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 .......

on my earliest seasonal observations on the pair nesting on Zero Avenue here in Surrey,  They use the hunting perch located on our Blaine, WA property.   If I remember correctly the earliest observed mating was October 6 and I am sure it was the very first day the pair returned to their territory.  Then mating continues through the fall, winter and increases in frequency as the laying becomes imminent.  So from this perspective if the pair has been seen mating frequently and recently this may well be a prelude to the appearance of the first egg.

However, eagles like some other species I know,  but will remain unnamed, obviously mate for reasons other than fertilizing the egg. The time sperm remains viable in the oviduct or infundibulum is not precisely known and it is probably variable.  However, in other species the viability of sperm drops off after about 10 to 15 days.  Generally it is believed that the effective mating takes place about 4 to 10 days before an egg is laid. 

The egg journey down the oviduct is about 3 days.  It is during the first day immediately following when the yolk erupts from the ovary and is "swallowed up" by the infundibulum that most fertilization takes place. The sperm have been 'hiding' in the folds of the infundibilum waiting for their big moment.  Once the yolk is fertilized, the outer egg membrane seals off,  preventing further sperm entry and the fertilized egg now starts down the oviduct.  It is during this 3 or 4 day journey down the oviduct that the yolk gets surrounded by various layers of protein and then the different layers that make up the shell. 

Then woopie -- an egg is laid.  In fact the process continues so a new egg is laid every 3 days until that female's full clutch of 2 or 3 eggs is deposited.

The interesting item here is that eagles will mate over a 10 month period but the only mating that results in fertilization happens during a two week period preceding and during the period while the eggs are being laid.  I suspect some of this mating is what we scientists would call "bonding behavior" to reinforce their territory and their relationship that must tolerate two killers coming together to procreate.  Others of us might more superficially just call this having a good old time. 

Obviously the biology that drives us drives the turtle, the rabbit, the eagle or the human. We however like to complicate each issue with altruistic and not-so-altruistic belief-based explanations.  Of course this whole process of procreation and survival rates, so finely tuned in nature to try and work to sustainable systems, is totally disrupted by our greed-driven technological capacities.  But that is another story - the story!

David Hancock

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