The excavated nests were scanned with a medical x-ray CT scanner (Somatom Sensation16, Siemens, Erlangen, Germany) with 120-kV and 150-mA settings and with a voxel size of 0.55 × 0.55 × 0.6 mm and 0.58 × 0.58 × 0.3 mm for the Senegal and Guinea nests, respectively. The tomographic images were filtered with a nonlocal means edge-preserving filter, followed by their segmentation into two phases (solid and air) with a seeded watershed algorithm based on the grayscale gradient and grayscale intensity of each voxel using Avizo-9 software. The segmented image was used for three-dimensional visualization using Avizo-9 software. To estimate the thickness of inner solid walls and channels, the solid phase was first isolated. A sequential dilation of 12 pixels and an erosion of 32 pixels were applied on the solid phase of the Senegal nest, whereas a sequential dilation of 20 pixels and an erosion of 50 pixels were applied on the solid phase of the Guinea nest. The resultant image was applied as a mask on all the channels, which isolated the internal channels by removing the channels near the outer wall (Fig. 2). The reason for using a sequential dilation and erosion mask was to analyze the thickness profiles away from the outer walls and to focus on the internal features. We then estimated the width of these isolated inner channels using ImageJ software, which is based on the Euclidean distance and medial axis. Similarly, we estimated the thickness of the inner solids (Fig. 2). The thickness of the outer wall was measured manually from the tomographic images at more than 300 points across the entire outer wall of the nests.

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