Neoptile feathers contribute to outline concealment of precocial chicks

Veronika A Rohr,Dirk Metzler,Tamara Volkmer,Clemens Küpper

doi:10.1038/s41598-021-84227-4

Abstract

Camouflage is a widespread strategy to increase survival. The cryptic plumage colouration of precocial chicks improves camouflage often through disruptive colouration. Here, we examine whether and how fringed neoptile feathers conceal the outline of chicks. We first conducted a digital experiment to test two potential mechanisms for outline concealment through appendages: (1) reduction of edge intensity and (2) luminance transition. Local Edge Intensity Analysis showed that appendages decreased edge intensity whereas a mean luminance comparison revealed that the appendages created an intermediate transition zone to conceal the object’s outline. For edge intensity, the outline diffusion was strongest for a vision system with low spatial acuity, which is characteristic of many mammalian chick predators. We then analysed photographs of young snowy plover (Charadrius nivosus) chicks to examine whether feathers increase outline concealment in a natural setting. Consistent with better camouflage, the outline of digitally cropped chicks with protruding feathers showed lower edge intensities than the outline of chicks without those feathers. However, the observed mean luminance changes did not indicate better concealment. Taken together, our results suggest that thin skin appendages such as neoptile feathers improve camouflage. As skin appendages are widespread, this mechanism may apply to many organisms.

Highlights

Background complexity and spatial acuityIntroducing background complexity (Scenario 4) resulted in similar mean edge intensities of the HEI pixels for 256 appendages as in the basic scenario for large squares
For the object without appendages, 1.59% corresponded to pixels that made up a distinct modal area with a mean edge intensity of 2.7 (Fig. 1a, ‘0’)
Adding appendages reduced the mean edge intensities of the HEI pixels with the lowest mean edge intensity reached in the image with an intermediate number of 256 appendages (Fig. 1a,b)

Summary

Introduction

Introducing background complexity (Scenario 4) resulted in similar mean edge intensities of the HEI pixels for 256 appendages as in the basic scenario for large squares. The region of interest (ROI) on the background with small squares showed slightly higher mean edge intensities for the HEI pixels than for the background with large squares. More appendages did not lead to such a pronounced increase of mean edge intensities as in the basic scenario (Fig. 1e). At a spatial acuity of 10 cpd, the minimum mean edge intensity of the HEI pixels in the image with 256 appendages was only half of the value obtained in the basic scenario (Fig. 1f). Background complexity and spatial acuity did not affect the mean luminance of the appendage region

Methods

Results

Discussion

Conclusion