Eagle Biology

GENERAL OVERVIEW

The bald eagle is much more than just the symbol of America: it personifies strength, majesty and independence. Yet, its very existence is dependent upon how humans manage the environment that is shared with all wildlife species. When chosen as a national symbol in 1782, it was found abundantly throughout the contiguous United States. Historically, it has been listed as a federally endangered species due largely to human ignorance and persecution by pesticides, careless shootings, car and powerline collisions and loss of habitat for nesting and foraging. Since DDT was banned in 1972, Florida’s eagle population has increased more than 300%. In 2007, bald eagles were removed from the Endangered Species list, having been declared officially “recovered”. Today, bald eagles are still protected by the Florida Fish and Wildlife Conservation Commission (FWC) and the US Fish and Wildlife Service (USFWS) under state (F.A.C. 68A-16.002) and federal laws (Bald and Golden Eagle Protection Act and the Migratory Bird Treaty Act).

The bald eagle is native to our continent and is one of the largest birds in North America. The Latin name “Haliaeetus leucocephalus” means “white-headed sea eagle”, referencing its genus classification. The word “bald” refers to an old English use of the word meaning “white” since the head is distinctly feathered. Birds commonly confused with bald eagles include Ospreys, Turkey Vultures, and Golden Eagles, the latter only occasionally sighted in northern Florida during the winter migratory months.

(Female-left / Male-right)
Like other birds of prey, bald eagles exhibit “reversed sexual size dimorphism”, and females are larger than males. Female eagles in Florida weigh from 8-12 pounds and have a wingspread up to eight feet. Males are smaller, weighing 6-10 pounds, with a wingspread of six feet. Both sexes have the characteristic white head and tail, yellow beaks and eyes upon maturity, a gradual process of four to six years. Young eagles, called juveniles, are uniformly brown and larger in size than adults due to longer wing and tail feathers. Immature and subadult refer to plumage sequences after the first year and before adulthood.

Adaptations for survival include keen eyesight that can identify objects three to four times farther away than humans, operating much like a telescope for focusing; powerful feet and claws called “talons” to capture and kill prey; an elongated beak with a sharp tip adept at tearing food; and well-developed muscles in the legs and supporting the beak that aid in prey destruction. Eagles tear and swallow their food in large pieces, temporarily storing it in an area below the throat called the “crop” before actual digestion. The crop is a noticeable bulge when full that can store over two pounds of prey when food is plentiful.

Vocalizations: An eagle’s call is quite distinctive, ranging from a short staccato note, a whining call, and a high pitched scream of descending notes. Vocalizations serve several purposes including greeting, solicitation, territorial defense, threat, and begging calls for food (juveniles).

Longevity: Eagles can live in the wild from 15-25 years and up to 38 years in capitivity. Eagles follow a pattern typical of raptors, with lower juvenile survival followed by increasing survival into adulthood.

Diet: Eagles are opportunistic scavengers with a prey base that includes fish, squirrels, wading birds, ducks and road-killed animals called carrion. Fish is also pirated from Ospreys in spectacular aerial acrobatics.

THREATS and TERRITORIAL FIGHTS

Threats: The bald eagle has no natural predators. Their biggest enemy is humans. Causes of bald eagle mortality include collisions with cars and power lines, electrocution, gunshot wounds and poisoning. Lead is highly toxic and a preventable cause of death if bullets/shot and fishing weights were converted to non-lead material. http://www.biologicaldiversity.org/campaigns/get_the_lead_out/ Eagles may also succumb to avian pox, mosquito borne illnesses and food contamination. Loss of nesting and foraging habitat through urban development seriously jeopardizes nesting success for eagles in Florida. These birds are strongly territorial, particularly during nesting season, and are known to engage in battles over nesting habitats, causing injury and even death. In the last decade, Florida has seen an increase in eagle mortality and injury due to territorial fights.

Territorial Fights: Although eagles are highly social during the non-nesting season, intraspecies fighting with aggressive aerial territory battles can occur among eagles throughout the nesting season. Territorial defense behavior includes:

Threat Vocalization: One or both adults will emit a high pitched scream, often repeatedly, at the trespassing eagle.

Circling Display: The defenders will soar over the unwanted eagle until it leaves the area. Note: you may observe this display if an immature eagle in non-threatening plumage enters the nesting area.

Territorial Chase: This is one of the most common and potentially dangerous of all territorial displays. Territory defenders pursue the invader until he/she leaves the area or a fight ensues. This can be a fight to the death. The state of Florida has witnessed an upswing in eagle mortality/injury due to territorial disputes as habitat diminishes. Typically, talon wounds are inflicted on the legs, lower abdomen, chest and head areas, and in severe disputes, mortality occurs.

When mature (age 4-5), eagles return to the vicinity of their original nest sites to breed. That is why there are concentrations of active nests located around large lakes throughout Florida. An area’s carrying capacity is reached when the habitat is saturated with eagles for the amount of food and nest sites that it can support. This causes a natural dispersal of birds into new areas for breeding, including historic ranges and non-traditional nest sites in urban areas.

Adult Plumage Sequence

  • 1  Year 1 Year The juvenile plumage is an overall brown color with few cream specs under the primaries and tail.
  • 1.5  Year 1.5 Year After the first year molt, the birds show more mottling than before and yellow color appears at the base of the beak.
  • 2.5 - 3 Year 2.5 - 3 Year At this age the crown (top of the head) turns smokey-gray and the iris color is light cream instead of dark brown. The overall plumage is highly mottled with light cream to white feathers.
  • 3 - 4Year 3 - 4Year This bird is approaching the third molting time. At this point most of the beak is covered in yellow and so is the coloring of the iris.
  • 4 - 5 Year 4 - 5 Year The bird now shows a white head with brown specs and the tail are mostly white with some light brown flecking near the base of the feathers. There is a well defined dark band at the tip of the tail feathers. At this age birds can be highly variable with some only showing the white heads, but still wearing a dark tail, or viceversa.
  • 4.5 - 5.5 Year 4.5 - 5.5 Year By now the bird has gone through the fourth molt and shows more of their definitive plumage (solid white head and tail).




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