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.