Behavioural plasticity at the spatial-social interface: Predation risk modulates density-dependent breeding dispersion

The spatial organisation of breeding populations fundamentally shapes individual fitness and scales up to influence broader population dynamics. While spatial patterns are often attributed to passive habitat filtering, they may alternatively emerge from active behavioural plasticity at the spatial-social interface, governed by density-dependent trade-offs. To test these competing mechanisms, we leveraged long-term, large-scale monitoring data of a loosely social, ground-nesting wader, the northern lapwing (Vanellus vanellus), across human-modified agricultural landscapes. We quantified fine-scale spatial breeding dispersion using a novel segment-count metric, the Normalised Spatial Dispersion Index (NSDI), and applied generalised additive mixed models (GAMMs) to evaluate the relative influence of local conspecific density, avian predation risk, habitat composition, and pre-breeding winter weather. We found no support for passive habitat filtering; broad-scale habitat composition did not predict spatial dispersion. Instead, spatial organisation was primarily driven by density-dependent biotic interactions. Under low predation risk, lapwings dispersed as local density increased, mitigating intraspecific competition. However, under high predation risk, this relationship inverted: lapwings aggregated at high local densities to enable communal defence, but dispersed at low densities to minimize nest detectability. Furthermore, warmer pre-breeding winter temperatures drove spatial clustering, likely by prematurely drying soils and forcing individuals into restricted micro-habitat bottlenecks. Our results demonstrate that spatial breeding strategies in loosely social species are highly plastic and strictly context-dependent. This plasticity dictates that group-level component Allee effects, such as the failure of communal defence, emerge dynamically rather than as fixed demographic traits. Conservation monitoring must account for this behavioural flexibility, as localized population collapses may accelerate if climate-driven habitat bottlenecks force aggregations under intense predation pressure. Integrating the spatial-social interface with Allee effect theory is therefore critical for understanding species persistence in changing landscapes.