Chat Moderators > Decorah Eagle Mods Want You To Know -- Eagle Education

Growing Up...Developmental Milestones

<< < (3/5) > >>

jfrancl:
10 to 11 weeks old!  Now it's all about preparing to fledge!  As we have mentioned, along with flight feather development, the last stage of development for a bald eaglet is neurological and behavioral, learning coordinated movements.  At 10  -11 weeks old the eaglet will soon be ready for flight.  Typically fledge occurs between 10 to 13 weeks of age.  Researcher Gary Bortolotti stated that males 1st flight average 78 days (68-84 day range), females 1st flight average82 days (78-88 day range).  They are now exercising their wings regularly, hovering and may have started to branch.  These active behaviors strengthen their pectoral muscles needed for flight.  As the eaglets approach fledging the adults spend less time around their young and the eaglets themselves do not seem very interested in their parents unless they arrive with a meal.  When the nestlings spot the parents coming toward the nest, they usually start their loud screaming whines and squeeing!  We have been asked in chat about their changing feeding schedules.  When the chicks are young, peak energy demand occurs in the late stage of rapid growth when metabolic demand is high.   After terminal size is reached, energy demand declines to maintenance alone and resembles that of adults feeding schedule. We have also been asked if parents will disallow food for a few days around this age to encourage fledge.  Some researchers state that this is not true and the eaglets fledge when they feel confident in doing so. 

The juvenile eaglet's eyes are a dark brown, beak and cere are a black tone and feathers are a dark brown to black in color and the feet and foot pads are yellow.   Weight varies but a ballpark of 10 lbs with a range of approximately 2 lbs in either direction would be safe to say. Determining gender with a 100% accuracy at this point can only be done through DNA analysis or blood work.

Mark Stalmaster reports that at 10 to 11 weeks old , defensive behavior towards other birds or humans nearing the nest might include retreat to the opposite side of the nest, or jumping from the tree or attempting to fly away.  This can lead to severe consequences for a still flightless bird.  He also categorizes 3 stages following fledge.  These include early, intermediate and late stages.  In the early stage (10 - 12 weeks old), once most of their flight feathers have fully developed at about 95%, they may attempt leaving the nest.  The timing of fledge can be affected by several circumstances.  Disturbance as I mentioned earlier in the post, gender - males more likely to fledge earlier than females, and a single eaglet may fledge sooner than one who has nestmates.  Birds that fledge at a younger age are more likely to land on the ground before trying again.  Parents do not force fledge and as stated above, fledging occurs only when the eaglets are physically prepared to leave the nest. 

During these final weeks before fledge, the eaglets flap, jump and generally move around the nest a great deal and spend an increasing amount of time on support branches of the nest.   According to Gary Bortolotti, males tend to be noticeably more active in this respect than females as is typically the case with raptors.  The period just before fledge, nest site imprinting is on the upswing and will peak after their first flight.

Stay tuned for Finn's informative post on week 12!

jfrancl:
One quick observation I'd like to add to Finn's wonderful post on week 12 and fledge.
 
During the fledglings time on the wing observing their parents hunt and fish, we have also seen dad teach them how to soar!  This seems to occur a couple weeks after fledge, at least at this nest site.  in 2013, Our observations:  Dad starting low circling his way up in thermals, purposefully and deliberately.  Rising up strong and steady and effortlessly.  One of his fledglings followed just beneath him, not knowing exactly what to do, flapping then a little soaring, flapping, soaring, while still rising up under dad.  Then another fledgling did the same under its sibling.  Flapping, soaring, flapping, soaring!  Now with 2 eaglets under dad and rising, the third eaglet joined in under sibling #2.  Flapping, soaring, flapping, soaring!  Now all under dad in a column formation flapping, soaring.  As they rose higher, more soaring than flapping.  By George, I think they've got it!  It was breathtaking to see how dad taught his eaglets how to Flap, Glide, Soar!!

jfrancl:
Our last stage in the milestones thread is dispersal!

Dispersal can be defined as movements that have no fixed direction or distance, resulting in a mixing of individuals from different areas but don't necessarily bring about any change in overall distribution.  This occurs when the young eagle is no longer dependent on its parents for food. Dispersal can lead to range extension and can have important genetic consequences (reducing inbreeding, promotes gene exchange).  There are several types of dispersal.  Natal dispersal, breeding dispersal and non-breeding dispersal.  In following our Decorah juvenile, we will cover natal dispersal, which generally encompasses much larger areas than the other forms of dispersal.   

The degree of nest site fidelity varies between species and between sexes in which females usually disperse further than males and larger species such as bald eagles tend to disperse further than smaller species.  Movements of juvenile bald eagles has not been well documented in the past as their movements are more nomadic than purposeful at first.  Many juvenile eagles will migrate south if their natal nest is in the north or time to go north if raised in southern latitudes. A flyway is a path birds congregate along to travel. There are four major flyways in the US. Pacific Flyway, Central Flyway, Mississippi Flyway and Atlantic Flyway. Tracking using telemetry offers researchers the opportunity to document the daily movements of individuals for many consecutive years.

Mark Stalmaster stated that approximately six to ten weeks after fledging young eaglets will begin to break family ties and leave the nesting area.  Dispersal times vary depending on the individual bird, some leaving sooner, some later. By this time they are more or less sufficient, able to fly with ease and acquire prey on their own.  In some populations that are not migratory, they may remain in the vicinity for several years.  And even migratory juveniles may return to the general area of their natal nest location, and perhaps establish a territory of their own and continue the cycle of breeding.  Colder weather increases eagles food requirements and shorter days give them less time to obtain sufficient food. In addition to learning hunting techniques the juvenile will learn what type of prey to hunt and what not to pursue though some prey recognition is obtained while in the nest. First year eagles are notorious for attacking just about anything that floats or moves.  Whether they are successful or not in capturing prey is a different story.  Most first year eagles will feast mainly on carrion or stealing prey from other eagles and other species of birds.

Closer to home, D1 and Four are both part of the largest longitudinal study of bald eagles ever undertaken.  We know that D1 has traveled North to Hudson Bay in Canada starting in 2012 and back to the greater Decorah area each year.  Here is a post pertaining to this study that Decorah Eagles Chat room moderator FinnBMD entered in the Flap, Glide, Soar thread in RRP forum.  Page 11,  post number 151:

http://www.raptorresource.org/forum/index.php/topic,1745.150.html#lastPost

We know that young eagles may move in more random directions than adults who have developed strong habits, returning along the same routes to the same wintering and nesting areas year after year. Movement pathways also depend on the season. Gary Bortolotti stated that: "For immature eagles in their first fall, migration and movements appear to be, to a considerable extent, a reflection of whither the wind blows. For adults, wind probably has no influence on the eventual target of their migration, generally summer and winter habitats with which they are familiar. Nevertheless, the wind strongly affects the route they take and other aspects of the pattern and timing of their movements."

As D1 has been following a similar path going north and returning south, it will be interesting to see in which direction Four will travel in the coming years!  Will she follow her sister's migration pattern or will she venture in a totally different direction?  Only time will tell!

T40cfr403:
This quote comes from the Raptor Education Foundation's Facebook page:

"I have been told by a long-term eagle biologist that typically the very EARLIEST a young golden eagle will start hunting on her own is approximately 90 days after fledging. However, most eagles rely upon carrion and the kills of older, more experienced eagles, until they are around 2 1/2 to 3 years of age. The status of local prey populations makes a huge difference in the learning curve."

This sounds reasonable to me but do you think it could also apply to bald eagles?
 

jfrancl:
Hi T40 and thanks for your comment.

According to Gary Bortolotti, male bald eagles fledge at about 78 days old and female bald eagles fledge at about 82 days old.  They stay in the area with their parents until they are no longer dependent on parents for food.  That learning period can vary greatly but an average is between 4 weeks and up to 2 months after fledge. 

The eaglet first learns to recognize foods in the nest; when it fledges and follows the parents, it learns by watching them and other eagles. It continues to learn by observation and through trial and error. The predator's instinct to pursue prey is innate and is fine-tuned through learning and experience.

Their first year of independence is the toughest for them and they will mostly rely on carrion and stealing tecniques for their meals.  Fine tuning these techniques takes time and proficiency in hunting make not be accomplished until they reach 2-3 years old.  Of couse bald eagles are mainly fish eaters and goldens rely more on ground prey but the availability of prey would probably play a part in the success or failure of honing their survival skills. 

In a study done in Labrador, Canada, it was stated that juvenile migration might be initiated by changes in foraging opportunities with birds departing an area if prey is not readily available.  It stated: "While the four eaglets (out of 5 being studied) travelled independently of one another some went in a similar direction moving between lakes and rivers and exploiting aquatic environments.  They made several stopovers along the way, most near larger water bodies such as the Gulf of St Lawrence, Ontario Lake and Lake Champlain.  Gerard believed that juvenile BEs followed rivers and lakes as these areas represented a more favorable habitat.  We suspect that the Labrador eaglets gained experience during their autumn migration and their flight routes and strategies were more defined and less nomadic in the spring."

Navigation

[0] Message Index

[#] Next page

[*] Previous page