here we test these alternatives in breeding female Great Tits (Parus major). We combine field measures of hissing nest defense with standardized TI assays to evaluate whether these behaviors are linked within a phenotypic association. This integrative approach links behavioral ecology, mechanistic antipredator theory, and personality research, and it clarifies how multiple defenses covary within individuals in a natural system.

2. Materials and Methods

2.1. Study Area and Species

Fieldwork was conducted in May 2023–2024 near Krāslava, Latvia (54°58′ N, 27°10′ E) [18]. Our site is located in the hemiboreal/boreo-nemoral forest zone, characterized by a humid continental climate. The stands are dominated by Scots pine (Pinus sylvestris), Norway spruce (Picea abies), and silver birch (Betula pendula). The Great Tit is a widespread Palearctic secondary cavity-nester that readily uses nest boxes. Typical clutches are 8–12 eggs, the female incubates (c. 13–14 days), nestlings fledge after 18–20 days, and females often produce snake-like hissing at the nest [19,20,21]. Nest boxes were installed in 55–75-year-old Scots pine stands. Nest-box dimensions are: internal floor area 120 × 120 mm, total height 280 mm, and entrance hole diameter 30 mm. Adjacent nest boxes of Great Tits were never closer than 170 m from each other (229 ± 12 m, mean ± SD). No female was studied in both study years, and we considered only the first clutches of Great Tits. In our study area, all nest predation in 2023–2024 was attributable to Great Spotted Woodpeckers (Dendrocopos major). Pine martens (Martes martes), typically major predators of hole-nesting birds, were not detected, which may reflect increased hunting pressure in the preceding winters [22].

2.2. Predator Intrusion (Hissing) Assay

We monitored Great Tits during the egg-laying and incubation stage. Nest boxes were checked daily during egg-laying stages by digital inspection endoscopes (Yinama, Pickify, Shenzhen, China) to determine clutch initiation date and clutch size. Following standard Great Tit phenology, we considered full incubation to begin on the day the last (or penultimate) egg was laid. The hissing assay was scheduled for incubation day 3–5.

To elicit nest defense, we used a taxidermic Great Spotted Woodpecker, a principal predator of Tits in northern Europe [5,8]. After confirming that the female had returned from a foraging bout, an observer approached quietly and inserted the model’s head into the nest-box entrance. The stimulus was presented for up to 60 s or until the female ceased hissing, whichever occurred first. We recorded (i) whether the female hissed (yes/no) and (ii) the number of hissing calls during the presentation (test duration was constant at 60 s for silent females). Observers moved ≥50 m away immediately after the trial. To estimate the repeatability of the hissing behavior, we repeated this procedure 4 or 5 days later on the same females.

2.3. Tonic Immobility (TI) Assay

We measured tonic immobility using a standard protocol [12,13,15]. Females were captured with mist nets when nestlings were 3–4 days old to minimize the risk of nest abandonment. Upon capture, the birds were banded with standard aluminum rings, weighed, and their wing and tarsus length were measured. No other samples such as the blood, feathers or feces were collected; therefore, the handling procedure before the TI measurement did not exceed 90 s.

A single induction was performed by placing the bird supine on the experimenter’s open left palm and gently restraining the head and sternum for 10 s. The restraint hand was then removed, and TI duration (s) was timed with a stopwatch until the bird righted itself and flew off. If the bird righted immediately (<1 s), TI was scored as 0 s. Each female was tested once and released at the capture site.

2.4. Statistical Analyses

The data on the number of hissing calls were converted to binary data, dividing the Tits into groups of no hissing calls and those with hissing calls. To analyze the influence of the hissing call group on the tonic immobility duration, we used the Tweedie general linear model as implemented in R 4.5.1 [23] and its library ‘statmod’ [24]. The initial model included, as independent variables, hissing call group (yes/no hiss) and year interaction (as data came from two years), but year was removed from the model as an insignificant variable. Because we also recorded hissing counts, we ran a complementary model with negative binomial regression (log link) for number of calls (offset = log test duration), predicting call rate from TI duration and year. Model diagnostics (residual plots, dispersion, and influence) were inspected; significance was assessed with likelihood-ratio tests and 95% CIs. Effect sizes are reported as estimated marginal means with CIs. Repeatability of the two measurements of the number of hissing calls was evaluated with the R library ‘rptR’ [25], using the Poisson distribution for the number of calls and the binomial distribution for the hissing call group (yes/no hiss).

3. Results

Of 141 incubating females, 105 (74.5%) hissed when a taxidermic Great Spotted Woodpecker was presented at the nest-box entrance, whereas 36 (25.5%) did not. Hissing females produced 2.97 ± 3.15 calls (mean ± SD) during the presentation of the nest intruder. The number of calls given was highly repeatable between two trials (R = 0.734, D₁ = 126, p < 0.0001), the same for the hissing call group (R = 0.998, D₁ = 203, p < 0.0001).

Tonic immobility (TI) duration differed sharply between groups (χ²₁ = 246.1, p < 0.0001): non-hissing females remained still for 0–302 s (median 52.8 s), whereas the TI duration of hissing females was 0–2 s (median 0 s) (Figure 1).

Nest failure was lower in hissing female nests than non-hissing female nests (10/105 = 9.5% vs. 12/36 = 33.3%; Fisher’s exact test, p = 0.0023; odds ratio for failure = 0.21, 95% CI 0.08–0.55), indicating substantially lower odds of predation for hissing female nests. In all depredated Great Tit nest boxes in which nestlings were killed, Woodpeckers had either enlarged the entrance hole or bored a hole through the side wall. No females were found dead at depredated nests, suggesting that predators could not kill the females.

4. Discussion

Previous work shows that cavity-nesting Tits emit snake-like hissing calls [26] that deter predators [27,28] and increase the survival of incubating females and fledglings [5,29]. We show that incubating Great Tit females that produce hissing calls at the nest exhibit near-zero tonic immobility (TI) under restraint and experience substantially lower nest failure than non-hissing females. Hissing was common (74.5%): hissers produced on average 3 calls, and their TI durations were significantly shorter compared with TI durations in non-hissers. Consistent with a fitness benefit, hissing female nests failed less often. However, the incubating females were not killed in either group. Together, these patterns indicate a tight, personality-like [30,31] association between proactive nest defense, lower restraint-induced immobility, and higher reproductive success in a woodpecker-dominated nest-predator environment.

Previous studies have demonstrated that Woodpeckers are a significant source of nest failure for Tits and other cavity nesting passerines, sometimes accounting for a large percentage of predation [32,33]. The nest survival contrast aligns with the natural predator context of our sites: in both study years we detected nest predation almost exclusively by Great Spotted Woodpeckers, a well-documented predator of hole-nesters in Europe [5,8,9]. Hissing in Tits acoustically mimics snake hisses and can deter or delay entry at the nest entrance [5,6,7]. A brief hesitation by a Woodpecker could be sufficient for the female to escalate defense or for the predator to abandon the attempt. We therefore interpret the lower failure rate of hissing female nests as evidence that this display is effective against Woodpeckers specifically. Efficacy is unlikely to be universal across predator guilds (e.g., mustelids, which are mostly hunting at night) [5,8]. Predator community composition should therefore modulate the strength and direction of selection on hissing behavior.

Because TI was measured once per female, we consider the link between hissing behavior and TI as a phenotypic association. We cannot partition among- versus within-individual covariance [30,31], and therefore we do not claim evidence for a behavioral syndrome sensu personality studies [3,4]. Instead, we view the personality interpretation as a hypothesis for future work with repeated measures and mixed-effects models. Ecologically, if woodpeckers dominate nest predation, selection may favor proactive defenders; under other predator regimes (e.g., high mustelid pressure), more passive strategies could be advantageous by potentially maintaining variation within populations.

The strong negative association between hissing and TI duration is consistent with the notion that these behaviors sit at opposite ends of a proactive–reactive coping continuum. Hissing is a conspicuous, proactive defense expressed at close quarters, whereas TI indexes fearfulness or passive coping when restrained and is widely used as a behavioral measure of fearfulness in birds [12,13,16]. Shorter TI in hissers could reflect differences in baseline or stress-induced HPA activity, neuromodulatory tone, or sensorimotor thresholds for righting responses. Physiological covariation between fear measures and stress endpoints is well discussed in the animal welfare literature [13,16,34]. Alternatively, repeated experience with predators or humans could both promote hissing and reduce TI through habituation. Disentangling intrinsic from experience-dependent mechanisms will require physiological assays (e.g., baseline and acute corticosterone and HR/HRV), repeated-measures designs to assess within-individual repeatability across stages, and, ideally, experimental manipulations.

Several caveats merit consideration. First, hissing was measured during incubation whereas TI was assayed when nestlings were 3–4 days old. Personality traits can be stable across breeding stages, but stage-specific effects are possible [8,9]. Second, we did not include potential covariates such as age, body condition, laying date, time of day, or prior disturbance. These factors could influence hissing propensity, TI, and survival [10,35]. Third, the nest survival analysis is observational. Unmeasured site-level factors correlated with hissing (e.g., habitat structure that affects Woodpecker foraging) could contribute to the association [5,10]. Finally, we used a single predator model (Woodpecker), and strategies might differ with other threats (e.g., mustelids and conspecific intruders) [9,22]. These limitations argue for cautious interpretation of causality, as studies have shown lower breeding success in non-hissing females in a mammalian predator-dominated environment [36].

Despite these caveats, the behavioral and fitness patterns are congruent, and the effects are large. The TI contrast is extreme (medians for yes/no hiss: 0 vs. 52.8 s), and the nest survival difference is substantial. We analyzed TI with a distribution appropriate for zero-inflated, right-skewed durations, and we verified significance with likelihood-based tests. Future work could bolster inference with mixed-effects models incorporating year and site, and with sensitivity checks that include relevant covariates.

Taken together, our results suggest that proactive nest defense can yield measurable reproductive benefits when the dominant predator can be deterred at the cavity entrance [37]. In tits, the defense is likely multimodal: the conspicuous hiss may delay or repel the intruder, and in some cases, is accompanied by pecking from inside the cavity. Thus, rather than a pure ‘acoustic bluff,’ hissing appears to mark a propensity for escalation to actual defensive aggression, with predator- and context-dependent efficacy. Trade-offs likely exist. For example, conspicuous defense might impose energetic or injury costs. They may also elevate risk with predators insensitive to hissing, helping to maintain behavioral diversity within populations [1]. At the population level, shifts in predator communities (e.g., changes in woodpecker abundance or hunting pressure on mustelids) should reshape the adaptive landscape of defense strategies and, over time, the distribution of antipredator behaviors.

Several extensions follow directly from our results. First, replicate the design across sites that differ in predator communities to test for context dependence of the hissing–nest survival link. Second, pair Woodpecker presentations with alternative stimuli (e.g., pine marten model and conspecific intruder) to assess the specificity of hissing and its generalization across threats. Third, measure physiological correlates (corticosterone and HR/HRV) and repeat TI within individuals across breeding stages to quantify stability and heritability. Fourth, expand the defense profile to include additional strategies (pecking, body blocking, and alarm calling) and analyze them jointly as a multivariate defense syndrome [25,32]. Finally, track female fates across years to test whether proactive defenders also enjoy higher adult survival or recruitment, completing the fitness portrait.

5. Conclusions

According to our findings, incubating Great Tit females that hiss are less immobile when restrained and suffer markedly lower nest failure under Woodpecker predation. These results connect a proactive nest defense display with reduced fearfulness and higher reproductive success, integrating behavioral ecology, personality research, and predator-specific mechanisms. By showing how defense strategies covary and translate into fitness differences in a natural setting, this study clarifies the structure and consequences of avian antipredator behaviors.