Biology Reference Index Page


 Raptors: Biology & Conservation


General Conservation Issues


Bald Eagle Biology                           

 Life Cycle Summary


Chehalis-Harrison River Bald Eagle Congregation & Feasting Notes 


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Bald Eagle Laying & Hatching Sequences

Bald Eagle Biology

Hi All:   RE use of words  '"synchronized"  vs "asynchronized" to define Bald Eagle laying & hatching sequences.

I don't think I would ever have chosen these words for defining the laying and hatching sequence of an individual pair of eagles. 

Bald Eagles generally lay their 2 or 3 eggs 3 days apart.  After the mean 36 days of incubation then each egg would hatch 3 days after the previous egg.  However, that assumes the female initiated incubation with the laying of the 1st egg.  If the female did not initiate incubation until the clutch was finished then the eggs would all have a similar start date for embryonic development and for hatching.  This latter example is customary for precocial species like wood ducks or geese whose broods need to all leave the nest together and follow mom.

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More Thoughts on Embryonic Egg Development

Bald Eagle Biology

Embryonic development initiates when the temperature of the eggs gets to a certain level -- approaching the standard temp of about 99.5 degrees F or  37.5 degrees C  The following url give a wondrous visual opportunity to follow the development in a chicken egg -- that would hatch in 21 days, not the 35 days of the eagle:


From viewing that brief look at the eagle lifting up and swallowing the egg content I feel the content had about 10 - 12 days of development -- and then rotting deterioration.  This would equate it to about the 6 - 8 days of the chicken embryo. My reasoning for this is:  a) An infertile egg simply dries up in the shell under the heat of the incubating birds.  b) Usually the infertile egg does not go bad or look rotten in this period of time.  c) It is the fertile egg, the one undergoing cell division, bringing in oxygen and germs from the outside through the pores that sucks in bacteria that can take over and rot the egg once the embryo dies and is not growing.  So what I believe I saw on the video was a fertile egg that had initiated development but died. The dead embryo then starts to be attacked by bacteria and deteriorates.  What we saw the female take out of the broken shell appeared to me to have some "degraded" substance more like a dead embryo than an infertile yolk.

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Re Delta 2, Lafarge, White Rock and Mission, BC Nests

Bald Eagle Biology

Hi All:  The season at times progresses slowly but the season is indeed advancing.

The Delta 2 pair, like so many pairs each year, give us predictability and change at the same time.  The eagle is largely driven by the light cycle -- each bird may respond differently but each bird is relatively constant in its migration dates, egg laying dates etc. from year to year.  The next pair may be a week or two or even three different but each pair will remain fairly consistent year to year. A prime example is the Lafarge pair that has laid its first and subsequent eggs on exactly the same day for each of the last 3 years.  This tells me that at least the female is the same bird in each year.  The White Rock pair in 2011 and 2012 laid its first egg on March 13 but this year changed by 12 days.  I suspect those 2 consistent years were due to the same female occupying the nest but this year I believe at least the female, with a new light periodicity, has taken over the territory.  From some of the recent observations on the male's behaviour, or appropriate lack thereof, the male may have also changed. if only we had banded birds to tell!

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Eagle Eggs: when and how they develop

Bald Eagle Biology

The question has come up from recent observations:  "Does egg laying immediately follow mating?"

Simply put -- No!  But of course we are dealing with live creatures so variability is expected.   As many people have now observed many of our bald eagles start to breed immediately upon their return to the nesting territory from the brief fall or winter flight north.

I have commented several times .......

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Update - Development of Bald Eagle Etiquette for the Chehalis Flats Harrison Mills.

Bald Eagle Biology

Prepared for the ‘coalition of the concerned’ – The Chehalis Flats Protection Group!     

Nov. 20, 2012    Updated Dec. 6, 2012

By David Hancock,     Bald Eagle Biologist,    Hancock Wildlife Foundation.

 Background of Chehalis Flats Bald Eagle & Fish Concerns: 

Fraser Valley Bald Eagle Festival for 20 years and in the past 7 years the Hancock Wildlife Foundation  have promoted the presence of bald eagles in the Harrison Mills area.  Most recently David Hancock reviewed various bald eagle winter concentration areas in British Columbia and Alaska and other North American sites.  The historic evidence clearly shows that over time many different areas hold varying numbers of wintering eagles.  The region’s largest concentrations have regularly been at the Chilkat River in Alaska and the Squamish River system near Brackendale, BC.  More recently the Chehalis – Harrison River complex in Harrison Mills has wintered the largest concentrations ever known.  

The reasons for these huge gatherings of eagles are largely two-fold:  weather conditions up north and salmon availability for the eagles throughout the area.  The food availability locally is driven by the incredible productivity of the Harrison River system salmon runs.  Historically we have seen that just having lots of spawned out carcasses does in itself not bring record numbers of eagles.  Normally the wintering eagles are dispersed all along the northwest coastal salmon rivers, feasting on the carcasses until they are eaten out or frozen under the ice.   On top of the influence of weather conditions is the prime numbers of salmon actually returning to the different rivers to spawn each season!  Fewer spawned-out salmon in the northern rivers simply means, regardless of impacting weather, that less poundage of salmon are there to feed eagles – or the next generation & Fish.



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