Monthly Archives: February 2017

Bald Eagles: Hunting and Food

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An eagle can rise on thermals and gain altitude until it is only a speck in the sky, and then it soars until it sees prey and can swoop down and make a kill. When an eagle spots a fish from the air, it begins to glide toward the water. As it nears its prey, it extends its legs and opens its talons. It soars just over the surface of the water and then plunges its legs into the water. The talons strike the fish, and the eagle immediately closes the talons, driving them deep into its prey. The eagle then flaps its wings to pull the fish out of the water and maintain enough speed to remain airborne. If the eagle cannot lift the fish, the bird may be dragged under water and forced to swim for shore. Eagles are strong swimmers, but if the water is cold, they may be overcome by hypothermia and drown.

 It is a common misconception that once an eagle grasps its prey with its talons, it cannot let go. While eagles can lock their talons, it is a voluntary action. An eagle can release a fish that is too heavy for it to lift, but sometimes it holds on anyway, perhaps deciding the prize is worth the swim to shore.

 Biologists estimate an eagle can only lift a maximum of four to five pounds, but since lift is dependent on both wing size and air speed, the faster the eagle flies, the greater its lift potential. An eagle that lands to grab a fish and then takes off again can manage less of a load than one that swoops down at a high rate of speed and plucks its prey from the water. Speed and momentum allow the eagle to carry more weight.

 An adult bald eagle needs between 0.5 lbs (.23 kg.) and 1.5 lbs (.68 kg) of food per day. A study done in Washington State found an eagle needs to consume between 6% and 11% of its body weight per day. If an eagle eats a three-pound (1.4 kg) fish one day, though, it does not need to eat again for a few days. Bald eagles living in coastal Alaska feed mainly on fish such as herring, flounder, pollock, and salmon. They may also prey upon seabirds, small mammals, sea urchins, clams, crabs, and carrion.

In the summer and fall on Kodiak Island, eagles congregate along salmon streams or near the ocean where salmon are likely to school. Large numbers of eagles also gather near fish canneries where they feed on the fishy discharge from the processing plants. Both mature and immature eagles feed on carrion, but research indicates young eagles are more dependent on carrion, and they eat carrion while they develop and hone their hunting skills. Adults, on the other hand, more actively hunt live prey, particularly fish.

 The bill and neck muscles of a bald eagle are adapted to allow the bird to gorge itself quickly. An eagle can eat a 1 lb. (.45 kg) fish in only four minutes, and it can hold onto a fish with one talon while it grips its perch with the other talon and tears apart the fish with its bill.

Eagles are the masters of their domain and consider any animal they can lift as a suitable meal. If an eagle flies over a seagull rookery, all the birds on the rookery take flight to chase away the menacing predator. Between fish, birds, voles, weasels, and hares, eagles on Kodiak Island have plenty to eat.

Don’t forget to sign up for my Mystery Newsletter. In March, Steven Levy, a respected author and historian from Anchorage, will be the guest author for my newsletter and will write about historical crimes in Alaska.

Bald Eagle: Flight and Vision

Bald Eagle, Birds, Kodiak Wildlife, , , , ,

If you look up on a windy day on Kodiak Island, you will likely see several eagles soaring high in the sky. Bald eagles are built for flight, particularly for soaring and gliding. An eagle expends a great deal of energy flapping its large wings, so to conserve energy when gaining and maintaining altitude, it utilizes thermal convection currents or “thermals,” which are columns of warm air generated by terrain such as mountain slopes. It has been estimated a bald eagle can reach flying speeds of 35-43 mph (56-70 kph) when gliding and flapping and 30 mph (48 kph) while carrying a fish. While not known as particularly fast fliers, eagles can soar and glide for hours at a time.

     The construction of an eagle’s wings and tail make soaring and gliding possible. The wings are long and broad and are covered by a layer of lightweight feathers arranged to streamline the wing. The primary feathers, or primaries, provide lift and control an eagle’s flight during turning, diving, and braking. An eagle can tilt and rotate individual feathers to maneuver and brake. The tail also assists in braking and stabilizes the eagle when it dives toward prey. While soaring, tail feathers spread wide to maximize surface area and increase the effect of updrafts and thermals.

     When an eagle finds an air current or a thermal, it can gain altitude without flapping its wings. If it is dead calm with no air currents moving up or down, eagles cannot soar, and that is why you see more eagles soaring on windy days or sunny afternoons and sitting on their perches on calm, cool mornings.

     When a young eagle first leaves the nest, its wing and tail feathers are longer than those of an adult. As an eagle matures, its wing and tail feathers become shorter and narrower with each successive molt. The larger wings of a juvenile make it easier for the bird to catch an updraft or weak thermal and to fly slower and in tighter circles than an adult. The downside of the larger wings and tail is the juvenile rises slower, sinks faster, and cannot soar as far as the adult. Adult bald eagles can flap their wings faster and fly at a greater speed than immature eagles, making them more efficient at chasing down live prey.

     Female bald eagles are larger than males, and while their wings are also slightly larger, the larger wing size does not make up for the increased weight of the female. Therefore, females require more wind or stronger thermals than males to be able to gain altitude and soar. Since thermals are weaker during the morning and evening hours, females are more likely to remain on their perches during these times and soar when it’s windy or in the afternoon when thermals are stronger.

     An eagle’s large wings make landings and takeoffs tricky, and landing on a perch is something eagles manage to do gracefully only after much practice.   A newly-fledged juvenile looks very awkward when it tries to land on a perch and may even crash land or swing upside down if it grabs the perch while it still has too much forward momentum.

     An eagle’s acute vision allows it to see prey while soaring high in the air. The eyes of an eagle are larger than those of an adult human, and an eagle’s eyesight is at least four times sharper than that of a human with perfect vision. An eagle flying at an altitude of several hundred feet can spot a fish under water. The eyes are protected by a nictating membrane, and each eye has two fovae or centers of focus, letting the bird see both forward and to the side at the same time. Eagles have binocular vision, so they can perceive depth, allowing them to judge how far away their prey is when they begin a dive.

Next week, I’ll write about what bald eagles eat and how they hunt.  Once again, I want to remind you to sign up for my free monthly Mystery Newsletter and read about true crime in Alaska.

    

Bald Eagle Body and Feathers

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Female bald eagles are slightly larger than males. Males range in body length from 30 to 34 inches (76.2 to 86.4 cm), while females measure 35 to 37 inches (89 to 94 cm). The wingspan of a male stretches from 72 to 85 inches (182.9 to 215.9 cm), while a female’s wingspan ranges from 79 to 90 inches (200.7 to 228.6 cm). Bald eagles weigh between 8 and 14 lbs. (3.6 to 6.4 kgs.).

The skeleton of a bald eagle weighs 0.5 lbs (250 to 350 grams), which is only 5 to 6% of the total weight of the bird. The bones are extremely light, because they are hollow, and the feathers weigh twice as much as the bones.

The bald eagle’s average body temperature is 106°F (41°C). They don’t sweat, so they cool themselves in other ways, such as panting, holding their wings away from their bodies, and perching in the shade. In cold weather, an eagle’s skin is protected by feathers which are lined with down. Their feet consist mainly of tendon and are cold-resistant, and little blood flows to the bill, which is mostly nonliving material.

The beak, talons, and feathers of an eagle are made of keratin, the same material as in our fingernails and hair. Because of this, the beak and talons continuously grow and are worn down through usage. An eagle’s beak can be used as a weapon and is sharp enough to slice skin, but is also delicate enough to groom a mate’s feathers and feed a chick. The talons are important for defense and hunting.

An eagle’s call is a high-pitched, whining scream that is broken into a series of notes. They don’t have vocal cords, so sound is produced in a bony chamber called the syrinx, located where the trachea divides before the lungs. Scientists have differentiated four different calls. Eagles are most vocal when they are threatened, annoyed, or mating.

When eagles first fledge, they are mostly brown, except under the wings, which are mostly white. As immature eagles grow, their body coloration changes and they molt and replace feathers each summer. As juveniles mature, their feathers become a mottled brown and white. By three-and-one-half years, the head and tail are nearly all white, and by four-and-one-half, immature eagles are nearly indistinguishable from adults.

Bald eagles have 7000 feathers. Feathers protect them from both heat and cold and offer a barrier to snow and rain. They have several layers of feathers that tightly overlap and provide a solid covering. It is because of this coat of feathers that eagles can spend winters in extremely cold climates. Depending on the ambient temperature, an eagle can rotate its feathers to reduce or increase their insulating effect. They puff up their feathers for a variety of reasons, including preening, to insulate themselves from cold temperatures, to make themselves appear larger when threatened, and when they are sick.

Feathers are of course also necessary for flight and for gliding and soaring. Like the bones, the feathers are hollow and lightweight, but they are structurally very strong. The primaries, the large feathers along the tips of the wings, provide lift and are the main controls for flight. An eagle twists these feathers to brake, turn, and maneuver. The tail feathers are also important for flying, maneuvering, and landing and for stabilizing an eagle when it dives toward prey.

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Red Squirrel (Tamiasciurus hudsonicus)

Kodiak Wildlife, , , ,
Photo by Tony Ross

Two species of squirrels live on the islands of the Kodiak Archipelago. The Arctic ground squirrel (Spermophilus parryii) was introduced a few hundred years ago[1], and the more common red squirrel (Tamiasciurus hudsonicus) was introduced in 1952.[2] The red squirrel population is slowly spreading across Kodiak Island, and biologists estimate at least 10,000 to 15,000 red squirrels now live on Kodiak.[2]

Red squirrels range across Canada and Alaska and south into the North Central and Northeastern United States, west to the Rocky Mountains.[3] Their range in Alaska extends through most of the forested areas, from the Brooks Range through south central and southeast Alaska.[4]

Red squirrels are members of the rodent family, and the species Tamiasciurus hudsonicus has been divided into 25 subspecies.[3] A red squirrel is small, measuring approximately 12 inches (30 cm) in length, with its long, bushy tail accounting for a third of the total length. A large adult may weigh 8.4 ounces (240 g).[5] In the summer, it has a pale red to olive-gray coat with a black line along each side. It is creamy white or buff-colored on its underside. In the winter, it has reddish-brown ear tufts and a bright rusty red strip along the back, while the black stripes along the side fade or disappear. In all seasons, a red squirrel has a white ring around each eye.[5]

Red squirrels are territorial and vigorously defend their territories.[3] A squirrel’s territory can range in size one-half acre to six acres, and each squirrel knows its territory well and may have several nests and food caches within the boundaries of its territory.[4] Red squirrels build their nests in trees. The nests are usually between 10 to 60 ft. (3-18 m) above the ground and are either constructed inside a tree cavity or out of a mass of twigs, leaves, moss, and lichens inside the dense foliage of a branch.[4,5}

Red squirrels mainly eat the seeds of conifer cones.[5] A spruce forest covers the north side of Kodiak Island, and the squirrels living in this forest eat the seeds of spruce cones. Since few spruce trees grow on the rest of the island, though, squirrels in other areas eat and cache alder cones. When collecting cones, a red squirrel cuts green cones from a tree and allows them to fall to the ground. The squirrel then gathers the fallen cones and buries them in one or several caches in its territory. By collecting only green cones, the squirrel knows the seeds are still in the cones.[5]

A squirrel may collect several bushels of cones in a cache, and a cache may be as big as 15 by 18 ft. with a depth of 3 ft. (5 x 6 x 1 m). In addition to one large cache, a squirrel often has several smaller caches in its territory.[4] Besides seeds, red squirrels also eat berries, buds, fungi, insects and bird eggs.[4] They do most of their food collecting during the day but may also be active on moonlit nights.[4] Red squirrels do not hibernate but instead depend on their stored food caches to make it through the winter. In regions with heavy snow, they may dig elaborate snow tunnels to reach their caches.[5]

Photo by Tony Ross

Red squirrels can climb trees with ease. They run up and down the trunks and along branches and can jump as far as 8 ft. (2.4 m) from one branch to another. On the ground, they walk or run and can run as fast as 14 mph (22.5 km/hr) over short distances.[5]

Red squirrels are solitary animals except during the breeding season, when males leave their territories, and females allow males to enter their territories.[4] A female has a one-day estrous period, and one to ten males may pursue her during that time. The dominant male will approach the female while uttering quiet vocalizations. Copulation is brief but may be repeated several times until the female becomes aggressive.[5] After a gestation of 36 to 40 days, the female gives birth to three to seven young. The young are blind and hairless at birth and weigh only ¼ oz. (7 g).[4] The young develop slowly and do not open their eyes until they are 27 days old. By 30 days of age, they are fully furred and begin to leave the nest. When young red squirrels are 9 to 11 weeks old, they begin to establish their territories.[5]

A red squirrel will emit a long rattling buzz when another squirrel enters its territory, and this call is often accompanied by tail-jerking and foot stamping. Neighboring squirrels may respond with similar calls. A slowly repeated “whuuk” call is an alarm call announcing the approach of a predator.[5] Biologists have noted red squirrels often produce a high-frequency alarm call when they detect an avian predator and a low, barking call when they sense the approach of a land predator.[4] In addition to vocalizations, red squirrels use posturing and chemical signals to communicate.[5]                                                                                                                                  

Red squirrels have a high mortality rate, and only 22% survive to one year of age. Females that survive their first year have a life expectancy of 2.3 years and a maximum lifespan of 8 years.[3] Red squirrels may be preyed upon by hawks, owls, eagles, bobcats, coyotes, weasels, minks, foxes, raccoons, and fishers. In the long term, habitat loss is the biggest threat to red squirrels.[4,5}

Beginning next week, I’ll write a few posts about bald eagles.  As always, I appreciate you stopping by to read my blog and would love hearing from you.  Please leave a comment!