Facts About Eagles

ABOUT BALD EAGLES:

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Q: How big are eagles?

A: Eagles within the Chesapeake Bay weigh between 7 and 12 pounds. Females are 30% heavier than males and have a stockier appearance. Females weigh 10-12 pound. Males weigh 7-8 pounds. Wing span is 6-8 feet. As with many animal species, body size in eagles increases from the southern to northern part of their range. Birds in Florida are quite small compared to birds in eastern Canada. Birds in the Chesapeake Bay are in the middle of the size range.

Q: What is the lifespan of an eagle in the wild?

A: The oldest recorded bird in the wild is 29 years. This is biased low because longevity records are from banding. It is likely that birds can live well into their 30s and maybe beyond.

Q: When does an eagle reach maturity?

A: Most eagles begin to breed for the first time in their fifth year. Though we have had birds breeding as young as 3 years old, many individuals may not breed until they are well past 5 years old.

Q: When do an eagle’s head and tail feathers turn white?

A: There is a transition in plumage over the first 5 years. Each successive molt being closer to adult plumage. Most birds attain the classic adult pattern between their 4th and 5th year. Some may have residual brown for more years or may never entirely lose it.

Q: When does an eagle’s beak turn yellow?

A: Bill color is like plumage in that it moves closer to classic color as they are closer to sexual maturity. Most birds have a clean yellow bill by their fourth year. Some individuals take longer.

Q: Why are females larger than males?

A: Our current thinking is that sexual size dimorphism in raptors is mostly driven by differences in gender roles. Males often do more of the hunting for brood rearing and so do more flying. Their body design is more sleek, lighter with lower wing loading which makes flight less energetically expensive.

Q: What do eagles eat?

A: Bald eagles are opportunistic foragers. They eat a wide range of prey including reptiles, birds, mammals, and fish. During brood rearing here in the Chesapeake nearly 90% of prey is fish (including quite a bit of shad and catfish). They also eat quite a few muskrat and turtles. In the winter their diet moves to a greater portion of waterfowl and gulls.

Q: How do eagles kill their prey?

A: Depending on the prey type, eagles typically capture prey with their talons and then clip the spinal chord with their bill.

Q: What is the heaviest prey an eagle can lift?

A: As a general rule of thumb, most raptors cannot fly very far with a load more than 30% of their own body weight. For eagles here in the Chesapeake Bay that would be 3-4 pounds. In terms of just lift, they can lift off the ground about half of their body weight.

Q: Why do eagles eat fish head first?

A: Most of the spines of fish are oriented backward so will onlygo down one way. When adults are feeding young chicks however they will often eat the hard parts themselves and feed the softer tissues to the chicks so they do not get caught in their crops.

Q: How many eggs do eagles lay?

A: Bald eagles typically lay between 1 and 3 eggs. Here in the Chesapeake Bay the average clutch size is about 2.3. Four-egg clutches have been documented here in the Bay but are rare. A couple of five-egg clutches were documented in the early 1900s during the collecting days but it is not clear if these large clutches were laid by a single female.

Q: What is the average brood size? The largest known brood?

A: The Chesapeake Bay supports one of the most productive eagle populations throughout the species range. Our rate of 3-chick broods is the highest in the country ranging from 10-20% annually. Average brood size for successful nests typically averages around 1.8. The largest brood recorded for this species is 4. There have been 3, 4-chick broods documented in the Bay including 1 in 1986 and 2 in 2011.

Q: Does a bald eagle have adversaries? Predators?

A: Historically, man was the greatest threat to eagles here in the Chesapeake. Currently, that threat is much diminished. Beyond man, the greatest threat to eagles is other eagles. Virtually all of the nest and chick guarding that we see is to protect them from other eagles. Great Horned Owls take over 3-5% of nests annually here in the Bay. Raccoons take both eggs and chicks from active nests.

Q: Do eagles mate for life?

A: Our current thinking is that mate fidelity is very high in this species meaning that the divorce rate is low. However, we have no good studies to estimate rates and divorce and cheating are likely increasing along with population recovery. More than 90% of birds likely stay with partners until they die at which time they will recruit another mate. Adults have a 10% annual mortality rate so about 1 in 10 birds should be recruiting a new mate annually.

Q: When do eagles mate/lay their eggs?

A: Breeding season changes with latitude such that northern pairs breed considerably later than southern pairs. Just within the Chesapeake Bay there is a 5 day difference in laying date between the James and Potomac rivers (a distance of about 100 miles). Along the James, nest building begins to intensify in October, we see courtship from late December through mid-January, and most pairs have laid eggs by mid-February. Virtually all pairs have laid by the end of February. The earliest pairs here on the James are on eggs by mid-December.

Q: What is a nest bowl? What is it made of?

A: The general lining of the nest is often referred to as a bowl because of its shape. Nests are lined with fine materials including marsh grass, field grasses, corn husks, pine straw, etc. These are fine materials that are soft. After the lining is completed, most pairs will form an egg cup on that surface that is composed of very fine plant material with good insulating qualities.

Q: How big are the eggs? What color? How fragile?

A: Eagle eggs are about the size of a baseball. They are white to beige in color with a matte finish. The shell is fairly thick and can take the adult weight, particularly if they are laying on a soft surface like the nest lining.

Q: What keeps the adult from breaking the eggs?

A: Adults can break eggs if flustered or startled and they step on them in the wrong way. Typically adults are careful when walking around eggs and position themselves in a way so as not to break them. When adults are incubating eggs the egg cup is made so that it is mounded around the eggs and takes much of the weight. The lining under the eggs also has some give and this also protects them from breakage.

Q: What keeps the eggs from freezing?

A: The egg cup which is made of fine grasses has great insulating qualities. Warmth is provided by the adults but the construction of the nest helps. During cold days the adults are covering and incubating the eggs virtually 100% of the time. On warm days with good sun the adults may take breaks from incubating and allow the sun to keep the eggs warm.

Q: What is a brood patch?

A: A brood patch is an area of bare skin on the abdomen of some birds that becomes highly vascularized during incubation. Many species lose feathers to clear this area. The increased blood supply to the skin which increases the warmth in this area. The brood patch is placed in direct contact with eggs for more efficient heat transfer.

Q: How long are eggs incubated?

A: Incubation time for bald eagles averages about 35 days.

Q: What are male/female eagles’ duties in raising young?

A: Male and female eagles can perform all of the jobs related to chick rearing but for most pairs they do have roles. In the early period after hatching the male does the bulk of the hunting providing prey to the brood. The male also stands guard nearby and is responsible for territory and nest defense. During this early period, the female does most of the direct brooding and most of the feeding of chicks.

Q: Why are eggs laid & hatched asynchronously?

A: Eagle eggs are quite large and it takes a considerable amount of energy for the female to produce a clutch. This constraint along with space limitation within the female has led to the strategy of serial laying. Many birds have asynchronous laying but synchronous hatching because they delay incubation until the last egg is laid. Development does not begin until the onset of incubation so this delay serves to synchronize the brood. Eagles typically initiate incubation with the first egg which leads to asynchronous hatching. The reason for this is not clear but it may relate to ambient temperatures at the time of laying. They may not have a choice if they want to keep the eggs viable.

Q: How do eagles keep their nests clean?

A: Like humans, eagle pairs vary considerably in nest cleanliness. Some pairs are very messy and others maintain clean nests by removing old prey remains and regularly bringing in fresh nest material.

Q: How can a bird so big fly?

A: Bald eagles are only about a quarter of the mass of Andean Condors the most massive flighted bird. They fly like other birds by gaining lift from their wing surface. The wing has a concave underside and convex outerside such that wind passing over it creates upward lift. If the lift is greater than the mass they will rise.

Q: Why do big chicks pick on little chicks? Why don’t the parents stop this?

A: The occurrence of a dominance hierarchy varies from brood to brood. Typically broods that have more than enough food coming into the nest do not form strong dominance hierarchies. It is when food is in short supply that contests are ongoing to get enough food. Dominance in these food-scarce situations serves to insure that at least 1 chick will survive (if there is not enough food and it is split evenly they will all die). It is a natural mechanism to fit the brood size to the available food.

Q: Where do eagles sleep? What keeps them from falling off their perch?

A: During the breeding season, adults sleep either at the nest or on a branch in the nest tree or nearby tree. Eagles have a specialized mechanism in their foot that allows them to lock it in position so they can sleep without controlling it. This is similar in concept to a horse sleeping standing up.

Q: Do male & female eagles both incubate the eggs?

A: Both adults incubate eggs, though for most pairs this is not evenly divided and the female typically incubates through the night.

ABOUT BALD EAGLE ANATOMY:

Q: Do eagles blink? What is the nictitating membrane?

A: Eagles do blink by closing the outer upper and lower lids like we do. The nictitating membrane is a clear membrane that is moved back and forth over the surface of the eye often times very rapidly. It functions to keep the surface moist and to clear off debris. Reptiles and birds have these membranes, humans do not.

Q: What is the crop? How does it relate to the stomach and the gizzard?

A: Birds have a two-part stomach including the forestomach or the crop and the hind stomach or gizzard. The crop is a storage space for food. The gizzard is highly muscular and depending on the species processes or crushes the food.

Q: What is a pellet/casting?

A: Pellets are bones, feathers, hair, scales and other materials that are not easily digested. These materials are not moved through the digestive system but held in the crop and regurgitated as boluses or pellets. Casting is the regurgitation of a pellet.

Q: How does an eagle’s eyesight compare to human’s or other birds’ eyesight?

A: Eagles are very visually oriented and it is their most developed sense. The old saying is that if an eagle could read it would be able to read a newspaper at 100 yards. We don’t know if this is true but the point to that statement is that they have very well developed visual acuity meaning that they can detect movement and other fine details important to them at a great distance. We still know relatively little about other species senses because we can’t actually experience them. We can compare their anatomy but there is more to sensory perception that just the receptor system.

Q: Why is there a hole in an eagle’s tongue?

A: The hole in the tongue is an opening to the respiratory system.

Q: What are those ridges in the tongue?

A: The ridges or arrow-shaped structures on the tongue aid in keeping the food moving backward toward the esophagus.

Q: What is the beak made of?

A: The bill is made of keratin similar to human fingernails.

Q: What are the talons made of? How many talons? What is the hallux?

A: The talons are also made of keratin. They have a talon at the end of each toe which means 4 on each foot. Eagles have 3 toes that orient forward and 1 that orients backward. The hallux is on the one oriented backward.

Q: How many feathers does an eagle have?

A: Eagles have several thousand feathers that account for about 5% of their body mass.

Q: Where are an eagle’s ears? How well does it hear?

A: Like all birds, eagles do not have external ears like we do. Their ears are small holes on either side of the head back behind the eyes. They are under the feathers so are not visible unless you look for them. Eagles have excellent hearing.

Q: Are those nostrils on the eagle’s beak? How well can it smell?

A: There are nostrils or nares at the base of the bill. Like with humans, this is one way the bird can breathe. Most birds are not believed to have a very well-developed sense of smell. That is the general thinking with eagles. They do not use this sense to locate prey. Supporting this view is that they have a relatively undeveloped olfactory bulb compared to birds like turkey vultures that have a very well-developed sense of smell.

Q: What keeps an eagle from freezing?

A: Eagles are warm-blooded like humans meaning that their body temperature is maintained within a relatively narrow range. There is a complex physiological system that governs temperature regulation.

Q: What is thermoregulation?

A: Thermoregulation is the ability to maintain a near constant body temperature. Bald eagles do not have this ability at hatching and that is why they require consistent brooding. They develop this ability around 15 days of age and this is a developmental milestone.

ABOUT JUVENILE BALD EAGLES:

Q: What are pin feathers?

A:Pin feathers are new feathers that are in the process of growing. They are at times called blood feathers because they are still connected to the blood supply and this connection is what fuels their growth. After they are fully grown the blood supply retreats and the feather is inert like a fingernail.

Q: What is hovering?

A: Hovering is flying in a fixed position like treading water.

Q: What is branching? When will it begin?

A: Branching is a developmental milestone when the chicks begin to venture out of the nest and into the surrounding limbs. It typically begins at about 9-10 weeks of development.

Q: What is fledging? When will it begin?

A: Fledging is an event rather than an ongoing process. It is the time when birds make their first flight. In bald eagles this typically occurs around 12 weeks of age.

Q: Will the eaglets return to the nest once they have fledged?

A: After fledging eaglets return to the nest to be fed by adults, to roost at night, and to loaf. Their association with the nest and the natal territory will begin to wane as they learn to forage on their own and they begin to roost elsewhere. The length of this transition to independence varies considerably between individuals from a couple of weeks to several months.

Q: How long do parents continue to feed the nestlings?

A: Adults will continue to tear food and feed chicks until they have acquired the ability to self-feed. Self-feeding is a developmental milestone that typically begins when the chicks are around 40 days old.

Q: When does a juvenile become a sub-adult?

A: Unfortunately, these terms are interchangeable in common usage. In the bird literature, juvenile typically refers to a bird in a particular early-stage plumage. The most common usage of juvenile with bald eagles is a bird that is in its first year. The most common usage of sub-adult is a bird after its juvenile year that has not attained adult plumage.