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.

I do not consider that Bald eagles closely "synchronize" their egg laying.  Adjacent pairs can be a month apart. On the other hand many eagle pairs precisely repeat, year after year, the same seasonality -- arriving back from migration, laying first egg etc. on the same or near same day. The adjacent pair is more likely to be 2 - 5 weeks different than synchronous with its neighbor.

Many raptors have a different strategy.  Generally they initiate incubation with the laying of the first egg so the chicks hatch at the same delayed period as they were laid - in eagles about 3 days apart.  One of the big debates in biology is why have many species of predators evolved a system that often results in sibling mortality of the later hatched chicks?   Starting incubation with the 1st egg insures protection of the 1st egg against predation by passing ravens.

An alternative argument, particularly among those species that lay many eggs, is that this is "the species" insurance policy to facilitate large survivability during years of food abundance but gives further assurance during years of low food supply that the oldest chick(s) who are dominant get sufficient food to survive.  In the years of low food availability only one or a few of the chicks survive by being the "food bullies" while the smaller last hatching chicks quickly wither and die.  In other words the species decision has been it is better to raise 1 or 2 chicks than have everybody die of simultaneous starvation.

The eagles have generally evolved a slight modification on the above strategies.  While wilderness eagles along the British Columbia coast generally initiate incubation with the 1st egg, we have recently seen our "urbanized eagles"  show a slight modification.  These urban-suburban eagles seem to have modified their strategy.  They sometimes seem to "partially cover the 1st egg" (offering protection from ravens?) yet don't seem to sit so tightly or consistently during the period between the 1st egg and the 2nd to not initiate embryonic development, and giving the 2nd eggs a "more similar incubation period to the 1st".  Is this some kind of adaptive behavior to the urban environment where ravens are fewer and where food supply is often more abundant than in wilderness areas? 

Both elements may well be playing apart. As we saw a couple of years back, our Sidney Ma was away just less than 1 minute (53 seconds!!) when a raven took an egg. Yet, it seems apparent, though not spelled out scientifically, that the urban eagles seem to raise more young per nest than wilderness eagles.

Note:  I have always applied the terms "synchronous" vs "asynchronous" to how one bird of a group encouraged others nearby to have similar timing to their breeding.  Concentrating the breeding cycle to a shorter season has several advantages.  For example in flocking geese, a shorter breeding cycle insures more adults are peaking in their breeding cycle to insure fertility, and then the resulting clutches and broods offer a shorter period when they are vulnerable to predators.  The predator has fewer days when eggs or chicks are most exposed. The same argument is given for the concentration or synchronous breeding and calving of wildebeest in Africa -- the helpless young are concentrated over a very short period reducing predation.

I see our discussion forums have used the terms for slightly different meanings.  Certainly various breeding behaviors, like "synchronous calls" between male and female eagles would surely be an activity to stimulate common timing of the pair.  They need to be building nests, undertaking effective "copulations", etc. during the narrow time period prior to the egg emerging from the ovary to entering the fallopian tube etc.  We know that eagles will copulate from the day of their arriving back on the breeding grounds (Oct. 6 was my earliest observation) from their northern migration to the days they depart after fledging their young (late July).  Somewhere in that 10 months of mating is surely a narrower window when fertility is possible  -- that mating is not just fun and bonding!

David Hancock

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