Owl Adaptations
Owls have always held a special spot of interest for people, both birders and nonbirders alike. As they should, this group of birds can be elusive, secretive, adding to their allure. Many are nocturnal, making them a special unexpected surprise for those who don't purposely seek them out.
Owls have evolved so many unique adaptations to suit their lifestyle, the need to be able to navigate in almost pitch black darkness, to hunt in darkness, to catch enough prey to survive but also to provide enough for their young. To do so, at the very most basic aspect, they must be able to see trees and obstacles and avoid colliding with them. To successfully hunt mice, small mammals and other prey, they must be able to locate them in a field, forest or under a foot of snow, they must be able to pinpoint the exact location, and then pounce, without their intended prey knowing of the impending ambush. While this may sound like a simple task, it's not, and owls have evolved a handle of adaptations to allow them to survive and thrive under this lifestyle. So let's explore the world of owl adaptations.
While owls are predators, they aren't immune to becoming prey themselves, especially if they are asleep and vulnerable to predation during the daylight hours, especially with it being a time when most predatory animals are active, like hawks for example. To help keep themselves safe during this time, owls have evolved some of the most amazing cryptically patterned plumage. Mottled patterns of whites, grays, browns and blacks along their feathers help them look nearly identical to the bark on many trees to the point where they nearly disappear when they are perched against a tree's trunk. You can really get the feel of this amazing camouflage when you look at the back of species such as the Great Horned Owl, Screech Owls or a Great Gray Owl. Even the diurnal snowy owl blends perfectly with its arctic home. Its clean white plumage with varying amounts of black streaking helps it blend into the snow and rocky ground it may be perched on. Barn owls have a light brown coloring on their back which blends perfectly in their grassland and field hunting grounds. Various species are different colors depending on their prefered habitat, their ability to blend into their environment is truly amazing.
Several species also have special adaptations to help break up their body shape as a bonus. Look at the top of the head of several different owl species and you will notice two small clumps of a few feathers there, looking similar to having ears. About 50 species of owls are known to have these ear tufts, which are not actually ears, but clumps of small feathers that can be held erect or payed flat against their head. While they don’t aid in hearing, instead, they serve several other different purposes. One being that these tufts help break up the owl silhouette and allow them to blend into their surroundings even more, looking similar to branches or twigs. On top of that, these feather tufts can be raised or lowered, making them a way to communicate to other owls, to help make themselves look bigger, a bigger threat, to other owls or potential predators. To communicate with other owls, whether that is to communicate to a potential rival, to let them know that this territory is already occupied or courtship with a potential mate, many owls also have another way to communicate. That comes in the shape of a white patch of feather on their throat, below their bills, often hidden until needed. This group of white feathers is called the gular pouch, while normally hidden, when hooting or calling this pouch is in a way inflated and becomes quite visible, especially when the owl itself is typically a darker shade of gray or brown. In the dark, this contract of color really stands out and is used as another method to communicate with other nearby owls.
Camouflage and being able to blend into their surroundings isn't the only key to an owl's success. To survive, they must be able to successfully hunt at night, in near total darkness. One way to successfully catch prey at night is to have incredible eyesight. One look at an owl and you will notice that their eyes are particularly large for their head size, especially if you compare them to other birds like hawks or eagles, which themselves have incredible eyesight. If our eyes were proportional to the same size as an owls, ours would be the size of a softball! Another comparison of owls eyes vs ours is that an owl's eyes make up to 5% of an owl's body weight, while our eyes are only about .0003 percent of our body weight. Fun fact, since owl eyes are so big, if you peaked into the ears of some species, you would actually be able to see the back of their eyes!
Another cool thing about an owl's eye is that generally, if you look at its color, you can tell what time of day that species is going to be most active. Though this is not true in all cases. If their eyes are brown, like Barred Owls or Barn Owls, they are most likely to be nocturnal, or active at night, if they are yellow or orange, they are most likely to be diurnal, active during the day, or crepuscular, most active at dawn or dusk. It is believed that by having dark eyes as a nocturnal bird, that it helps with their camouflage and being able to go unnoticed by both predators and prey.
But an owl's eyes differ from many birds in even the most simple of ways, it’s shape. While most eyes are round like a ball, owls' eyes are more of a tube shape, and inside these eyes are two types of photoreceptors: rods and cones. Rods are extremely sensitive and are important for seeing in low light conditions, while cones need much more light to be useful and are responsible for being able to see colors. Owls have a rod to cone ratio of about 30 to one, while humans have only about 20 rods for every cone. Most diurnal bird species, meaning those active during the day, like hawks, eagles and falcons, have two fovea, which are areas of the retina that are associated with high photoreceptor density which then translates to high visual acuity. Owls on the other hand, only have one fovea with a high concentration of rods, this combination leads them to have excellent vision in extremely low conditions but at the expense of having great color vision.
Owls also have another neat trick to make use of every tiny trace of light in their environment. On the back of the inside of their eyes, is a layer called the tapetum lucidum. This layer can catch any light that passes through the retina and bounces it back to the rods in its eyes that we talked about earlier, allowing them to see in near total darkness. It is believed that some owl species can be up to 100 times more sensitive to low light than us.
Their eyes are also forward facing, perfect for tracking movement and successfully catching prey. Where the field of view overlaps on both eyes, they have binocular vision, having a large binocular vision field of view can be seen in many predatory species. This perception gives them a 3-dimensional view of their world, allowing them to better gauge of distance, and depth perception. This field of view for owls is about 110 degrees, with 70 of that being binocular vision.
To have room for this great vision, owls need to have very large eyes. This leads to several other problems though. To keep these large eyes in the correct spot and position in the skull, they are held in place by bony structures called sclerotic rings. These sclerotic rings, though, don't allow their eyes any room to move. Their eyes are, for all purposes, locked into their skulls, to look left or right, up or down, they literally have to move their head in that direction.
This leads to an issue with them possibly having a narrow field of view, relying completely on how far in each direction an owl can turn its head. Owls, again, have come up with an amazing way to fix this issue.
So how do they get around this limited field of view, with a special skeletal structure in their neck! While us humans have seven neck bones, owls have 14! The extra bones allow them a greater range of motion, up to 270 degrees each way. Humans, for example, can only turn our heads about 90 degrees each way. On top of that, the neck bones at the base of their neck have special adaptations. If we somehow were able to turn our heads to that degree, we would have a big issue, our bones would punch blood vessels and block blood from getting to and from our brain. So how do owls get around this issue? One way is that their carotid artery, instead of being on the side of the neck, like us, are actually located just in front of the spine. By being there, they experience much less twisting whenever they turn their neck. On top of this, there is extra space for the arteries, the neck bones have larger cavities for them to pass through. While humans this area is just big enough for them to through, in owls, this cavity is about 10x bigger than the space that is taken up by the arteries. In addition, in owls, this space seems to also be filled with an air sac to further protect these vulnerable blood supply. Combined, these features allow a proper blood supply to their head, even when it’s turned 270 degrees one way, without damage.
Another recent discovery is that the carotid arteries on owls have wide segments in them just under the base of their skull. Researchers have found that these segments could dilate and fill with blood, creating a form of reservoir for blood to avoid disruptions in its flow.
Another important sense that owls have is their hearing. Birds, unlike most other animals, don't have external ears. Instead, they had what's more like holes or slits in the side of their skull. Each slit that makes up the opening of their ears is a flap of skin, that is called an ear conch, which then opens up into a large ear canal. The flap over the ear can open or close by using the muscles that are located under the ring of feathers that are around their face.
To help enhance their hearing even more, the feathers on an owl's face form a facial disk. These feathers around their eyes act as a bit of a satellite dish, funneling sounds towards their ear openings. Even their bill is in a form that helps direct sound to their ears, by being short and pointed downwards, it actually increases the surface area where sound waves get collected by their facial disk.
While some owl species have these ear openings symmetrically on their head, like our ears are directly across from each other, some owl species have asymmetrically placed ears. This appears as one ear being higher up on one side of an owl's head and the other ear being lower on the head. This unique feature can be easily seen if you look at the skull of one of these owl species that have these asymmetrically placed ears. There is actually a big benefit for having this odd ear placement. This ear placement actually helps the owl judge to a greater extent of where a sound is coming from, especially on a vertical axis. For example, if an owl hears a possible prey species, like a mouse, even if it is under snow, the sound would reach one ear before the other, but if an owl continues to turn its head until the sound hits both ears at the same time, it would have pinpointed the exact location of where the sound was coming from. This amazing adaptation can greatly increase its success rate of catching its intended target. This can really be seen in nocturnal species like the Boreal Owl and Barn Owl. To properly translate how the sound is received and processed where it is coming from by creating a mental image, owls brains have a complex medulla, which is the part of the brain that is associated with hearing. For example, a Barn Owls medulla is believed to have at minimum 95,000 neurons, which is about three times as many as a crow has. A great demonstration of how important this sense is to owls.
If you look at what animals an owl typically prey on, you may notice that many have large ears, mice and rabbits being good examples. Nocturnal mammals also rely on a heightened sense of hearing to avoid predators while they are out and about at night. So for an owl to successfully catch these animals, they have to be able to hunt while not alerting potential prey to their presence. That leads to one of the adaptations that owls are most famous for, their silent flight. Owls proportionally have quite large wings for their bodies, they seem to fly almost effortlessly thanks to that and their light bodies, leading to less wind resistance going over their wings and giving them the ability to fly a lot slower. Their flight feathers are specially adapted to allow for this silent flight, with each having a fringe along the outer edge, called combs, and on the surface and trailing edge is a velvety fur-like texture. The leading fringe on these feathers soften the flow of air as it goes over the wings as the owl flies, breaking up the air that makes sounds as it goes over the feathers. The soft fur-like texture that covers the feathers helps to absorb and disrupt the noise from the air and that the feathers make as they move against each other. There is quite the amount of surface area of contact with each feather and the ones on either side of it, that friction between them would create quite the amount of noise if it wasn’t for these adaptations. Think about how noisy most birds are in flight when they fly overhead or nearby, even little finches can make quite the noise when they fly by you. This quiet flight also has another advantage, when they are swooping in for the catch, they aren’t creating any additional sounds that would interfere with the continuing to hear their potential prey, flying in completely silent, not alerting their prey until the moment they catch their target.
So what about owl species that primarily prey on species that don’t have good hearing, like fish or insects though? Studies have been done on the connection between an owl's wing combs compared to what their primary diet looks like. Fish and insect eating owls were found to have narrower combs, which means that these owls have less noise-canceling qualities to their feathers and wouldn’t be as silent as other mammal eating-owls. You don’t really need to be as quiet when you are hunting fish that are underwater or hunting insects that don’t have good hearring. Some species even have little to no comb structure on their flight feathers, like the Burrowing Owl or the Tawny Fish Owl.
Also, when you look at the noise-canceling structures on owls feathers, you will notice that many diurnal species, those active during the day, like Snowy Owls also have a much smaller comb size on their feathers than nocturnal owl species.
Another little adaptation I want to highlight is called a pectinate claw, this is present only in barn owls. This specific tool is usually located on the middle longest talon. This adaptation isn’t just found on barn owls though, it can also be seen on species such as herons, egrets, bitterns and nightjars. This claw is special in that it has ridges on the inner edge that look very similar to a comb. These ridges are believed to help with preening and straightening facial feathers. This is especially important for barn owls as their facial disk and the feathers on it help direct sound to their ears for hunting.
Overall, Owls are a perfect example of some of the unique adaptations that can be found in the world of birds, of how they survive and support their lifestyle of hunting at night. I hope you learned a few things about this amazing group of birds.
References:
https://www.bbc.com/news/science-environment-21279609.amp
https://www.owlresearchinstitute.org/adaptations
https://abcbirds.org/blog/owl-eyes/
https://www.audubon.org/news/the-silent-flight-owls-explained
This is so interesting, thank you! Biology is truly amazing.
Excellent, informative post. Thank you for sharing.