Bald Eagles

Bald Eagles

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LIFE HISTORY

Life History or Annual Cycle

References: Life History

References: Feathers & Molt

References: Movement & Migration

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Summary: Life History or Annual Cycle

The Life History of a bird is an annually repeated cycle, a genetically hard-wired program adapted over eons of time. A species’ physiological and behaviorial features that ensure its survival are known as Life-History traits.
Life-History traits engender Life-History strategies that play out in a repeated series of stages that for most species mirrors the Earth’s annual cycle of changing seasons. The stages include:
- Reproduction
- Molt
- Migration
All birds undergo Reproduction and Molt, usually yearly. Migration is widespread, but it is not essential to all species for their survival, including Bald Eagles. I use the term Movement instead of Migration for this stage in the Life History of Bald Eagles.
The availability of food is the ultimate factor in the evolution of all the stages of a species’ Life History.
The Reproduction stage occurs during the time of year when food is most abundant for both growing chicks and their parents. It proceeds through several phases:
- The Preparation phase, which involves claiming a territory, nest-building, and pair-bonding. The Gonads undergo a long process of regrowth, called gonadal recrudescence, since they contract and become unfunctional (gonadal regression) after the previous breeding season. For Bald Eagles, Preparation can last for 8-16 weeks.
- The Maturity phase, which includes two sub-phases:
  - The Sexual sub-phase, when the nest is finalized, and eggs develop, ovulate, are fertilized, and are laid. For Bald Eagles, this sub-phase can occupy 3-4 weeks.
  - The Parental sub-phase, which entails incubation of eggs and brooding of chicks. For Bald Eagles, this sub-phase can take 15-18 weeks.
- Termination, when juveniles fledge and disperse, adult molt and gonadal regression proceed, and adults transition into a Movement stage. For Bald Eagles, this phase can be as short as 1 week or last up to about 8 weeks.
The Molt stage for Bald Eagles occurs only once annually.
- For breeding adults it begins about halfway through incubation.
- It is a partial molt each year, during which all feathers molt except the remiges (flight feathers of the wings). From 4-7 of the primaries and 3-11 of the secondaries on each wing are replaced annually, in a pattern during which only 1-3 feathers that are not adjacent to each other molt at any one time.
- Juvenile eagles keep their first contour feathers for a year and have their first Molt beginning in the first spring after they hatch.
Migration across long distances in search of food is not essential for Bald Eagles, but most breeders undertake some Movement away from their immediate nesting area after their young fledge.
- Bald Eagles breed from late fall into early spring, earlier than most North American species.
  - Their Movements away from their nest territories begin from late winter into spring, at a time when other birds are beginning to return from their non-breeding habitats in the south.
  - Bald Eagles then return to their nesting territories in the late summer and fall, when other species are departing on Migration.
- Limited data is available about the Movements of Bald Eagles, but tracking studies indicate that they travel in different directions, not exclusively south, depending on location of their breeding area and food availability at their destinations.
- Bald Eagles are partial migrants in that some individuals or populations make long-distance journeys while some individuals or populations do not. They follow several strategies of Movement:
  - Local movements with little or limited travel from an area.
  - Nomadic movements with unpredictable directions, distances, destinations, and durations.
  - Migratory movements from one discrete area to another.
- Some breeding adults are year-round residents of their local nesting territory, but most undertake some Movement. Some stay within a few miles of their nesting area, while others travel further, to areas with more abundant food.
- Non-breeding adults, or floaters, tend to be nomadic until they can claim a mate and establish a territory. Some of them remain in a local area, perhaps near a mated pair’s territory awaiting an opportunity for a takeover.
- Sub-adult Bald Eagles generally are wide travelers, some nomadic, and some more migratory.
- Juveniles are the most adventurous, more nomadic than adults and sub-adults and moving greater distances.
- Migratory Bald Eagles seek areas with readily accessible food, often gathering in convocations at open waterways. They move only during daylight hours when thermals and winds are active, generally alone or with only one or two other eagles, often mixed in with hawks.

HOME
BLOG
BIOLOGY
BREEDING
EAGLET GROWTH
- EAGLET DAILY PHOTOS & MILESTONES
- REFERENCES
BALD EAGLE NEST CAMS
GLOSSARY
CONTACT

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