Hatch-year 2024 Eagle Q& A

How does SOAR choose a release location?

With every bird, the key to a release site is appropriate habitat for that bird! Before release, we look for a major travel corridor to make it easy for them to find a new territory / open space. River corridors are often used. Weather plays a factor, too. We want to give these rehabilitated birds every chance at continued success, so will not release right before a winter storm or during one of Iowa’s blazing hot spells. Releases are timed and located to create the least disturbance to resident wild birds.

Adult eagles with a territory “know” where home is. Here’s one rehab story to help explain this. In 2006, a bald eagle was admitted from Mills County, Iowa with a USGS (US Geologic Survey) band. We learned that she was banded as a nestling in Shell, Missouri in 1986!  She had a fractured humerus — which our veterinarian pinned. The wing healed nicely, she spent eight weeks in intensive care. After a time in the 100’ flight pen she was ready to go. This eagle was released on SOAR property. In March 2009 a call came in from the Conservation Officer in Mills County, he had a dead eagle with a band — yes the same eagle. From this report, we know that, with at least this eagle, she had fidelity to her nesting territory and flew over 100 miles back to Mills County after release. (Liver biopsy showed lead poisoning as cause of death. She was 23 years old and in perfect feather.)

Kay worries about releasing inexperienced hatch-year raptors. Because SOAR sits in the midst of a diverse, quality habitat with no eagle nests within several miles, many of these young birds are released at SOAR. With the 20×60′ flight pen that has a “hack window” in the far end, this flight pen has been used as a place for soft-release. A soft-release is one where after the ready patient is used to eating and navigating in that flight pen, the hack window / door is then opened and the bird can leave when ready. This flight pen is then left empty, with the window open, and food available until Kay is sure that the released birds are not returning. These birds can also indicate they are not ready by not leaving. Appropriate food (i.e. a roadkill deer) can also be left at the pond on the property. A soft release is more like leaving the nest.

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Will SOAR release the eaglets near their natal nests?

By the time the eagles have improved their flight skills and increased their stamina in both the 20×60′ and 100′ flight pens, the parents of all the hatch-year 2024 eaglets will likely have moved on from rearing young. Once the nest is empty, the adult’s territorial behavior breaks down, they are no longer bound to their territory. This behavior is referred to as post-breeding dispersal behavior. Photoperiod plays a big role in bird behavior and with the longer days, molting might be next up for the adult eagles.

Will the eaglets be tracked post-release?

bald eagle sized bird band

The only way to track a raptor post-release is to either place a USGS band on their leg or have a backpack-style telemetry unit fitted to the bird. Both of these activities require a US Fish and Wildlife Service permit (read that as permission). SOAR has neither a bird banding permit nor a telemetry permit. A USGS band typically only gives two sets of data – the age of the bird and date and place the band was attached and then the age and date and location where the band was encountered. The second encounter is usually when the bird is found dead.

telemetry unit

The telemetry unit is attached to a backpack style harness of teflon ribbon.

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When do bald eagles get their white head and tail feathers?

The adult bald eagle with the white head and tail is easy to identify! But, what about those big brownish colored birds? Some are dark, some are speckled, and some just look bigger than an eagle. These birds are considered immature, juvenile, or sub-adult eagles until they have reached their adult coloration or plumage. The layers of feathers that cover a bird, the pattern, color, and arrangement of those feathers are the bird’s ‘plumage.’

The plumages are acquired by feather ‘molts’ that begin each spring and are completed by late fall and will carry the bird through the winter months. Feathers are similar to our own hair and nail structure and are made of keratin (a protein). If the feathers (or our hair) become damaged or worn, they need to be replaced. Hormones and seasonal changes trigger feather molts. Also, if a bird loses a complete feather, it will immediately be replaced without waiting for their molting period. Feathers need to be in the best shape possible for them to be in top flying condition. Annual molts must occur over a long period of time and in a pattern so that they can continue to fly and hunt for food. This is quite different from waterfowl that will molt their feathers all at once and are left flightless for a long period of time. Understanding basic molt patterns assist in identification of a species, as well as determining an age.

As young, immature eagles age, their eyes (iris) will begin to lighten, going from a dark brown to yellow and their beak changes from a charcoal black to a brilliant yellow. These changes occur over years of time, sometimes we overlook the younger generations until they become mature with their stately appearance.

  • The first four weeks, young eagles are covered in fluffy white down.
  • The basic black and brown plumage appears at about five weeks of age.
  • At around ten weeks of age, young eagles are fully feathered with dark eyes and charcoal color beak. A hatch-year eagle (HY) will keep its chocolate brown plumage through the winter until spring. Many times, this age of eagle can easily be confused with the golden eagle.
  • In their second year (older than hatch-year but not two years), the plumage begins to start mottling with more white speckling appearing throughout the body. The eyes and beak show a very slight lightening of color.
  • Juvenile eagles in their 3rd year have a noticeable ‘belly band’ and more white mottling. The beak and eyes are beginning to show some notable fading.
  • During an eagle’s 4th year, the head and tail feathers are showing the characteristic color and look of the adult bald eagle. Yellow is beginning to appear on the beak with eyes changing to a much lighter brown to soft yellow.
  • Most eagles molt into their white head and tail feathers during their 5th year. Some may still show some blotches of brown or grey as they each will molt differently. The beak becomes vibrant yellow and the eyes will have changed to a pale yellow.

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How can you tell a bald eagle’s sex?

Male and female bald eagles have the same coloration. The difference is expressed in their size.

As part of ongoing data collection looking at bald eagles and lead exposure in birds admitted to wildlife rehabilitators in Iowa, SOAR started collecting beak depth measurements. A research paper from Garcelon et al (Journal of Wildlife Management 49(3):1985) is the basis for SOAR to use beak depth measurement for a gender determination. The Garcelon paper discussed 12 different measurements taken on bald eagles, but that beak depth showed no overlap in values between the sexes. This is the least invasive way to determine gender. A metric dial caliper is used and the measurement is taken from the top of the upper beak (maxilla) next to the cere to the bottom point on the lower beak (mandible).  The measurement is expressed in centimeters (cm).

In the bald eagles measured in the Garcelon et al study, male bald eagles had a beak depth range of 3.0 cm to 3.349 cm, while the female bald eagle beak depth ranged from 3.35 cm to 3.6 cm. All the eagles in the study were from North Central United States and Canada and information should be transferable to bald eagles in Iowa. Beak depths can, of course, be less than 3.0 cm and greater than 3.6 cm.

Also remember that female bald eagles will weigh more than males and that northern bald eagles are larger than those from the southern U.S.

Liberty beak depth line

 

measuring beak depth

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