Tag Archives: Bald Eagles in Alaska

Bald Eagles: Hunting and Food

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

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

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.

Sign up now for my free monthly Mystery letter.  The next edition will be mailed out on February 15th.

 

 

 

 

 

Bald Eagle (Haliaeetus leucocephalus)

Bald Eagle

The bald eagle (Haliaeetus leucocephalus) is only found in North America. Its range stretches from northern Mexico to Canada and Alaska and covers all the continental United States. Due to a variety of factors, including the use of the pesticide DDT, bald eagles nearly became extinct in the contiguous United States by the 1950s. The 1940 Bald Eagle Protection Act prohibited commercial trapping and killing of bald and golden eagles, and more significantly, DDT was banned in 1972 when it was proven the pesticide interfered with the eagle’s calcium metabolism, causing either sterility or unhealthy eggs with brittle shells. In 1973, Congress passed the Endangered Species Act, and the bald eagle was listed as an endangered species. In 1995, when eagle populations in the continental U.S. began to rebound, the bald eagle was removed from the endangered species list and transferred to the threatened species list. On June 28, 2007, bald eagles were removed from the List of Endangered and Threatened Wildlife.dsc_0005-2

While Alaska’s eagles were never threatened by the use of DDT, Alaska has its own nefarious history with bald eagles. In 1917, commercial salmon fishermen convinced the Alaska Territorial Legislature that eagles killed large numbers of salmon and were competing with the fishermen’s livelihoods. This claim was later shown to be false, but the legislature enacted a bounty system on eagles that paid two dollars to anyone who turned in a pair of eagle legs. This bounty system lasted for thirty-six years and led to the killing of a confirmed 120,195 eagles, plus countless others that were never turned in for a bounty. The bounty system ended in 1953, and when Alaska became a state in 1959, its bald eagles were officially protected under the Federal Bald Eagle Protection Act of 1940. Alaska’s eagle population is now considered healthy, and one-half of the world’s 70,000 bald eagles live in Alaska. Twenty-five-hundred bald eagles reside on the Kodiak National Wildlife Refuge.

dsc_0118-2

The genus Haliaeetus, the sea eagles (in Latin, hali means salt and aeetus means eagle), includes eight of the sixty species of eagles. The sea eagles live along sea coasts, lakes, and river shores. The species Haliaeetus leucocephalus (leuco means white and cephalus means head) consists of two subspecies. The southern bald eagle, Haliaeetus leucocephalus leucocephalus, is found from Baja California and Texas to South Carolina and Florida, south of 40 degrees north latitude. The northern bald eagle, Haliaeetus leucocephalus alascanus occurs north of 40 degrees north latitude. Northern bald eagles are larger than their southern cousins

Eagles are well-insulated by their feathers and are good at regulating their body temperature. Unlike many birds, they do not need to migrate to warmer areas each winter, but in many parts of the country they do migrate, sometimes long distances, in response to varying food supplies. Due to its abundant year-round food supply, Kodiak has a non-migratory eagle population. Furthermore, hundreds of eagles from the Alaskan mainland migrate to Kodiak for the winter months. Effluent from canneries and fish processing plants in the town of Kodiak provides a consistent source of food for these birds in the winter months, and hundreds of eagles can be seen in town perched in trees, on cannery rooftops, on the edges of dumpsters, and even on pickup trucks. In Uyak Bay and other remote bays on Kodiak, eagles stay near their nests all winter, feeding on fish and winter-killed deer among other things.

Since the ban on DDT and related pesticides in 1972, bald eagle populations around the country have rebounded to some degree. The bald eagle population in Alaska is healthy and stable and has never been listed as endangered or threatened by the Federal Government. Eagles in Alaska never suffered the scourge of DDT poisoning, and even now in most areas, they live in a relatively contaminant-free environment.dsc_0099-2

The Bald Eagle Protection Act imposes a fine of $10,000 and two years imprisonment for anyone who harms a bald or golden eagle. It is illegal to even have an eagle feather in your possession without a proper permit. Nevertheless, humans are still responsible for many bald eagle deaths. On Kodiak, Refuge biologists have recovered eagles that have starved to death, been killed by airplanes and cars, caught in traps, and oiled by fish slime or fossil fuels. The Exxon Valdez oil spill killed hundreds of eagles in Alaska. In January 2008, fifty eagles swooped down on a dump truck filled with fish guts outside a Kodiak seafood processing plant. Twenty of the eagles were drowned or crushed, and the rest were so slimed they had to be cleaned. Bait left unattended on a fishing boat can cause a frenzy when eagles land and start fighting over their find. If their feathers become oiled by fish slime, they become less-waterproof, and then if the eagle falls into the water, it is more susceptible to hypothermia.kodiak_alaska_microgrid_508

In 2009, the Kodiak Electric Association (KEA) erected three wind turbines on Pillar Mountain near the town of Kodiak and added an additional three turbines in 2012. Many people worried the turbines would be a danger to eagles since turbines elsewhere in the U.S. kill an estimated 573,000 birds a year. KEA funded a study to address the concerns, and researchers determined that eagles went out of their way to avoid crossing the ridge among the turbines. No eagles were killed during the study, and according to avian biologist, Robin Corcoran, she has never received a report of a dead eagle near the turbines.

Eagles do die from electrocution on Kodiak. Many of the power poles near town are fitted with devices designed to protect eagles, but in January 2011, an eagle was electrocuted when she landed on the lowest of three cross bars on a power pole. That particular crossbar did not have a protective device because utility authorities believed there was not enough room for an eagle to land on it. The dead eagle had been banded years earlier by the Alaska Department of Fish and Game, so biologists knew she was 25 years old, the second-oldest bald eagle documented in Alaska, and one of the oldest-documented eagles in the country.

While these manmade disasters are tragic, they are uncommon and do not appear to be a threat to Alaska’s bald eagle population. A greater and less-obvious threat is the destruction of eagle-nesting habitat by logging and commercial and residential development. Eagles tend to nest in large, old trees that are not easily or quickly replaced once they are removed.

Once an eagle attains its adult plumage, it is impossible to determine its age unless it has been leg-banded by biologists. For this reason, we have limited data on the life span of bald eagles. Biologists believe 50-70% of all juvenile bald eagles die in their first year, and as many as 90% die before they are fully mature. Eagles in captivity may live 40 to 50 years. The oldest documented eagle resided in Stephentown, New York and lived to be 48-years old. On average, eagles live 15 to 20 years in the wild, and the oldest documented wild eagle was a 32-year-old bird from Maine. Alaska’s oldest eagle was a 28-year-old from the Chilkat Valley.

Eagles are fascinating birds and have been studied a great deal. Later this winter, I will dedicate more posts to eagle biology. Please leave a comment if you have a question or anything you would like to share about eagles.

Don’t forget to sign up for my free, monthly newsletter if you would like to read about true crime in Alaska.