Behavioural Diversity: Conditional Movement Tactics in the Ruff (Calidris pugnax)

Abstract

Understanding the movement behaviour of male ruffs (Calidris pugnax) during the breeding season requires integrating recent telemetry data with long-standing theory on conditional reproductive strategies, lek dynamics, and behavioural polymorphism. A large-scale tracking study revealed extensive within-season movements among many males, with individuals visiting 1 to 23 sites, but also documented prolonged residency, with site tenures exceeding 40 days. Such variation is not contradictory but expected in a species whose reproductive system combines genetically fixed alternative strategies, governed by a supergene, with flexible conditional tactics expressed in response to ecological and social cues. Here, I synthesize movement ecology, state-dependent decision models, lekking theory, and previous empirical work to show that spatial behaviour in ruffs reflects a continuum of tactics rather than a homogeneous nomadic mode. Telemetry data thereby enrich our understanding of how individuals navigate fluctuating environments, competitive pressures, and mating opportunities. Embracing behavioural heterogeneity is essential for interpreting movement patterns and for understanding how reproductive diversity evolves and is maintained in lekking systems.

1. Introduction

Understanding movement patterns in species with complex mating systems requires integrating empirical data with the theoretical frameworks that govern behavioural variation. This is particularly true in lekking species, where spatial behaviour during the breeding season is the surface expression of underlying ecological and evolutionary processes. The recent contribution of Kempenaers et al. (2025) provides a comprehensive telemetry dataset gathered for male ruffs (Calidris pugnax) [1]. Their study reports that males visited between 1 and 23 distinct sites during the breeding period, with a median of 11, and that tenures at a site often exceeded 40 days. These findings are important and timely, and they offer a remarkable opportunity to open a debate on lek fidelity, reproductive decision-making, and the interplay between conditional tactics and genetically determined strategies.

Interpreting these movement data requires framing them within the extensive behavioural ecology literature on ruffs and within general theory about alternative reproductive tactics (ARTs), conditional strategies, and movement ecology. Without such a scaffold, there is a risk that a single behavioural pattern such as mobility may be overgeneralized, thereby overshadowing the species’ intrinsic behavioural heterogeneity.

In this paper, I embed the results of Kempenaers et al. [1] within four analytical layers: (1) the genetic and behavioural architecture of ruff reproductive strategies, (2) the ecological and social logic of lekking systems, (3) movement ecology and state-dependent behavioural rules, and (4) the empirical history of lek fidelity and spatial variation among males. This integration reveals a picture in which mobility and fidelity coexist as expected outcomes of a conditional strategy. It also shows that interpreting the species as uniformly nomadic, or treating high mobility as its defining reproductive trait, does not accurately reflect the breadth of behavioural variation documented both in the telemetry data and in decades of field research.

Crucially, the two perspectives, the empirical pattern documented in [1] and the mechanistic interpretation developed here, are fully compatible but not equivalent: the former describes the spatial distribution of movements, whereas the latter explains the evolutionary and ecological mechanisms that generate this distribution.

By framing the movement patterns documented in [1] within established evolutionary and ecological theory, I do not contradict the authors’ empirical findings but clarify their mechanistic underpinnings. The two perspectives are therefore fully compatible: their study provides a detailed description of the spatial pattern, whereas the present analysis develops the explanatory framework that accounts for why such patterns should arise in a lekking species characterized by behavioural polymorphism and strong competitive asymmetries. Understanding how mechanisms generate patterns is essential for interpreting the biological significance of the movement diversity revealed by telemetry.

2. The Genetic Architecture of Behavioural Variation: Strategies and Supergenes

The mating system of the ruff is structured by a remarkable supergene inversion, producing three distinct male morphs, independents, satellites, and faeders, each characterized by divergent morphologies, hormone profiles, and competitive behaviours [2,3,4]. This genetic architecture imposes deep constraints on behavioural repertoires, yet it does not prescribe spatial tactics in a deterministic way. As highlighted by Gross [5] and reaffirmed in Shuster & Wade [6], genetically determined reproductive strategies coexist with environmentally modulated tactics that individuals deploy conditionally.

It is therefore essential to distinguish genetically determined reproductive strategies, governed by the supergene, from the behavioural tactics expressed within each strategy. This conceptual distinction, formalized by Gross [5] and Shuster & Wade [6], makes clear that all males share the same conditional decision process for evaluating whether to remain at or leave a display site. Divergence in spatial behaviour is thus not evidence for different strategies, but the expected outcome of variation in the internal states and encounter histories that shape each male’s tactic selection.

The telemetry dataset of Kempenaers et al. [1] is consistent with this theoretical framework. Morph identity did not predict whether a male would be strongly mobile or nearly sedentary, reinforcing the idea that movement tactic is not a fixed component of the morph strategy but a flexible, state-dependent response. This aligns with the view presented in Baguette et al. [7], who argued that morphological and behavioural polymorphism in ruffs produces ecological and social gradients of opportunity that individuals navigate using a wide repertoire of context-dependent decisions. The genetic strategy thus defines the starting point for behavioural expression, but the tactic ultimately observed—remaining on a lek, sampling multiple sites, or shifting repeatedly across the landscape—is shaped by ecological contingency.

3. Lekking as a System of Behavioural Divergence Rather than Uniformity

3.1. Fidelity, Dominance, and the Social Topography of Leks

In lekking species, reproductive success is strongly skewed, with only a subset of males achieving most copulations [8]. These males are often those who occupy central positions on stable leks, integrate themselves into the social structure, and attract females through predictable presence. A fundamental prediction of lekking theory is that high-performing males will show strong spatial fidelity [8].

This prediction is supported by long-term observational studies in ruffs. Widemo [9] demonstrated that successful males exhibit persistent lek fidelity, both within and across years. Early field studies by Hogan-Warburg [10] similarly documented males returning to the same traditional lek sites across multiple breeding seasons. Van Rhijn’s monograph [11] further synthesized these observations and reported that a subset of males show multi-year fidelity to a single lek, typically over two to four consecutive breeding seasons. Importantly, this long-term site fidelity was not evenly distributed across males: dominant or high-status individuals were consistently more faithful, whereas young or subordinate males were more likely to shift among leks between years. Vervoort & Kempenaers [12] found that some males maintain weeks-long stable attendance at a single lek within a season, while others move more widely across display sites.

These results resonate strongly with the patterns observed by Kempenaers et al. [1], who report that a small but non-negligible fraction of tracked males remained at a single site for the entire breeding period. Such individuals are therefore not anomalies but fit squarely within a recurrent, long-standing pattern of spatial fidelity documented over six decades of research on the species. In evolutionary terms, these stationary males are not exceptions. They represent the fitness peak of the lek system’s despotic structure, where only a few males maintain the spatial and social advantages necessary to achieve high mating success.

3.2. Why Mobility and Fidelity Must Coexist in a Conditional System

The coexistence of highly mobile males and highly sedentary males follows directly from ecological and social principles. Males who face unfavourable conditions like limited female attendance, social harassment by dominants, or deteriorating site quality should explore alternative opportunities [5]. Conversely, males who find themselves in advantageous positions have strong incentives to remain. Conditional strategies therefore generate a bimodal or broadly distributed behavioural spectrum, not a uniform pattern [5].

The telemetry results of Kempenaers et al. are entirely consistent with this logic. Reporting that the median number of sites visited was 11 does not imply behavioural homogeneity; it expresses the central tendency of a heterogeneous distribution. While the median number of sites visited during a breeding season was 11, the distribution of site use was indeed far from uniform: males visited as few as 1 and up to 23 distinct potential sites within a single season, and the duration of tenure at individual sites spanned from about 0.02 days (~30 min) to ~41.7 days, with a median tenure of 2 days. This wide range of both number of sites visited and site tenures illustrates a highly heterogeneous spatial behaviour. The presence of individuals visiting only 1 or 2 sites with site tenures exceeding 40 days testifies to the existence of alternative stable tactics consistent with the broader evolutionary ecology of the species.

4. Movement Ecology: Mechanistic Insights into Spatial Tactics

4.1. Internal States and External Drivers

The movement ecology paradigm [13] allows us to situate the spatial behaviour of male ruffs within a mechanistic framework in which movement arises from the joint action of internal state (energetic reserves, hormonal condition, competitive ability), navigation and motion capacities, and external ecological drivers such as habitat structure, resource distribution, conspecific density, and female availability. Crucially, this framework predicts that movement is not a uniform species-level trait but the emergent outcome of individual-specific combinations of state, information, and ecological opportunity.

This conceptual lens directly clarifies the patterns reported by Kempenaers et al. [1]. Their telemetry data show that movement decisions may be closely linked to the shifting distribution of fertile females, an iconic example of an external driver shaping movement trajectories. Such dynamics fit squarely within movement ecology: males should remain resident when expected reproductive payoff is high and relocate when that payoff declines. At the same time, the pronounced among-individual variation documented in [1], with some males moving extensively and others remaining stationary for over 40 days, illustrates the role of heterogeneity in internal states and decision rules. Variation in energetic condition, dominance status, hormonal milieu, experience, or information about the landscape likely alters how each individual integrates external cues into movement decisions [13]. In this sense, the movement ecology paradigm not only accommodates but predicts the wide behavioural spectrum revealed by the telemetry data.

4.2. State-Dependent Decisions and Ecological Predictability

State-dependent behavioural models developed by McNamara & Houston [14] provide the natural extension of the movement ecology framework by specifying how individuals integrate internal state, environmental cues, and expectations of future payoff into concrete decisions. These models predict that even minimal differences among individuals—such as variation in energetic condition, dominance rank, hormone levels, prior social investment, or informational reliability—will lead to divergent behavioural trajectories, even under superficially similar external conditions.

The dataset in [1] illustrates this principle with striking clarity. Some males persisted at a site even when indicators of local opportunity were declining—for example, when the number of fertile females decreased or when conspecific competition intensified. Such persistence is expected when an individual’s internal valuation of the site remains high: males that have accumulated social capital, hold advantageous positions within the lek, or have invested substantially in residency may experience a high expected payoff from staying, either because their prior success increases the likelihood of future success or because the costs of relocating outweigh potential gains elsewhere [5]. Conversely, other males departed rapidly after similar declines, consistent with different expectations of future payoff, lower social status, fewer accrued benefits, or a more risk-prone tactic [5].

This spectrum, from prolonged fidelity to repeated relocation, is therefore not an anomaly requiring reconciliation but the predicted signature of state-dependent decision-making under uncertainty. Movement heterogeneity arises naturally because individuals differ in their internal state and in the decision thresholds that translate environmental change into action [5,13,14]. The telemetry data thus do not merely align with state-dependent theory: they empirically demonstrate its core prediction that identical ecological conditions can elicit distinct, yet equally adaptive, behavioural responses.

4.3. The Role of Environmental Heterogeneity

The northern European breeding landscape of ruffs consists of ephemeral wetlands, shallow flooded meadows, and seasonally inundated grasslands whose suitability can shift dramatically within days and between years [10,11]. This volatility is well documented for freshwater meadow systems, where precipitation, water-table fluctuations, early-season vegetation growth, and grazing pressure alter habitat structure at very short intervals. Such systems are therefore classic examples of temporally stochastic habitats, where the profitability of micro-sites is expected to vary rapidly and unpredictably.

Within this ecological context, the ideal despotic distribution (IDD) [15] offers a mechanistic basis for predicting how males should distribute themselves. The IDD explicitly states that dominant individuals occupy the highest-quality patches, while subordinates are displaced to inferior or degraded sites. When habitat quality is temporally unstable, as is typical of ephemeral wetlands, the IDD further predicts frequent positional reshuffling as individuals respond to shifting resource landscapes. This provides a formal explanation for why movement, both within and between years, sometimes extensive, should be common in a lekking species breeding in such habitats.

Lekking theory reinforces this expectation. Studies of ruffs and other lekking birds show that mating success depends heavily on local female attendance, visibility, and social positioning, all of which are sensitive to environmental change [8,16]. When a lek’s microhabitat deteriorates, through vegetation growth, water-level changes, predator intrusion, or a decline in fertile female availability, the expected reproductive payoff decreases. Under such conditions, theory predicts that dominant males may remain if they can still monopolize a local advantage, whereas subordinates should be more likely to relocate in search of a display site offering a more favourable combination of competition and female visitation [8,14]. These predictions follow directly from frequency-dependent selection and payoff asymmetry, two cornerstones of lekking theory.

Movement ecology [13] provides the final mechanistic layer by formalizing how organisms integrate internal state, motion and navigation capacities, and environmental drivers when updating movement decisions. In dynamic environments, this framework predicts that individuals must repeatedly reassess habitat suitability and modify their spatial behaviour accordingly; movement is therefore not a mere by-product of habitat change but a functional adaptation to spatiotemporal heterogeneity. This also clarifies why males should monitor changes in habitat structure, female distribution, and social competition: all three directly determine the expected reproductive payoff of a display site. Dominant individuals may remain faithful to a site as long as its local conditions continue to yield high expected returns, particularly when accumulated social capital or positional advantages buffer them against short-term fluctuations. In contrast, marginal males, whose payoffs are lower and more sensitive to competition are predicted by state-dependent decision rules [14] to relocate once the expected value of staying falls below that of exploring alternative sites. Mobility and fidelity are thus not mutually exclusive behavioural modes but context-dependent expressions of the same decision-making architecture. Telemetry results [1] do not contradict earlier observations of fidelity; rather, they provide the temporal resolution needed to reveal how individuals shift along the continuum of available tactics as internal state and environmental drivers fluctuate.

Seen through these combined lenses, the patterns reported in [1] are not unexpected. Extensive movements by many males are consistent with the necessity of tracking rapidly shifting ecological and social conditions, while prolonged residency among a minority reflects situations in which a site remains sufficiently profitable for high-status males to maintain a stable position. Movement thus emerges simultaneously as a requirement imposed by the ecological context and as an expression of behavioural plasticity shaped by competitive asymmetries, environmental stochasticity, and individual state.

5. Revisiting the Telemetry Data: Behavioural Diversity, Not Homogeneity

Kempenaers et al.’s dataset [1] contains crucial signals about behavioural diversity. The range from 1 to 23 sites visited is itself highly informative. This range reflects not random noise but structured, ecologically meaningful variation. Individuals at the lower end represent resident tactics, while those at the upper end represent exploratory tactics. The median value (11) reflects neither dominance nor typicality; it merely summarizes a highly heterogeneous distribution, with individual males visiting as few as 1 and up to 23 sites during a single breeding season.

Moreover, the presence of males who remained at a single site for over 40 days, as well as the between-year fidelity to the same site mentioned above, underscores the persistence of residency as a viable tactic. These individuals likely represent those who gained early access to favourable conditions, perhaps advantageous social positions or high female attendance, and thus followed rules predicting that staying maximizes expected fitness. Fidelity, therefore, is unlikely to be a remnant of a traditional lek system but likely reflects a continuing, adaptive behavioural tactic.

The diversity documented in [1] aligns closely with the synthesis proposed by Baguette et al. [7], who emphasized that behavioural variance is not peripheral but integral to understanding how ruffs navigate mating opportunities. Their analysis highlighted the multiplicity of decision-making pathways made available by the species’ morphological and behavioural polymorphism. The telemetry data in [1] reinforce this view, demonstrating that the species expresses a continuum of spatial tactics shaped by the interplay of fixed genetic architecture and flexible decision rules.

6. Conclusions: Recognizing Movement Diversity as the Core of Ruff Reproductive Behaviour

The synthesis of telemetry data [1] and behavioural ecology leads to a coherent understanding. All males share a conditional reproductive strategy in which spatial behaviour is context-dependent. Movement is not a fixed trait but a decision layered upon morphological and social constraints. Fidelity reflects favourable conditions rather than behavioural rigidity. Mobility reflects uncertainty, competitive displacement, or environmental change rather than a species-wide predisposition.

The ruff mating system therefore exemplifies how inherited strategies and conditional tactics jointly shape behavioural diversity. Telemetry data enrich this picture by revealing fine-scale spatial patterns, but their interpretation must remain aligned with ecological and evolutionary theory. The extensive movements documented in [1] provide important new insights into the spatial ecology of male ruffs. Yet these movements represent only one facet of a complex behavioural system defined by polymorphism, conditionality, and ecological contingency. When framed within the broader theoretical and empirical context, it becomes clear that ruffs do not express a homogeneous reproductive mode. Instead, they exhibit a rich diversity of spatial tactics, from prolonged residency to extensive sampling, each adaptive under specific ecological and social conditions.

Movement in ruffs is thus best understood not as a uniform species-level trait but as one expression of a dynamic, context-sensitive strategy. Recognizing this diversity enriches our understanding of behavioural evolution, the ecology of leks, and the interplay between genetic and environmental influences on mating systems. The telemetry data [1] do not diminish the complexity of ruff behaviour; they deepen it, providing an empirical foundation for interpreting movement as part of a broader repertoire of reproductive tactics shaped by ecological and evolutionary pressures.