Two different views of rainforest habitats in Queensland, Australia, showing the difference in leaf contrast seen through the ultraviolet channel (VMS, top) and the green channel (LMS, bottom) of a multispectral camera used to mimic avian vision. [Image: Nat. Commun., doi: 10.1038/s41467-018-08142-5, licensed under http://creativecommons.org/licenses/by/4.0/]
Birds, like most animals with color vision (but unlike humans), can see UV wavelengths of light in addition to red, green and blue. However, not much is known about the role UV vision plays in helping these animals navigate their environments. Now, researchers from the Lund Vision Group at Lund University, Sweden, say that they have designed a multispectral camera with UV, red, blue and green channels to study how UV vision creates contrast, which helps birds see details in their monochromatic leafy habitats (Nat. Commun., doi: 10.1038/s41467-018-08142-5).
The researchers say that their multispectral camera is the first that allows for the in situ observation of different environments through different color channels. They hope that the camera, coupled with a new optical model, will help reveal more about the role of specular reflection and adaptive coloration in vision studies not only in birds, but in other animals as well.
Bird’s-eye view
Lund researchers Cynthia Tedore and Dan-Eric Nilsson developed a multispectral camera with rotating red-, green-, blue- and UV-channel filters that mimic the four different types of color-vision cones in a bird’s retina. They took their camera into three avian habitats that ranged from sparsely vegetated to densely vegetated to study how UV-cone spectral sensitivities, compared to red, blue and green sensitivities, help birds capture a more detailed picture of their environment.
Tedore and Nilsson found that it was difficult to distinguish one leaf from another when they viewed the habitats through the camera’s green channel, because both sides of a leaf transmit and reflect nearly the same amount of green light. However, when they viewed the same habitats through the UV channel, they discovered that most of the UV wavelengths that hit the waxy top of a leaf are reflected back in a single direction, making the surface appear lighter than the textured underside.
The researchers say that the contrast created by this UV specular reflection adds more depth and detail to a visual field filled with green. That, they suggest, makes it easier for birds to find flight paths, food and ideal locations for nests.
Specular reflection and vision systems
The Lund team concluded that UV specular reflection—and not the more commonly studied diffuse reflection—is what enhances structural details in a bird’s vision system, allowing them to better navigate their vegetated habitats.
Using data from their multispectral camera studies, Tedore and Nilsson developed an optical model to calculate how UV specular reflections could shift depending on the density of the vegetated environment. Their calculations showed that environments with thick, closed canopies would favor birds (or other animals) with vision systems containing shorter-wavelength UV photoreceptors, and environments with more open spaces favored birds with vision systems spectrally tuned to longer UV wavelengths.