1 INTRODUCTION

Camouflage is one of the most common anti-predator strategies exhibited by animals, as reducing the ability of predators to detect or distinguish a target from its background reduces the risk of predation (Cott, 1940; Cuthill, 2019; Endler, 1981). Ground-nesting birds are no exception to this, with many of their species exhibiting camouflage at various phases in their life history (Stevens et al., 2017, 2017; Stoddard et al., 2016). One notable phase where camouflage has evolved is that of the egg (Kilner, 2006; Westmoreland, 2008). The comparative openness and accessibility of ground-nesting wader nests, such as coursers (Cursoriinae) and plovers (Charadriinae), renders them particularly vulnerable to predation. When predators approach, adults abandon their nests (Blumstein, 2003; Wilson-Aggarwal et al., 2016), relying on the patterns of their eggs to camouflage them while the parent(s) harass or distract the predator (Armstrong, 1954; Simmons, 1951). The nests of species that rely more on the parents’ plumage for camouflage have been shown to have less camouflaged eggs. Whereas in other ground-nesting species the eggs can be occluded by either burying them, or by relying on vegetation from surrounding hedgerows, scrub or forest understory (Bravo et al., 2022; Masero et al., 2012; Stevens et al., 2017; Stoddard et al., 2011; Troscianko et al., 2016a). Occlusion by natural structures is arguably one of the most effective forms of camouflage (Troscianko et al., 2016a). Partial occlusion can mask important visual cues for detection and recognition such as an object’s outline, size and identifiable morphological features (limbs, eyes, etc) (DiPietro et al., 2002; Sharman et al., 2018; Sovrano and Bisazza, 2008; Tvardíková and Fuchs, 2010). While total occlusion forces observers to rely on other sensory cues to detect the occluded object, providing that the source of occlusion isn’t also recognisable e.g., nesting material or incubating parent (Bailey et al., 2015; Broughton and Parry, 2019; Stevens et al., 2017).
Occlusion isn’t without costs. The openness of the nests of ground-nesting birds is thought to be a balance between multiple trade-offs. Nest predation, parent predation, thermoregulation and both habitat and nesting material availability all contribute to the appearance of ground-nesting bird nests (Gillis et al., 2012; Kubelka et al., 2019; Mainwaring et al., 2014; Stevens et al., 2017; Swaisgood et al., 2018). Local vegetation height has been shown to influence nest site selection and predation risk of ground-nesting birds; taller vegetation results in shorter flushing distances, higher nest survival and greater parent predation risk (Bertholdt et al., 2017; Gómez-Serrano & López-López, 2014). A study using periscopes to assess the visibility of dogs and humans from the perspective of nesting Kentish plovers (Charadrius alexandrines) showed that sites selected by parents offered greater predator visibility at the cost of increased nest predation risk (Gómez-Serrano and López-López, 2014).
When measuring nest camouflage, the visual ecology of the observing parents and predators should be considered. Visual modelling using colour-calibrated images has increasingly been used to assess animal camouflage from different visual systems, accounting for differences in observer colour reception and spatial acuity (Caves et al., 2018; Maia et al., 2013; van den Berg et al., 2020). These measures have been used to show that camouflage from local background pattern match can predict nest survival in ground-nesting birds (Troscianko et al., 2016b). However, an aspect of visual ecology rarely considered is predator height in combination with distance and habitat structure. The height of an animal’s eye relative to its object of interest changes the angles and distances required for the object to be occluded by surrounding structures (Martin, 2011). A nest that appears exposed from a human height may be undetectable to a smaller mammalian predator even at closer distances, while an avian predator excluded to the edge of a field by harassing parents may be at too great a distance to detect a clutch of eggs (Gómez-Serrano and López-López, 2014). Microhabitat selection likely helps balance the trade-offs between predator and nest visibility (Gómez-Serrano and López-López, 2014; Lovell et al., 2013; Stoddard et al., 2016). By selecting areas of surrounding local elevation ground-nesting birds should be able to increase visibility of predators. Combined with the depression of the scrape, local elevation should paradoxically decrease nest visibility; requiring a greater viewing angle to be seen unobstructed by approaching predators.
Ground-nesting waders are in decline across their range due to habitat loss, agricultural intensification, reduced prey availability and elevated predation risk from mesopredators, such as foxes, mustelids, corvids and raptors (Evans, 2004; Galbraith, 1988; Roos et al., 2018; Vickery et al., 2004). Consequently, mechanisms of further understanding the habitat features that both encourage nesting and minimise predation are of increasing conservation interest, as predation is typically the leading cause of nest mortality (Baines, 1990; Ricklefs, 1969; Teunissen et al., 2008). Just as camera quality has advanced colour analyses of visual scenes, increasing accessibility of terrestrial and aerial 3D scanners allow for the measurement of topography and vegetation structure at different spatial scales (de Vries et al., 2021; Hill et al., 2014; Li et al., 2022). Terrestrial scanners have even been used to compare the volume and shape of bowl nesting birds, though these were taken in vitro (Simonov and Matantseva, 2020). 3D scanning allows for a more complete measure of local 3D composition than more traditional chart and ruler based measurements of vegetation height and cover (Gómez-Serrano and López-López, 2014; Gregg, 1991; Pendleton and Nickerson, 1951).
In this study, we used hand-held 3D scanners and colour-calibrated images to measure the shape and appearance of northern lapwing (Vanellus vanellus ) nests in pastoral, arable and wet grassland sites. The goal was to investigate how the 3D and colour environment influences lapwing nesting decisions. The methods of habitat management and local variation should also influence the colour and 3D composition of the habitat, changing the occlusion of nests, the number of distractive structures with similar 3D shape to the nests and the colour match of the nests. We hypothesised that lapwing should favour backgrounds of higher local elevation, greater surrounding 3D variation at scales relative to the size of their nests, and which are more obstructed from the perspectives of their predators. We also compared the distances where modelled occlusion and acuity influence detectability by predators and investigated whether background match and occlusion predict predation in lapwing. A full breakdown of the 3D scanning methods and scripts required is provided within our supplementary material, including methods for using photogrammetry generated point clouds in place of 3D scanners.