The functional significance of the dorso-ventral gradient in plumage pigmentation (‘countershading’) in seabirds has remained unclear since the topic first received interest over 100 years ago. A range of hypotheses have been proposed to explain this, including thermoregulation, abrasion resistance, camouflage and social signalling; however, in no case has a firm conclusion been reached. The aim of this thesis was to ascertain if white ventral coloration could function as hunting camouflage in marine birds, and under which environmental conditions would white plumage be least visible against the sky, from the perspective of aquatic prey. I recorded videos of white spherical targets suspended above, and resting on, the water surface from an underwater perspective. I measured the luminance contrast ratios between the target and the background (sky) and compared ratios across different environmental conditions. I also executed two detection experiments with human participants: one in a realistic simulated pelagic environment, and one where participants were shown frames from the videos taken in the field. Of the video frames taken when it was sunny, 79% had targets with low visibility (<5% luminance contrast), compared to only 0.3% when it was cloudy. More than 90% of frames taken when it was sunny and windy had low visibility targets. In 23% of the frames analysed the target was totally undetectable; wind then cloud cover explained the most variance in undetectability. In the detection experiments, participants generally took longer to detect targets and were less accurate when it was sunny compared to cloudy. Direct sunlight causes glare, obscuring nearby objects from view, potentially causing undetectability of targets. Higher wind speed causes increased surface rippling and wave height, which affects refraction at the water surface and can lead to distortion of the above water image. Contrary to the conclusions in published papers using light-field modelling, white ventral plumage can potentially conceal seabirds from their prey under some conditions.
Countershading in Seabirds
Sands, D. J. (Author). 29 Sept 2020
Student thesis: Master's Thesis › Master of Science by Research (MScR)