AT WILD BALD EAGLE NESTS,
2006-2020

© elfruler 2020

Lost Eggs

From 2006-2020 at the nests observed, there were a total of 388 nests with first clutches of eggs (not counting second clutches), producing a total of 910 eggs (See Table 1):

• • 1-egg clutches produced 27 eggs.
  • 2-egg clutches produced 488 eggs.
  • 3-egg clutches produced 378 eggs.
  • 4-egg clutches produced 12 eggs.
  • One 5-egg clutch produced 5 eggs.

Through all years, 20.8% of the 910 eggs were lost. Table 3 enumerates these.

Table 3

Click on the Pop-Out button to open in a new tab.

Loading...

Taking too long?

Reload document

| Open in new tab

• • 1-egg clutches lost 55.6% of their eggs.
  • 2-egg clutches lost 21.1% of their eggs.
  • 3-egg clutches lost 16.7% of their eggs.
  • 4-egg clutches lost 25% of their eggs.
  • The single 5-egg clutch lost all of its eggs.

The causes of the losses include external events, such as bad weather, a fallen nest, Bald Eagle intruders, and intrusions by other animals, which can lead to broken, abandoned, or predated eggs. Other causes include accidents (such as a misstep by a parent) and hatching failure (when the egg begins to hatch but fails to complete the process). Often, but not always, these events can be observed on cam, and even when they can be viewed, the effect on the eggs is not always clear. Among the lost eggs at these nests:

• • 12.7% were lost as a result of intruders.
  • 5.3% fell to predation.
  • 4.2% experienced hatching failure.
  • 3.7% were abandoned.
  • 3.2% succumbed as a result of bad weather.
  • 1.6% were lost when the nest fell.
  • 1.1% broke as a result of an accident.

Eggs that were lost without observable cause make up the majority of the lost eggs:

• • 29.6% failed to hatch.
  • 21.7% broke before hatching.
  • 7.4% were lost for unknown reasons.
  • 4.8% disappeared.

An egg that fails to hatch is either infertile or nonviable. (See discussion of unhatched eggs here):

• • Infertility is a reproductive problem with one or both parents that prevents the female’s ovum from being fertilized by the male’s sperm. In the data in Table 3, 1.1% of eggs were found by laboratory examination to be infertile.
  • Nonviability is a problem with the embryo that prevents it from developing properly, which can result from myriad reasons, including external events such as enumerated above. In the data in Table 3, 3.7% of eggs were found either by laboratory examination or by evidence visible on cam (such as a partially developed embryo in a broken egg) to be nonviable.

Failed First Clutches

Of 389 first clutches of eggs observed, 11.3% were unsuccessful, resulting in the loss of all eggs, as listed in Table 4.

Table 4

Click on the Pop-Out button to open in a new tab.

Loading...

Taking too long?

Reload document

| Open in new tab

1-egg first clutches failed at the highest rate, while none of the 4-egg first clutches failed:

• • 44.4% of 1-egg clutches failed.
  • 11.5% of 2-egg clutches failed.
  • 2.4% of 3-egg clutches failed.
  • None of 4-egg clutches failed.
  • The only 5-egg clutch failed.

The causes of failed clutches include some of those described above in reference to lost eggs, including external events, infertility, and nonviability, but a large number failed for unknown reasons. Table 4 enumerates these failed clutches at specific nests (referred to by abbreviated codes, which are identified at the end of the table) and gives the cause, if known. See this page for discussion of unhatched eggs.

The number of lost clutches varies from year to year. 2018 and 2020 saw the most lost clutches, with 7 lost each year. However, the highest percentage of lost first clutches occurred in 2015, when 22.2% of first clutches were lost. In 2007 and 2012 no first clutches were lost. 2009, 2010, and 2019 were good years, when 5%, 5.3%, and 7.9% clutches were lost respectively.

At nests marked in Table 4 with an asterisk (*) the adults later produced a second clutch of eggs, which is discussed here.