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Birds of Prey

Barred Owls Courting

2-11 barred owl 057Barred Owls call year-round but their vocalizations increase and expand in February when courtship begins. No longer are their calls limited to their year-round “who-cooks-for-you — who-cooks-for-you-all.” Males and females engage in “duets,” as well as many other vocalizations, including cackles, hoots, caws and gurgles. Those who sleep with open windows may feel like they are in the middle of a jungle inhabited by hundreds of raucous monkeys.

Barred Owl courtship is not strictly vocal. Male Barred Owls display by swaying back and forth and raising their wings, while sidling along a branch in close proximity to a female. Courtship feeding and mutual preening also occur prior to copulation. The nights of February are filled with amorous avian calls and gestures.

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Great Horned Owls Courting

1-22-15  great horned owl IMG_5973The intense hooting of Great Horned Owls begins in late December or early January, about a month before actual mating takes place. Males call during most seasons of the year, but the period when the males are hooting vigorously lasts for a month or six weeks. During the mating season the deep, rich tones of the males are occasionally interspersed with the higher and huskier notes of the females. The answering calls of the females are heard for only a week or two, toward the end of the six-week period.

Eventually, when a male and female approach each other, they do a sort of courtship “dance.” The male cocks his tail, swells his white bib (see photo), and with much bobbing and jerking utters a series of deep sonorous calls that elicit calling responses by the female. He cautiously approaches the female, continuing much tail-bobbing and posturing. The owls nod, bow, and spread their wings as well as shake their heads. Courting pairs have been observed engaging in high-pitched giggling, screaming, and bill-snapping. Mutual bill rubbing and preening also occurs. Copulation concludes the courtship ritual, with both owls hooting at a rate of 4 or 5 hoots per second throughout copulation, which lasts 4 – 7 seconds. (Photo: Great Horned Owl, in captivity)

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Owl Night Vision

1-5-15  barn owlIMG_6003Like humans, birds have a sensitive retina in the back of their eyes that absorbs incoming light, senses it, integrates the information in it, and sends this information on to their brain. An avian retina is much thicker than ours and contains more rod cells (for dim light vision) and cone cells (for acuity and color vision).

As most owls are active at night, their eyes must be very efficient at collecting and processing light. The eyes of owls are disproportionately large compared to the size of their skull, and enable them to collect as much light as possible. In addition, the retina of an owl’s eye has an abundance of light-sensitive rod cells — owls have almost a million rods per square millimeter compared to humans which have only about 200,000. Barn owls can see a mouse at 6 – 7 feet with an illumination of .00000073 foot-candles – the equivalent of humans seeing a mouse by the light of a match a mile away. Eye size, an abundance of rod cells and additional neural mechanisms provide owls with vision greater than that of most of their prey.

Since owls have extraordinary night vision, it is often thought that they are blind in strong light. This is not true, because their pupils have a wide range of adjustment, allowing the right amount of light to strike the retina. Some species of owls can actually see better than humans in bright light. (Photo: barn owl, in captivity)

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Overwintering Red-tailed Hawks

12-24-14 red-tailed hawk-juvenile 002Red-tailed hawks are “partial migrants” — some individuals are migratory, and others are not. Many Red-tails living in the northern portion of the species’ range in southern Canada and northern United States migrate to more southerly locations for the winter. A few northern birds, however, remain on their breeding territories even in the most severe winters.

Overwintering Red-tailed Hawks are generally easy to spot, as they often perch on dead trees overlooking open fields and on telephone poles next to highways, where they watch for prey. Mice, voles, squirrels, snowshoe hares and an occasional bird make up most of their diet. If you notice the coloration of a Red-tailed Hawk’s tail, it will tell you whether the bird is a juvenile or adult. Adults have rufous tails; juveniles have barred, brownish tails. Seconds after this photograph was taken this juvenile Red-tailed Hawk killed and consumed an American Crow while being mobbed by more than 50 other crows.

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Contents of One Barred Owl’s Stomach

11-7-14 vole, shrews and mouse 027Fantastic guesses, given you had no measurements to work with. Very creative, indeed. Yesterday’s mystery photo was a packed version of the bodies displayed today.

Owls swallow small prey, such as mice and voles, whole, while larger prey is torn into smaller pieces before being swallowed. Once eaten, prey goes directly into the owl’s stomach, as owls have no crop, and thus no ability to store food for later consumption.

Like other birds, owls have a stomach with two chambers — one is the glandular stomach, or proventriculus, (yesterday’s mystery photo) which produces enzymes, acids and mucus and begins the process of digestion. (Because the acids are weak, only the soft tissues are digested.) The second stomach is the muscular stomach, or gizzard, also called the ventriculus. The gizzard lacks digestive glands – it serves as a filter, holding back bones, fur, teeth and feathers that are difficult to digest. The soft parts of the food are ground by the gizzard’s muscular contractions, and allowed to pass through to the rest of the digestive system.

Several hours after an owl has eaten, the indigestible parts remaining in the gizzard are compressed into a pellet the same shape as the gizzard. The pellet travels back to the proventriculs and remains there for up to ten hours before being regurgitated. Because the stored pellet partially blocks the owl’s digestive system, new prey cannot be swallowed until the pellet is ejected. If more than one prey is eaten within several hours, the remains are consolidated into one pellet. (In this case, one very large pellet!)

Update: I left the contents of the deceased Barred Owl’s proventriculus outside last night, and a resident Barred Owl recycled the Meadow Vole and Masked Shrew.

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Juvenile Barred Owls Mastering Flight

barred owl-fractured wing 062Typically Barred Owls in northern New England hatch in May and fledge, or leave their nest, in June at approximately four to five weeks of age. Unlike most young birds, Barred Owl nestlings leave their nest before they can fly. They initially perch on the rim of the nest and then climb to a branch on the nest tree, eventually dropping to the ground and climbing a nearby leaning tree to perch. The parents feed their young from the time they hatch until late summer or early fall. The fledglings begin short flights at approximately 10 weeks of age, attaining longer flights by 12 weeks. The pictured Barred Owl may have been mastering flight when it fractured a wing and ended up on the ground, soaking wet and very vulnerable to predation. (Thanks to Bob Moyer for photo op and Vermont Institute of Natural Science’s Wildlife Rehab staff for setting wing.)

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Young Cooper’s Hawks Fledging

8-19-14 juvenile cooper's hawk2 306After a month of living in a nest that measures roughly 7 ½ inches across and 3 inches deep, Cooper’s Hawk nestlings are more than ready to stretch their wings. Although they’ve been dismembering prey (mostly birds and a few small mammals brought to them by their parents) since they were three weeks old, catching prey is a skill they have yet to acquire. For roughly ten days after they leave their nest, the young hawks return to it for continued prey deliveries (and for roosting). During this time the fledglings learn to catch their own prey and they become independent, but they continue to stay together near their nest for the next month or so. (Thanks to Marian Boudreault for photo op.)

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