Saturday, May 23, 2009

The Tiger in the Sky

Bubo virginianus, the great horned owl, is the most widely distributed owl in the Americas. It can be found in the tundra regions of Canada, cities and forests of the US, deserts of Mexico, tropical rain forests of Central and South America and almost any place in between. Although it is not the largest owl in size in America, both the great grey and snowy owl are larger, it is the largest in weight and ferocity. Commonly called the, "Tiger in the Sky" the great horned owl is the top aerial predator through out its range. But even though they are a fierce and formidable predator there is a softer side to the great horned owl, a side which many people do not see as a tender spouse and a loving and devoted parent.
GHO courtship begins around the beginning of the year, although it can begin earlier in warmer portions of their range. Young males, GHOs become sexually mature at around the age of two, go out and find a territory to claim. Once they have a territory they begin to advertise to available females by singing at dusk and dawn. While they sing they often puff out their chest, displaying the white patch of feathers beneath their chin, it is possible that this helps to signal his location to the female.
Interested females will respond to the males calls by perching close to him. The male will then try and impress her with a display of wing flaps, bill clacking, feather ruffling and short courtship flights around the female. The female will takes in the show and shows her interest by either watching the male or appearing to take no notice of him. The male will then approach the female slowly paying attention to her reactions. If she ruffles her feathers or seems disinterested he will begin the show again. If she appears receptive he will move in close and initiate mutual bill rubbing and preening. They may complete their bonding by flying off together.
Once the fun is over, and the pair has bonded, it is time to get to work. There is plenty for the new couple to do. Besides the day to day necessities, such as finding food, the pair will both defend their new territory. Most often they will avoid confrontations by singing territorial songs to warn off other owls. They will also need to find a nest. GHOs do not build their own nests so they need to borrow one or find another alternative. Usually they can find the nest of another large bird, such as a hawk, heron or crow, that is currently not in use, since they nest so early in the year most of these other birds are not around at this time. If a acceptable birds nest is not available then they will use alternatives such as hollow trees, squirrel nests, caves, or man made structures like towers or old barns. It is the males job to find potential nests and offer them to the female who will make the final decision.Once they have decided on a nest the couple will usually only do slight remodelling, lining the nest with feathers from the female of bits of fur from prey animals, before the females lays her eggs. The eggs are similar in size to chickens eggs, about 2.1 to 2.2 inches long and 1.8 to 1.9 inches long, and are dull to glossy white in color. The clutch can consist of one to six eggs, with two or three eggs being the most typical.
The eggs will take approximately 27 to 36 days to hatch. During that time they must be constantly incubated. If the eggs are left unattended for any period of time they may freeze and become addled and will never hatch. The female is the one who handles most of the incubation. The male will occasionally take over for her, particularly at dusk, so that she can go out and hunt but since male GHOs do not develop a brood patch it is likely that the male does very little of the incubation. A brood patch is a spot that some birds get when nesting where there are no feathers. Feathers provide birds with insulation that helps keep cold out and their own warmth in. So it is important for a bird that is incubating eggs to have a spot with no feathers, a brood patch, so that they can share their body warmth with their eggs. Typically the male perches in a tree nearby where he can keep an eye out and warn the female if danger approaches. When night approaches he will go out and find food for both himself and his hard working bride.
When the eggs finally hatch the young owls chicks are completely helpless. They weigh around a half of a pound and are covered with white down feathers which help to keep them warm. At this point they are unable to open their eyes, stand or feed themselves. The remnants of the egg tooth and the yolk sac are usually still visible on the chick.
A few hours after hatching the owl chicks are able to hear and sense movement in the nest which mean the presence of an adult. At this point they are totally dependent on their parents for food, warmth and protection. When they hear or feel an adult arrive they instinctively move to the noise with their mouths open and emit a raspy chirp to signal to the adults that they are hungry. If food is placed in their mouth they reflexively swallow it. After a couple of days they will be able to partially open their eyes but they will not fully open until they are 9 to 10 days old. During their first two weeks of life mom will spend most of her time with the chicks in the nest. Most of this time is spent keeping the new chicks warm, especially in places like Minnesota where it still may get below zero after the chicks hatch. While mom is brooding over the chicks it is dad's job to keep the family fed.
After the first couple of weeks the female spends less time at the nest. The chicks now getting into their 3rd or 4th week begin to develop their brown juvenile feathers. They have grown quite a bit and their wing primaries are beginning to emerge and their facial disks and horns are beginning to develop. They are much more active now, moving around the nest and frequently changing their positions. They are also more alert, often responding to planes flying over head, loud noises, or left overs in the nest.
The adults are still close by keeping a watchful eye on the nest. They still need to bring food to the young and depending on the size of the food they may still have to help feed the chicks. Rabbits are a favorite with the GHOs in this area but it is difficult for the chicks to tear it into pieces that they can swallow. So an adult, typically the female, will visit the next several times a day to feed the chicks.
By around their sixth week the young owls are getting rambunctious. They no longer spend much of their time in the nest. Instead they begin to climb out among the branches surrounding the nest. Often while they are branching they will flap their wings in preparation for fledging which is not far off. The young owls will climb among the branches day and night usually returning to the nest only when an adult arrives with food.
Branching is a dangerous time for the young owls. All of the noise of the young owls branching could attract a predator such as a hawk that would enjoy making the young owls into a meal. Also while hopping and climbing among the branches it is possible for the young owls to fall. A fall from a tall tree could lead to injury. Even if it is not injured a young owl on the ground is vulnerable to many different land predators. Fortunately branching also helps the owls learn to climb, so owls that are not injured in a fall can usually climb back up into the tree. Since GHOs hatch asynchronously, at two to six day intervals, older siblings are often more developed and begin branching while younger siblings may still be in the nest.
A couple weeks after they begin branching they will attempt to make their first flight. Their first flight are often awkward and very comical to see, particularly the landings. Often times they miss their intended perch and end up on the ground, in which case they usually are forced to climb back up. By the end of their first day of flight they usually have flying down pretty well, although landings typically still need work. For the next couple of weeks the young owls will continue to prefect their flying and landing skills. They typically stay near the nest site. The adult are usually nearby keeping a watchful eye out, although they rarely perch in the same tree. Even though the adults are still feeding the young owls at this point they use the feedings to train the young ones lessons that they will need to survive. Often they will bring the food in live and watch as the young owls learn how to kill their prey. Sometimes they will even let prey go giving the young an opportunity to practice their hunting. A couple of weeks after the young fledge the adults quit bringing them food and the young are left to find food on their own. By about ten to twelve weeks the young owls are able to fly as well as the adults. They will stay with their parents for the rest of the summer and sometimes into the fall. Later in the fall the parents will eject the young from their territory. The young will often stay in the area and become floaters until they mature and find a territory of their own and then the process will begin again with a new generation.


Gaelyn said...

What a marvelous and informative post! Great captures. Did you actually to follow this process? If so, that's is so COOL!

Patricia @ ButterYum said...

Very cool post!

Chris said...

Interesting post and very well illustrated. This bird is very impressive, huge eye and its gaze is gloups terrific!

Carol said...

I've had Hawks and Osprey nest close enough to the house to watch them raise their young..but no Owls. Nice post.

Anonymous said...

I love these owls. And your photos are excellent. Nice post.

BLS said...

Very informative post and great photos to go with the story. My favourite is the swooping owl. I was thinking the markings on the face resembled a knot on a branch or a large butterfly or moth.

Unknown said...

Wonderfully informative post and a terrific sequence of photos. Thanks!

Dawn Fine said...

This was a great post! Thanks for all the wonderful information and amazing photos.

Allison said...

Magnificent and informative post. The pictures are awesome!
Thanks for having me!

General Viagra said...

It looks like imposing animal, with those horn and its big chest. Also that kind of stare give me a creep like if any moment that animal can attack you.