Well, Owl Be...

​We are wrapping up virtual owl pellet dissections with Easton’s third graders and it has been so awesome to explore all of the incredible ways owls are able to thrive in their environments! Many of us hear owls calling at night, but seeing an owl is much more uncommon. This is in part to do with all of the adaptations that allow them to know what is going on around them while being essentially undetectable by others. When looking at the physical structure of owls, everything about them is to help them to be successful predators.

​​An adaptation is something an animal has on its body or something it does that makes it better able to survive in its environment. The first thing to consider when thinking about adaptations is an animal's environment. Over time, animals have evolved to fit perfectly into specific ecological niches, so when we go through the adaptations of owls, we need to think about how physical characteristics impact their successfulness as predators in the areas where they are found. One of the most notable owl adaptations is their very large eyes! 

The assumption that many of us make is that because owls have big eyes, they must have great vision. This is absolutely correct and not only do their big eyes equal great vision, but more specifically they allow for great night vision! Having larger eyes with larger pupils lets more light enter the eyeball. This means that in the middle of the night when things looks dark to most animals (humans too), owls can still see a lot of detail! In addition to being able to let in more light, owls' eyeballs have many more rod cells than most animals. 

​Rod cells are the part of the eye that is responsible for processing photons, or light particles. Humans have about 200,000 rods per square millimeter in their eyes, which sounds like a lot, right? Well, owls have about one million rods per square millimeter in their eye! Their large eye size coupled with the enormous amount of light detecting rods per square millimeter makes owls exceptional at seeing in the dark.

You're probably thinking that if owls have great vision at night, it must mean that they can’t see during the day… Not quite! Yes, owls' eyes do let in a lot more light during the day than other animals, but they have adaptations that compensate for this extra light intensity. Many times, owls will look as though they are sleeping or have their eyes half shut if they are seen during the day. This is because they are blocking out extra light with their eyelids, but the part of their eye that focuses on specific objects is angled so that it points toward the ground! Owls can block light coming in from above them (the sun) while still being able to see the ground with extreme clarity. 

Western Screech Owl. Image from Allaboutbirds.org

​We often assume that all owls are nocturnal, meaning that they are most active at night, but some owls are what we call diurnal, meaning that they are active during the day. The local Barred Owl is a species that can often be seen during the day on overcast days as well as when they are raising their young and teaching them how to hunt. Additionally, some owls are what we call crepuscular, which means they are most active at dusk and dawn!

Because of the size of owls' eyes, they have a special adaptation inside their skulls to keep their eyeballs from deflating! This modification is a bone called the sclerotic ring, which is actually inside the eyeball and offers much needed support to keep the eyes functioning at their best. Many birds and reptiles have sclerotic rings, making it one of the many shared traits that birds, reptiles and a number of dinosaurs have in common! Unlike other birds and reptiles, the owl's sclerotic rings are much larger and much sturdier. They are so sturdy, in fact, that owls are unable to move their eyes within their sockets to look from right to left and up and down. 

Sclerotic rings in owl skull

Because of the large support bones making owls unable to move their eyes, they are adapted to move their heads almost completely around to scan their surroundings with ease. Many of us grow up believing that owls can turn their heads completely around and, although this would be awesome, it is not entirely true. Owls can turn their heads around about 3/4 in either direction, which means they can see everything around them, but they can't see it all with one turn of the head. Owls have 14 neck bones, or vertebrae, which is one of the reasons they are able to turn their heads so far in either direction with ease. Most birds have between 13-25 vertebrae and humans only have seven!

Image from www.oregonzoo.org

Owl vision is only one aspect of what makes them such successful predators. They are also able to fly in complete silence due to modified, frayed wing tips, they have asymmetrical ears for pinpointing their prey, and hooked beaks for eating what they catch with their large talons! Not to mention the ability to produce a pellet in their first (of two) stomachs, the proventriculus, which is where the owl's body filters out all of the undigestible hard-parts like bones, fur and feathers of the animals they are eating. Once these hard-parts are filtered out, the nutrient-rich meat continues on to be digested in the owl's second stomach, the muscular stomach. All of the fur, feathers and bones are then compacted into a pellet in the proventriculus and regurgitated, or spit up, back out the mouth of the owl. Incredible!