It’s not only beauty that varies with the eye of the beholder. Unlike humans, birds can detect ultraviolet (UV) light – and it may help them navigate forests by enhancing the contrast of the foliage, according to European researchers.
Cynthia Tedore from the University of Hamburg in Germany and Dan-Eric Nilsson from Lund University in Sweden used a multispectral camera and optical modelling to simulate the visual spectrum of birds.
They took 173 sets of six photos – each through different filters across the visible spectrum, with and without UV vision – from assorted leafy habitats with different lighting conditions in southern Sweden and Queensland, Australia.
As reported in the journal Nature Communications, they found the UV light amplified the contrast between lower and upper leaf surfaces, making the position and orientation of the leaves stand out more clearly.
Although upper and lower leaf surfaces both reflect small proportions of UV light, they suggest the strong contrast could be explained by lack of UV transmission through the leaves, and light shining down from the sky being brighter than light directed upward.
They also found that specular reflections – reflection of light waves from smooth surfaces – from the waxy leaf cuticle enhanced the degree of contrast.
Specular reflections shift the appearance of leaf colour, they explain, creating maximum leaf contrast at short UV wavelengths in open forest canopies and long UV wavelengths under closed canopies.
Silhouettes created by the contrast make leaves and the geometry of the vegetation more visible, which the authors propose could help birds navigate complex leafy environments, search for prey, lay eggs and find refuge.
Humans can see what we call the visible spectrum of colours ranging from red to violet – a wavelength range of 400-700 nanometres. Many animals, insects, fish and birds can see a range of 300-700 nanometres, which includes the shorter UV rays.
Although destructive to human eyes, this UV vision is understood to have many adaptive benefits, including navigating and orienteering, detecting food and predators, assessing mates and communicating.
Tedore and Nilsson propose that their work offers new insights, opening up “the possibility for a whole new realm of inquiry into the importance of specular reflections for the evolution of vision and colour patterns in natural habitats.”