Bald Eagle Nesting Season -- a Review of Dates and Events

Bald Eagle Biology


Recap of Bald Eagle Nest Sequence:  territory defence, egg laying, hatching & fledging.


Bald eagles occupy almost the entire calendar year with their breeding cycle.  This cycle covers 10 to 11 months from territory occupancy or re-occupancy, territory defense, nest repair and building, egg laying, incubation and feeding of young through fledging, to also include a short   ... for more ....

 


 

    .... more ......        post-fledging period before the territory and young are abandoned.

Of course the timing of the above cycle to the local situation is something that has been worked out between the eagles and the young's survival for millenia.  To a large extent it is driven by both weather and the availability of food, particularly for feeding the young and for the fledged young to find.  Along the California, OR,  WA, BC to Alaska coast, this season can have almost a 5 month difference in starting date for egg laying -- one of the more specific and easily defined times.

Timing in Southern British Columbia:  the Sidney, Delta & Lafarge Nests.

Occupancy of Territory:     The adults arrive back from the wintering grounds from October through November.

Nest Building & Repair: can start the day of the birds' return and last through the entire nesting season.  The most intense repair period is January through March.

Courtship & Mating:   Mating can also take place from the day both birds return from the north in October and proceed with a build up in intensity and frequency as egg laying approaches in late February and March, but can still be engaged in through the nesting season.

Aerial courtship is spectacular with the pair often flying in tight unison with one bird slightly behind and above the other.  Large loops are flown over a considerable distance of the territory.  Usually the pair break off this courtship flight, return to the area of the nest tree and proceed with loud unison calls where the head is thrown back. As the intensity and head throwing becomes more synchronized there seems a greater likelihood of the male immediately mounting the female.

Mating always takes place with the female standing on a branch, the nest or some solid sub-straight.  It never takes place in the air.

Egg Laying:   Eggs take 3 or 4 days to move from the ovary through the oviduct where they collect the surrounding protein and membranes and finally the shell.  One to three eggs are laid by bald eagles though there is at least a record of four eggs appearing in a Montana nest. Other instances of 4 eggs were known to result from two females laying in the same nest during the same season.

Each egg is laid about 3 days apart.  Incubation starts with the laying of the first egg.  This is likely to offer full protection to the egg from encroaching crows or ravens and to ensure that the eggs don't hatch at the same time.  Each egg also takes 35 days of incubation -- plus or minus a few hours. This means that the chicks hatch 3 days apart. The synchronization of all of this is more marvelous than the small variation witnessed.

Different pairs are more closely synchronized from year to year than necessarily from neighbor to neighbor.  There can readily be a two week or even a month's difference in the above time of the cycle between two next door pairs.  And the difference between different years for the same pair is very small.  If a pair lays their first egg February 27 one year it is likely to be within a day or two of that date every year.  In fact seeing a couple of weeks' difference in the laying dates for a pair from one year to the other is a good indicator that one of the pair has been replaced by another -- and a new timing schedule is about to be developed for the new pair.

Hatching:   is a very delicate matter and seems to take place largely at night or early morning.  Since each egg takes 35 days to hatch and the eggs were laid 3 days apart, the chicks hatch 3 days apart. This does give both insurance and opportunity for success in the developing chicks.

Alternatively, with facilitating the survival of at least one chick, this staggered hatching can be a negative element for the second or third hatching chick in times of poor food availability.  If food is in short supply the first hatched chick, with a 3 day growth advantage, is going to out-compete his smaller siblings and take most or all of the food. The other siblings will perish.

We have been fortunate in our British Columbia region to have not had to witness sibling caused mortality, usually called fratricide.  Our parents have always come up with enough food to keep all the chicks growing -- not always at the rate some of our observers wished -- but we can be thankful all our chicks have survived.  As some of our regulars well know, we have had tense moments when food seemed in short supply and one chick appeared to languish.  But we -- they -- were lucky -- the parents and the habitat pervailed.

Growth and Fledging:  Growth in eagles, as in the evolution of all species,  is a marvelous lesson in parsimony.  The simplest path is best.   At every stage in the eaglets growth you see this lesson repeated. 

At hatching the huge feet and head and beak seem disproportionate.  But the little eaglet must to able to balance itself and hold up a 'huge bill' to both ingest food but also to peck at its sibling to insure it is fed first.  The big beak and leveraging feet make this work.  Remember, ma feeds only the most intense beggar -- not the smallest or hungriest.  To get fed you call but most importantly you reach out for the food and take it before a sibling gets it.  After the first chick is satiated it simply acquiesces and sits quietly by while the next biggest chick competes for ma's attention.  If the second gets enough, the third can be fed -- but not unless!  Sibling mortality is a great survival mechanism -- but not a great strategy to witness.

Shortly the development takes on another humorous tone.  The eagles develop an immense body and belly.  It appears so out of proportion considering the chick is not developing wings or feathers. Through the first 5 or 6 weeks the chicks have gone from the light 'whitish' down, that offered little or no insulation and requiring that a parent was almost constantly brooding, to initiating a thick grey and insulating down that started in after about 10 days of age.  With this insulation on warmer days both parents could be away briefly to find food for the rapidly expanding family.

But as inferred, by the 6th week the chick is only a large glob with a big bill attached to an enormous belly and splaying legs and feet. Stubby little wing butts show but appear deformed, quite useless for housing the incredible flight panels that are about to appear almost overnight. And then it happens.  The chick has literally full body weight but is grey with down and no feathers until the feather tracks, begin to reveal emerging brown feathers.  Within a few days,  about two more weeks,  the body is all of a sudden that of an eagle. Long large wing bones seem to be sprouting as you view the bird.

Then as the 8th week approaches the downy grey chick has evolved into a fully statured and frequently standing eagle.  But it is missing wings -- but not for long.  Within two weeks, now that the wing bones have grown down to full length, there is a place to hang the huge collection of wing primaries and secondaries.  Their growth is so rapid that a standing eaglet frequently droops its wings since the developing feathers are so full of blood that the wing is very heavy.  And the entire set of primary and secondary feathers is growing at the same time, each adjacent feather follicle is filled, about 3 inches in length, with the nourishing blood  to grow the shaft and veins. 

At this stage of growth, the eagles seem to be a bit more lethargic, probably an adaptation to not wanting to bang the developing follicles for risk of permanently damaging a feather.  Each grows with the side protection of its neighboring feather.  Then by the 11th and 12th weeks all that is left to do is reabsorb the blood and grow the last of the body feathers and outer leg covets.  And of course exercise.

The exercise can be difficult to achieve or fun to watch, depending upon the practice area available at each nest.  Those nests with open tops can offer the practicing eagles open space to jump around, pick up every little gust of wind to practice 'helicoptering' and the all important landings.  Nests located down in the canopy with lots of surrounding branches offer less open areas for practice but may give more branches for gaining jumping experience.

And all that invested experience is about to be put to the eaglet's greatest test -- its first flight -- its fledging flight.  In many wilderness areas the surrounding terrain is either water or dense tall forest.  Certainly eagles can swim but that is not confidence building on a first flight nor conducive to getting a second flight.  And the alternative of falling into the surrounding canopy and not being able to emerge is not a good option either.  The important item on the first flight is a good landing, the reason for all that jumping around, the vertical flights with fancy footwork on landing etc. etc. -- the two final weeks of exercise.

And, except for our city dwelling eagles, the urban and suburban birds that perhaps have some different new adjustments they are adapting to, the wilderness eagles fledge within a day or two of the 12 week period.  Again, like the 3 days between eggs, the 35 days of incubation, the 12 week growing period is more striking by the universal small variation between pairs than for any range in these periods.

Abandonment:   Our western eagles have evolved in the region of the greatest biological wonder on the continent -- the salmon spawn. Our eagles, like our First Nations people, are creatures of the salmon. For over half the year salmon drive the eagles' cycle, particularly the production of eagles, left to fend for themselves when the dead spawning carcasses are available along the coast in the millions of tons.

If you love
-- or even care for -- our eagles, our bears, our orcas, or your neighbors, then you must care about the health of the sea and its ability to produce salmon.  This is most certainly not assured today.

David Hancock

Tag: david hancock eggs eagle lafarge sidney biology reference

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