Friday, December 09, 2005

Cool Birds 2: the Great Horned Owl

We are blessed here in North America with one of the most fearsome as well as evocative birds - the Great Horned Owl. It is a familiar bird to most people, and yet probably only few have ever had the opportunity to witness them in closer quarters. They are by no means rare, but they are rarely seen or heard by the population at large - in most cases you do have to go looking for them to see them, often at night. The good part is that when you do see them, you will be enchanted by those huge forward-facing yellow eyes staring at you. And even if you don't see them, just hearing their hoots, especially if you're camping in the woods and you wake up to hear them at 4am as I have, can be just as exciting. There's something inexpressibly haunting about knowing there are highly skilled hunters prowling the tree canopy in the dead of night.

The Great Horned Owl belongs to the Family Strigidae, the "typical owls". This is a fairly large family, consisting of over 160 species worldwide, of which 18 live in North America. The Great Horned is not actually the largest North American owl (Snowy and Great Gray Owls are larger), but it is pretty large as owls go. They are most familiar for their resonant low-pitched hoot calls, which are what most people imagine owl calls to be - usually, two rapid hoots, followed by 2-4 slower hoots. They are nocturnal hunters, preferring to feed mostly on small mammals like rodents ands rabbits; passerine birds and sometimes game birds like quail or pheasant; and other small critters like lizards or scorpions. However, they are also capable of taking down even larger prey like skunks - at the RMRP we recently admitted a GHOW that anyone with a sense of smell would know had just been feeding on one. Great Horneds will also prey upon other birds of prey, especially their nestlings. I'll discuss shortly how they manage to do that effectively.

Great Horned Owl, Oct 16 2005
First, a couple images I've captured. Last September I snapped this shot of a GHOW at Cottonwood Hollow, just off Prospect Rd in Fort Collins near the Poudre River. It was kind enough to light onto a branch only yards from the trail, but as you can see it gave me the evil eye as I edged gingerly toward it for a closer look. Even though they are largely nocturnal, Great Horneds can be active during the day too, as this one was. One possibility is that this one was looking for a new nest site. GHOWs don't build their own nests, but rather seek abandoned ones belonging to other large birds. They can also nest in tree cavities, crevices, or stumps.

Great Horned Owl with 4 chicks, April 17, 2005
Speaking of nest sites - earlier in April while on a trip with the RMRP, I got this unfortunately blurry shot of a nesting GHOW in Frank State Wildlife Area, east of Fort Collins. Not easy to see, but there are 4 nestlings being watched over - an unusually high number for the species, which normally raise no more than 3 at a time. Our trip leaders remarked that this wasn't the only fruitful nest they'd seen recently. Apparently last spring was a good one for Great Horneds here.

The definitive outward physical feature of GHOWs is their ear tufts, often called horns, but technically referred to as plumicorns. They are the source of this bird's common English name, but these tufts are not actually associated with its ears. Owl ears are located on the sides of their heads, asymmetrically, so that they are better able to pinpoint sound direction with utmost precision, especially at night. The tufts, on the other hand, serve as both mood communicators to other owls (they can be erected or lowered), and as "shape interruptors", a useful adaptation that makes it harder for prey to identify them as a predatory bird. The idea here is that the "horns" make it more likely that a prey item will not recognize the body shape of the owl, if, say, it is perched nearby in low light.

One of the reasons GHOWs do as well as they do is that their nesting cycle starts considerably earlier than many other birds - some breed as early as January. Their thick plumage and diverse feeding habits allow them to overwinter in colder climes, and to survive late winter freezes. This basically gives their young, who are fledged by March or April, a head start on any other nesting birds in their area, making it all the easier to prey on them and especially their young. Their prey list includes not only small owls, but even the young of other birds of prey like Osprey or Red-tailed Hawks. This early breeding is a remarkable adaptation which helps us understand why their range is as large as it is - an expansive diet increases options especially in places where specific food sources are highly cyclical.

Another reason GHOWs do well, and one reason they are such a cool bird, is that their feathers come with an amazing feature whereby the barbs on the leading edge of the flight feathers are especially long and well-separated. This serves to reduce turbulence and therefore wing noise while in flight - a silencer, if you will. The reduction in flight noise is dramatic - while tending to GHOWs in the large flights (cages) at the RMRP, I'm stunned by how silent those that fly overhead from one high perch to the other are, even while flapping. Not only does this allow them to hear their prey more easily, it gives them an extra stealth factor while honing in on their target. This feature isn't unique to Great Horneds - in fact it is present in many owls, and even some nightjars - but it is one that for them has evolved to perfection.

Just because a bird is familiar doesn't mean it can't also be a cool bird. The Great Horned Owl is a perfect example of that - we in North America should be thrilled to have them right here lurking in our own neighborhoods.


Anonymous said...

Hi eric,
My name is Dave and we live in Colorado Springs in the Black Forest. We have been watching a pair of Great Horned owls that live on our property over the last 10 years. We have never seen them nest. This year, after not seeing the owls since last fall, we noticed a pair of owls in a nest. They were very "Fluffy" to the point we couldn't even see their horns. We know little about owls, so we did some internet research, including your site. These two owls seem very large to be "branchers". They appear about adult size. Then this morning, we saw an amazing site. A large flock of crows attacked the nest, waking me and my wife. From the middle of the woods, the older owl that we knew from before came out and killed one of the crows. There is also a Hawks nest, just 100 feet from the owl's nest and the male was out chasing away the crows, which finally left the area.
We were hoping you might be able to answer a couple of questions or direct us to the right place.

Could the 2 owls that are in the nest be branchers or are they young adults now with their own nest. When do owls loose their fluff.
Do hawks and owls get along or are they going to be attacking each others nest.
Are Owls communal, since it seems like "grandpa" is hanging around the young ones nest, perching on nearby branches, and at one time even at the nest.

dave and suzy

Anonymous said...

I live in Wolf Ranch, and my family and I have been hearing a Great Horned not far from our house. I hear it all the time in the morning, it's really exciting! I havent seen it yet, but I hope I can get the chance soon