Figure 2. 3D scan energy at different spatial scales relative to clutch size: x1/8, x1/4, x1/2, x1, x2, x4. The left-hand side shows the original height map and associated ROIs: eggs, clutch, rim and background. More elevated (positive) regions are shown as lighter, while less elevated regions are shown as darker. The right-hand images show the separated spatial scales with the position of the clutch marked by a black arrow. The eggs are most visible at x1/4 scale, the scrape at x1 scale and the nest elevation at x4 scale.

Clutch Occlusion & Visibility:

For each depth map, occlusion maps were created for 16 different observer orientations around the azimuth of the nest, from 0o-337.5o, in 22.5o intervals. To calculate occlusion, for each pixel in the clutch, the shallowest/minimum elevation angle (mA) that allowed it to be un-occluded was calculated from each of the 16 bearings, given the 3D depth profile, see Figure 1. Elevation angles above a pixel’s mA allow that pixel to be visible. For observer elevation angles between 0.50 - 600we measured the visibility. The visibility at a given polar angle was equal to the mean percentage of pixels un-occluded across the 16 bearings. The distance required to achieve viewing angles was calculated at fox height [0.4m] and a matrix of corvid flight heights [1.6m, 3.2m, 6.4m, 12.8m, 25.6m].

Colour Metrics:

Luminance ΔS and colour ΔS (JND) from the local (nest) and distal (background) surrounding each clutch was modelled for corvid vision and fox vision as a metric of camouflage, using the mica toolbox (Jacobs et al., 1993; Martin, 2017; Moher Alsady et al., 2016; Vorobyev and Osorio, 1998). Where higher ΔS values correspond with a poorer match. The Siddiqi method was used for ΔS luminance (Weber fraction 0.2) and RNL model for ΔS colour (Weber fraction of most abundant cone of 0.05) (Lind et al., 2013; Moher Alsady et al., 2016; Pretterer et al., 2004; Siddiqi et al., 2004; Vorobyev and Osorio, 1998). For each observer, we used the most phylogenetically relevant systems known. These were the common peafowl Pavo cristatus, for the corvid vision, and the red foxVulpes vulpes , for the fox vision (Jacobs et al., 1993; Malkemper, 2014; Malkemper and Peichl, 2018; Ödeen and Håstad, 2013).
ΔS values were measured for images acuity corrected for the hypotenuse distance required for a given series of polar viewing angles [1.875o, 2.5o, 3.75o, 5o, 7.5o, 10o, 15o, 20o, 30o and 40o], when at the height of the model observers (fox 0.4m, corvid 3.2m) (van den Berg et al., 2020). The polar viewing angles for corvid vision were adjusted post-hoc to the matrix of values used for clutch occlusion [1.6, 3.2, 6.4, 12.8 and 25.6 m], by calculating the polar angle and horizontal distance required to produce the same observer distance. Acuity correction was carried out using the known peak resolving power, magpie Pica pica 33.33 cp/d and red fox Vulpes vulpes 8 cp/d (Malkemper, 2014; Martin, 2017).