By Isabel Cantu
Many factors can drive a species to adapt. Of those in hotter climates with plenty of sunlight, thermoregulation is a top priority. Certain birds have developed a way to reflect near-infrared light, which is a much shorter wavelength of light than visible light. By reflecting it, birds stay cool and well-hydrated in places with exceptional heat. Considering how birds are especially vulnerable to heat, strategies like near-infrared reflection, or NIR reflection, make sense. In a study of 90 Australian bird species across the entire continent, the importance of this type of reflection is illustrated wonderfully. The ideal body temperature is tough to match when in an environment as hot and dry as the majority of Australia. In places with such high temperatures, many animals have to adapt with NIR reflectivity. Even across a single species, the rate of reflection can differ a lot depending on plumage, climate, and size.
At first, it might be expected that colors play a significant role in NIR reflectivity, but this has not been observed. Considering this targets infrared light, which cannot be seen, it makes sense that some species exhibit minimal color change while simultaneously showing extreme NIR reflectivity variation, as seen in the azure kingfisher. These birds do, however, show a difference in feather structure that seems to help reflect infrared light. Barb shape and barbule density are two aspects that have been observed to change. Barbs are the “branches” of a feather, growing out of the main shaft, whereas barbules grow out of barbs themselves. The rounder a feather’s barbs appear and the tighter the barbules are arranged, the more infrared light that will be reflected and prevented from reaching a bird’s skin. For smaller birds that are susceptible to overheating, this adaptation allows them to continue to hunt during the daytime in extreme heat.
NIR reflectivity has no connection to camouflage and does not serve to help with attracting mates, which explains why the colors we see in a species does not always indicate its level of reflectivity. However, there can be compromises. Birds may choose to sacrifice feathers with high NIR reflectivity in exchange for feathers that camouflage, but this isn’t common in places as hot as Australia. This is often seen in areas where infrared light isn’t as much of a problem, letting birds adapt with more visual characteristics.
In dry areas where water is scarce, reflecting as much infrared light as possible becomes even more of a priority. A study involving the hot Simpson Desert in Australia compared birds with a 40% NIR reflectivity and 10% reflectivity and saw that a significant amount of water was conserved in birds with plumage that could reflect more infrared light.
The impact an environment has on NIR reflectivity is not just present in birds. Sahara silver ants, for instance, have their distinctive color that helps reflect infrared light. Yet birds and their characteristics often show the most varied examples of how this reflectivity helps them survive. Temperature extremes and dry weather would otherwise kill many small, diurnal birds. By adapting to cut off the majority of infrared light reaching their bodies, a multitude of species have been able to thrive in some of the hottest places around.
No changes were made to the following image: https://commons.wikimedia.org/wiki/File:Azure_Kingfisher_-_Flickr_-_GregTheBusker.jpg, License: Creative Commons Legal Code
Q. Why is NIR reflectivity a great adaptation for birds?
1. Birds, especially smaller ones, are very susceptible to overheating. This is mostly the result of the size of their bodies and the way they reserve water, but dehydration and excessive heat can come quickly in a harsh environment. NIR reflectivity keeps a lot of this heat-related stress away from a bird.
Q. Is color associated with NIR reflectivity?
2. Color and NIR reflectivity are not correlated, but birds may adapt to have more of one than another depending on what is a priority in their environment. Birds in hot climates will almost certainly exhibit high levels of infrared reflectivity, whereas birds in more stable climates might opt for brighter colors for mating or dull colors for camouflage.
Q. Does feather structure impact NIR reflectivity?
3. Feather structure has a huge impact on how much infrared light a bird can reflect, barbs and barbules especially. It’s all about how much light can reach the skin—too much and a bird risks dying from the heat or dehydration. A tight arrangement of barbules and rounded barbs is a very effective structure to ward off excess heat.