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 What Drives Fledging and Migration or Dispersal?
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By: davidh (offline) on Sunday, August 22 2010 @ 02:49 PM EDT (Read 1419 times)  

RE: "Reluctant to Leave"!!

Comments on What Drives Fledging and Migration or Dispersal?

The end of July and throughout August on our northwest coast is the period of nest desertion but the process is challenging, danger filled and appears to be rapidly evolving to changing conditions.

The Observations:
Young bald eagles are largely abandoned in the nest at full growth and once the feathers are hard pinned and ready for flight. Only rarely do the parents feed them following fledging. In fact any feeding of young post-fledging I have witnessed was largely the young bullying the parents out of the parents' food, often a quarter a mile down the beach. In fact it is my interpretation that the aggressive bullying by the fledged chick, often attacking the parent to the extent of knocking them over on arrival with the food, is what actually facilitates the departure of the parents from the area.

Possible Interpretation: The 'abandonment' of the chicks and the territory is both a safety adaptation by the parents to avoid being attacked and hurt and part of the learning by the chicks to "out-bully" any other possessor of food. I see this bullying as the substitute for the lack of flying, tactical and hunting skills of the juveniles that may take a few years to hone down. Remember, there must be good reasons why it takes five years for a young bald eagle to reach the breeding age. I suggest this is largely because it takes five years for the young to develop sufficient hunting skills to ultimately be able to make a living for itself, its partner and young from a one to two mile territory around the nest.

The Process: The normal pattern of fledging, abandonment of chick and migration from area by parents and chicks in wilderness areas is this:

a) 2 -4 days before the 84th day after hatching the adults stop or slow down bringing food to the nest.

b) During this period the chicks have been exercising and developing both wing and foot agility -- so important in that first landing.

c) Finally the fledge -- the leaving of the nest and nest tree to a place a few hundred feet away -- the big danger should they goof the landing and fall into a thick understory.

d) The parents watch over the juveniles, still calling and perhaps defending the area from other eagles.

e) By day 5 - 7 post fledging of the last young, the adults abandon the area and the chicks.

f) During that 5 - 7 day period I had never seen food brought to the nest in wilderness areas. Certainly the feeding adults could occasionally be seen on regular feeding perches or adjacent beaches eating. Almost always they would be approached, more as an aggressive attack than "here I am to share your offering", by the kids. I have seen the juveniles at this stage literally knock over the parents in their eagerness to get to the food. I take this aggressive behavior as the "severing action" by the juveniles. The parents want no part of that unsafe aggression and this is the signal to the parents that parenting duties are over. They leave for the northern rivers.

g) The juveniles, after 4 to 7 days of further fasting or finding a few remains left in the area, then also initiate their migration.

h) Conclusion: The July - August nestling fledge, abandonment by parents and then departure from the area by the young is the normal process.

What we are witnessing is in my view just part of this natural weaning process. It is driven by evolution, by instinct, by the proven paths of success. We don't always see the 'kindness' elements and they in fact are not part of the equation. Survival is the game. And presumably, the parents don't wish to risk being hurt by the aggressive young and have learned to abandon the young at this stage. The young, those that survive and produce more offspring 5 years later, have built into their being that, at least for the coastal eaglets, that they must fly north to find the early dead salmon carcasses. Our coastal eagles can have access, somewhere along the coast to dead or spawning salmon from June though February.

Some differences may well have evolved for eaglets reared where they did not have access to the salmon cycle or another similar fish spawning cycle.

The Evolving Urban Eagle Difference: Those watching only the live cams of urban eagles might have even drawn another conclusion. We see some of the urban parents frequently bringing food back to the nest post fledging. The result is that the 'hunger driven' attacks on the parents seem to not be happening. Why? Well I have studied more wilderness living eagles than city or suburban birds but the biggest difference in habitats is quite obvious. That is security of resting and feeding places. In the wilderness areas, where almost every tree and section of beach is a safe resting or feeding area, the adult or young can land and feed and feel safe. In the urban areas they are so often driven off by dogs, women pushing baby carriages, joggers, cars etc. I suspect the key proven safe place is the nest so the parents continue to bring in their food to that safe place. This means that the young are not going through the pre-fledging weight loss as quickly, both a stimulus to going to where food might be and a stimulus to 'fledge'!

Then over the next two or even three weeks they should have been abandoned by the parents and undergone exercises with no or little food consumption, causing them to drop weight and easing the ability to fly and soar. Instead they are now still sitting in the nest -- perhaps not dropping the weight -- the key to easy soaring and a 500 to 1000 mile flight to a spawned out salmon carcass.

The urban eagle is undergoing changes.

What is driving these changes is another interesting question for which we have no definitive answer -- only hypothesis. One of my theories is that there is developing in these urban areas so much competition for nesting territories that the territory holders are coming back from the northern trip sooner and sooner. A couple of pairs may not even be leaving the nesting territory.

David Hancock

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