One of the easier hawks to identify in flight, Northern Harriers sport a white rump at the base of their tail which is readily visible as they glide low over fields and marshes seeking prey.
Unlike other hawks, Northern Harriers possess an owl-like facial disc of short, stiff feathers which direct sound towards the bird’s ears; they use their sense of hearing as well as their acute vision to locate small mammals and songbirds. Another distinction between Northern Harriers and most hawks is their sexual dimorphism – males are grey and females brown. (Photo: juvenile female Northern Harrier – note dark brown eyes which will become yellow with age)
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Although a Bald Eagle is massive, has excellent eye sight, powerful leg muscles and strong talons to grip prey with, their predilection for foraging for live prey (mostly fish but also mammals, birds and reptiles) isn’t as great as one might imagine. Often they (particularly immature eagles who lack experience foraging) resort to scavenging dead animals or stealing prey from other birds rather than capturing live prey. Marauded birds include ospreys and herons which are better at capturing live fish and aren’t particularly good at defending themselves.
What seems fitting given that eagles secure much of their food from other birds is that they in turn provide many birds and predators with a meal . Crows, ravens, coyotes, bobcats, and foxes all are known to move in when they become aware of an easy dinner. Eagles are said to be easily displaced by these species at scavenging sites. The pictured American Crows, however, are patiently waiting their turn. (Photo: American Crows waiting for 2nd year Bald Eagle to finish feeding on a fish; inset – Bald Eagle’s 5″-long tracks)
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It’s time to keep an eye out for Rough-legged Hawks (Buteo lagopus) which are currently migrating south from their breeding grounds on the arctic tundra to spend the winter in northern states. They look for habitat similar to the open habitat they left – agricultural fields, meadows, and airports fit the bill well. In Vermont, the Champlain Valley is a known winter destination. You see them on the ground, perched on fence posts, telephone poles and tree tops, as well as hovering in the air as they forage for small rodents below.
As to their common name, the “rough-legged” refers to the feathers that grow down their legs to the base of their toes, a clever adaptation for the cold climates they inhabit. Rough-legged Hawks come in two color morphs, light and dark. North America is the only place where the dark morph is found, and it is more common in the East than the light morph. The dark belly of the light morph Rough-legged Hawk pictured indicates that it is a female.
Roughly two months ago the courtship calls of Great Horned Owls could be heard throughout the U.S. as their breeding season began. Mating took place, eggs were laid and incubated for about a month, and several weeks ago those eggs hatched. Looking like balls of tan fluff, the downy chicks can now be seen with their mother in attendance.
Great Horned Owl chick appetites are voracious. At the ripe old age of two weeks they are capable of swallowing a mouse whole. The chicks weigh about an ounce when they are born, and for the first month or so, they gain roughly that much every day. In another month they’ll be taking short flights, but will remain with their parents throughout most of the summer.
Ospreys have returned to their breeding grounds in New England, where both courtship and copulation is taking place. The males engage in an undulating courtship display flight high over the nest site, often with fish or nesting material clutched in their dangling legs while they repeatedly issue forth screaming calls. This can go on for up to ten minutes or so before they descend to the nest. In addition, “courtship feeding” often takes place with the male providing his mate with food, often just prior to breeding.
Although an osprey pair copulates frequently (an average of 160 times per clutch), nearly half the time there is no cloacal contact. Most of the breeding takes place at or near the nest site. (Note the protective positioning of the male’s toes and talons as he mounts his mate.)
Naturally Curious is back! Different ecosystem (western vs. eastern Vermont) but same curiosity! This week’s posts are going to be devoted to the nesting behavior of the Osprey — the only raptor that plunge-dives feet first to catch live fish as its main prey source.
Ospreys nest within six to twelve miles of water (usually much closer). The male collects most of the nesting material and brings it to the nest site where the female arranges it. Sticks as large as an inch-and-a-half in diameter and three feet long are collected from the ground, or (less commonly) snapped off a tree while the Osprey is in flight. Nest-building continues throughout the incubation of the eggs as well as the brooding period — even if a nest fails, Ospreys will continue to add material to it.
Although nests built on platforms are relatively small, those built in trees or on the ground can be 10 -13 feet deep and 3 – 6 feet in diameter (the largest nests are most likely the result of several generations of nesting Ospreys). The shape of an Osprey nest changes during the breeding cycle. When the eggs are being incubated, the nest is bowl-shaped. After hatching the nest flattens out, but a rim of sticks is maintained. By the time the nestlings fledge (around 50-55 days) the nest is often completely flat.
Ospreys will reuse their nest year after year, saving themselves time and energy which allows earlier laying and more surviving young. Birds whose nests fail are likely to build alternate nests and use them in subsequent years. (Birds of the World Online).
Most owls do not bother to tear small prey such as mice and voles apart but instead swallow them whole. After eight to sixteen hours, all the nutrients available in the eaten prey have been absorbed by the bird. Owls cannot digest the fur, feathers, bones, teeth and nails of their prey, so these parts remain in the bird’s gizzard (specialized organ that grinds up food in most birds but serves as a filter for holding indigestible parts in birds of prey). This accumulation of indigestible parts takes on its pellet form (which is the shape of the gizzard) about eight hours after ingestion, but is sometimes retained by the owl for another six hours or so before being coughed up. As a rule, bones are on the inside of the pellet, and the fur and feathers form a soft coating on the outside.
The stored pellet partially blocks the entrance to the digestive system so it must be ejected before the owl can eat again. This process takes anywhere from a few seconds to several minutes. The owl appears to “yawn” several times before regurgitating the pellet. Note that the pictured Barred Owl has prey (a Deer or White-footed Mouse) in its talons, but out of necessity is getting rid of a pellet before devouring it.
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The migration of raptors has begun, and one of the first species to migrate in the fall is the Broad-winged Hawk (Buteo platypterus). While it is quite secretive when nesting, it is one of the more conspicuous species of birds when migrating. This is for two reasons. They are one of the few North American raptors that flocks during migration, and much of their migratory behavior is concentrated in the Northeast in a two-week period around the middle of September.
Migrating Broad-wings conserve energy by frequently soaring in thermals and mountain updrafts. Flocks of birds, or “kettles”, soar up the heated columns of air, peel off and glide to the next thermal where they repeat the process. Very little wing-flapping is necessary in order to cover a lot of ground. The flocks, or “kettles,” range from several individuals to thousands of birds (larger kettles generally occur nearer their Central and South America wintering grounds).
The number of birds migrating often grows following a cold front, when winds die down and thermals increase. Fall migration of Broad-wings in the Northeast is associated with good visibility, moderate favorable winds, high temperatures, and afternoons (vs. mornings). (Photo: juvenile Broad-winged Hawk)
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Only Naturally Curious readers would come up with flossing!
If lemmings are in short supply and you’re a Snowy Owl, head for tall grass where small rodents dwell. This juvenile female Snowy Owl successfully caught a Meadow Vole (along with a footful of grass) in its talons and proceeded to swallow the vole whole, along with some of the grass. However, most of the grass remained hanging from the owl’s mouth after the vole had been consumed, so it proceeded to grasp the grass with its foot and pull it out of its mouth (yesterday’s Mystery Photo).
Although many people are under the impression that hard weather forces Snowy Owls farther south some winters, the reason for Snowy Owl invasions or irruptions turns out to be linked to either prey population crashes in the north, high productivity breeding years (producing more predators than the prey can support) or a combination of the two. New research has shown that the abundance of Snowy Owls seen in the eastern U.S. during the winter of 2013-14 was the result of a particularly good nesting season on the Arctic tundra. A population boom of lemmings, the Snowy Owl’s primary food source, translated to a population boom of owls.
Unlike most young birds, Barred Owls fledge before they can fly. On average, they leave their nest when they are around eight weeks old, and don’t master flight until they’re about 12 weeks old. Fledging for a Barred Owl consists of climbing up out of the tree cavity where they spent their first two months, onto a nearby limb where its parents will tend to it. More often than not there is a nearby branch which it can hop to.
In the fledging depicted, the closest limb was a good 10-15 feet above the nesting cavity. With the help of its strong talons, beak and wings, the fledgling managed to scale the tree trunk up to a somewhat horizontal stub where it could get a good purchase. It may not have been the most graceful ascension, but the fact that it could manage to climb straight up for this distance without the assistance of any grasping fingers was impressive, to say the least. Fledged Barred Owls continue to be fed by their parents until they can fly and capture their own prey. (Photo: Barred Owl chick fledging – clockwise, starting from upper left)
For about a month the female Barred Owl sits on her 1-5 eggs (usually 2 or 3) until her white, downy chicks hatch, usually around the end of April or beginning of May. At about two weeks (mid-May) their natal down is being replaced by a white-tipped gray-buff secondary down, and primary feathers are beginning to grow in. They remain in their cavity nest for their first month.
It is pretty safe to assume that the adult owl in this photograph is a female as she is in constant attendance during the chicks’ first two weeks. The male brings her food which she tears into little bits so the chicks can swallow it. At the beginning of the third week, the female begins leaving the nest frequently to hunt. When she is at the nest, she often takes breaks from the kids and sits at the entrance surveying her surroundings. At this point the chicks start consuming prey (that is delivered to them) on their own. In a week or two the chicks will also start appearing at the cavity entrance. By their fourth or fifth week, the still-flightless chicks will leave the nest, but parental provision of food continues until fall.
Using their keen eyesight and sense of hearing, Barred Owls typically sit and perch on a branch, surveying the ground beneath them for whatever morsel appears, small mammals and birds being at the top of their list during the winter, with more emphasis on amphibians, reptiles, fish and invertebrates during the warmer months.
Several times I have observed a Barred Owl taking advantage of a vernal pool that was teaming with life, specifically Wood Frogs and Spring Peepers. Perched in a nearby tree, the owl kept an eye out for any sign of movement in the water. When ripples appeared near the shore it would swoop down to the water’s edge in an attempt to grasp a frog with its talons. Although it met with repeated failure, success was inevitable due to the plethora of distracted mating frogs.
Because they lack hair and feathers, frogs may be underestimated when scientists dissect owl pellets to see what Barred Owls eat. Given the frequency with which I have observed these feathered fishermen, I would think that might be very likely.
Red-tailed Hawks are on eggs, or soon will be. Whether their nest is in the canopy, on a building ledge, transmission tower or elsewhere, it usually has a commanding view of the surrounding area and unobstructed access from above.
Both members of a pair share in nest site selection, which is often in mixed woods adjacent to open fields. They build their nest together, working most diligently in the morning, and construction is completed within four to seven days. If a nest is re-used, which it often is, it is refurbished with sticks and greenery. Red-tails are very wary during this nest-preparation period and may discontinue nest-building if humans are detected, so should you come upon a nest during this stage, best to remove yourself quickly.
Great Gray Owls are impressive birds – at 27” in length, they are our largest owl (Great Horned Owl – 22”, Snowy – 23”) but at 2.4 pounds, not our heaviest (Great Horned Owl – 3.1 pounds, Snowy – 4 pounds). The feathers that make a Great Gray Owl look so massive are what keep it warm during winters in the northern boreal forests where it resides.
Most of a Great Gray Owl’s diet consists of rodents, and some winters, when prey is scarce, individuals wander south to southern Canada and northern U.S. to sustain themselves. Sometimes Great Gray Owls are highly irruptive, and the number of sightings in the Northeast is high. In the winter of 1978-79 there were over 150 sightings in New England and Quebec. While there were numerous sightings in southern Canada this winter, northern New England was visited by only a few individuals, including the one pictured (in central New Hampshire).
Striped Skunks do have predators other than Great Horned Owls (bobcats, foxes and coyotes-fishers have been known to prey on skunks, but very infrequently), but these predators have to be pretty desperate before they will prey on a skunk. Automobiles and disease kill more skunks than all of their predators put together, but Great Horned Owls have the distinction of being the primary predator of Striped Skunks.
Being a nocturnal hunter, a Great Horned Owl necessarily consumes prey which are nocturnal. Striped Skunks are active at night and are consumed by Great Horned Owls with regularity, even though a skunk can weigh up to three times as much as a Great Horned Owl (average GHO weighs a little over 3 pounds) and has a potent way of defending itself.
For many years scientists assumed that birds had a poor sense of smell because the area of a bird’s brain involved in smell is relatively small compared with the area found in mammals. However, recent research reveals that birds have a high number of active genes that are associated with smell, and many species may have an excellent sense of smell. It’s fairly safe to assume, however, from its consumption of skunks, that the Great Horned Owl’s sense of smell is not very well developed. In addition, if a skunk sprays, much of the odor is absorbed by the Great Horned Owl’s leg feathers, which extend down to its talons.
A favorite memory of mine is walking through a field at dusk and suddenly noticing a strong skunk-like smell coming from above, not below, me. A Great Horned Owl silently flew overhead, with only the tell-tale smell of a recently-captured skunk announcing its presence.
There has been an unusually high number of Barred Owl sightings reported in northern New England and New York this winter, primarily from the road and near bird feeders. This phenomenon, particularly with owls, is usually attributed to either a current lack of food or an abundance of food during the most recent breeding season resulting in a dramatic increase in the owl population.
In the case of Barred Owls, it is the former. Unlike Snowy Owls, which vary the size of their clutch depending on food availability, Barred Owls typically have two young, regardless of the size of the rodent population. Thus, a plethora of progeny can be eliminated as a viable explanation for the abundance of Barred Owl sightings this winter, which leaves a scarcity of food as the primary reason.
For several weeks there has been a thick crust on top of the snow, which makes hunting for mice and voles difficult for raptors. Because they are very territorial, Barred Owls rarely wander outside of their territory, even when food is scarce. Thus, especially in the past few weeks, they have been desperate to find small rodents. Roads are one reliable spot where mice, voles and shrews are exposed, and bird feeders are most definitely rodent magnets. Hopefully weather conditions will allow birds of prey access to the subnivean layer (space next to the ground where small rodents travel, shelter and breed) before too many more Barred Owls starve to death. (Source: Joan Collins, NYS Ornithological Association, UV-BIRDERS List) (Photo: Barred Owl with recent Northern Red-backed Vole catch.)
Short, powerful, rounded wings and a relatively long tail enable Sharp-shinned Hawks to maneuver in dense cover in pursuit of small birds, which compose 90% of their diet. Small mammals and insects are consumed, but not nearly as frequently as birds. The size of the birds eaten range from hummingbirds to Ruffed Grouse. Long legs and toes (especially middle toes) enable individuals to reach into vegetation and large eyes enhance its ability to catch fast-moving prey.
Sharp-shinned Hawks are familiar sights to those of us with bird feeders – this species is responsible for 35% of 1,138 predation incidents reported at feeders in continent-wide survey. In this photograph, a Blue Jay is successfully warding off an attack by a juvenile Sharp-shinned Hawk. (Photo by Jeannie Killam.)
The eyesight of a Bald Eagle is impressive. Part of the reason for their excellent vision is that these birds of prey have two centers of focus (foveae), which allow them to see both forward and to the side at the same time. Cone cells, one of three types of photoreceptor cells in the retina, perceive color, fine detail and rapid movement. In a human, the fovea has 200,000 cones per millimeter; in the central fovea of a Bald Eagle’s eye, there are about a million cones per millimeter. An eagle’s eye is almost as large as a human’s, but its sharpness is at least four times that of a person with perfect vision.
Bald eagles are capable of seeing fish in the water from several hundred feet above, while soaring, gliding or in flapping flight. (They locate and catch dead fish much more rapidly and efficiently than live fish, because dead fish float with their light underside up, making them easier to see.) It is very likely that a Bald Eagle can identify a rabbit moving almost a mile away. This would mean that an eagle flying at an altitude of 1000 feet over open country could spot prey over an area of almost 3 square miles from a fixed position. (photo: recently fledged, juvenile Bald Eagle)
Great Horned Owls are one of the earliest nesting birds — you can find them on nests in January, February and March, even in northern New England. Eggs are incubated for about a month, typically in March or April with young usually hatching in May or June. The nestlings remain in the nest for six or seven weeks before fledging. Unable to fly until they are ten or twelve weeks old, the fledglings follow their parents around and continue to be fed and cared for by their parents until fall. In late summer, when they have fledged but are still begging their parents for food, you can hear their distinctive calls. To know what to listen for, go to http://langelliott.com/mary-holland/great-horned_owl.mp3 (Sound recording © Lang Elliott – langelliott.com & miracleofnature.org.)
The fledging of flightless Barred Owl chicks takes place four or five weeks after they hatch. Typically they perch on the rim of the nest cavity before climbing to a nearby branch. If there are no branches close by, the chicks will drop to the ground and climb a nearby leaning tree, where they perch and are fed by their parents. Juvenile Barred Owls begin short flights at approximately 10 weeks of age, attaining longer flights by 12 weeks. They are now learning to hunt, but continue to be fed by their parents until late summer or early fall. (Thanks to Alfred Balch for photo op.)
For several weeks prior to their first flight from their nest, Bald Eagle nestlings practice flapping their wings to the point of lifting themselves up several inches into the air. This develops their wing muscles, flight coordination and landing ability.
At anywhere from eight to fourteen weeks of age, juvenile Bald Eagles fledge, or leave their nest. According to Birds of North American Online, up to half of young Bald Eagle nest departures are unsuccessful. The young land on the ground and may remain there for weeks before regaining flight ability. More often than not their parents will continue to feed them, but they are much more vulnerable to predation in this situation. Fledglings may continue to use their nest as a feeding platform for several weeks after leaving it, as they gain flight and foraging skills.
Young Barred Owls are fed by their parents from the day they hatch until late summer/early fall. During their first two weeks, food is delivered by the adult male to the adult female, in a bill-to-bill exchange. The female tears up the prey into swallowable bits and feeds them to her offspring. During this time the female does little hunting, but she begins to capture prey after about two weeks of brooding the young. At about this time, the young begin consuming whole prey on their own (see photo). Female prey deliveries are greatest immediately following sunset and immediately prior to sunrise, while male prey deliveries remain fairly constant throughout the night. (Photo: Barred Owl delivering Red-backed Vole to nestling.) (Thanks to Alfred Balch for photo op.)
Barred Owl eggs (usually two or three) are often laid in a tree cavity, where the adult female incubates them for roughly a month. Fuzzy, white, downy chicks hatch and remain inside the tree for four or five weeks while being fed by both parents. When the young owls are two or three weeks old, their white down is replaced with gray-buff secondary down, and they gain the strength to climb up the inside of the tree and peer out at the outside world. In and out they go, perching on the rim of the nest hole for half an hour or so as they await the arrival of their next meal, and then retreating back to the safety and warmth of their nest. (Thanks to Alfred Balch, naturalist extraordinaire, for photo op.)