This page presents data about times from egg-laying to hatch and  times between hatches recorded at wild Bald Eagle nest cams from 2006-2016.

The information here comes from JudyB’s charts, the Hancock Wildlife Forum, the Channel Islands EagleCAM Forum, nest cam websites, and my own observations. Scroll down for charts giving the raw data.

Determining the exact time of hatch is difficult for several reasons:

• A hatch may not be visible on the cam because the camera angle may not give a clear view into the nest cup, or an incubating parent, another egg or nestling, or nesting material may obscure the view.
• The online video stream may not have an embedded timestamp, or if present it may not be accurate.
• Most experts agree that a hatch has occurred when a chick is completely free of the shell, but some observers may record a time when the chick is only partially out of the shell.

Thus time of hatch may be approximate, or reported times may vary from one viewer to another. I have attempted to restrict my analysis to data that appear to be as reliable as possible, which includes dates and times for 44 breeding seasons at 31 nests with 2-egg clutches (N=44), and 28 breeding seasons at 20 nests with 3-egg clutches (N=28). In my judgment the overall analysis is plausible given the size of the sampling and consistency among the data.

General observations

Time from egg-laying to hatch among all 72 clutches:

• The average time was exactly 36.5 days (36 days 12 hours).
• The shortest time was about 34.5 days (34 days 11 hours 1 minute).
• The longest time was about 40.5 days (40 days 12 hours 17 minutes).

The interval between egg-laying and hatch almost always decreases for successive hatches in a clutch.

In 2-egg clutches, Egg 1 takes on average about 1.4 days longer to hatch than Egg 2:

In 3-egg clutches, Egg 1 takes on average almost 2 days longer to hatch than Egg 2 and almost 2.5 days longer to hatch than Egg 3:

The data also show that it takes longer for an eagle to lay a complete clutch of eggs (click here for data on egg-laying) than it does for those eggs to hatch:

The reason for these differences can be found in the incubation behavior of the parents.

• Bald Eagles’ eggs hatch successively in the order in which they were laid, called asynchronous hatching.
• The first chick to hatch will be larger and more developed than its sibling(s), which gives it an advantage in the competition for food in the nest.
• To help mitigate this disparity, the parents usually incubate the first egg intermittently until the second egg is laid, which slows the development of the embryos of the earlier eggs.
• This phenomenon is sometimes referred to as “delayed incubation,” but the parents usually do incubate most of the time, so it is actually the hatching that is delayed. A better description of the behavior might be “intermittent incubation.” Scientists use the term parental attentiveness in reference to the amount of time parents devote to incubating.
• Parents gradually increase their attentiveness until the clutch is complete.
• Since later eggs are incubated more consistently than earlier eggs, their development progresses more quickly and they hatch in less time.

Another way to see the effects of parental attentiveness on different eggs in a clutch is to consider the intervals between hatches.  In 3-egg clutches, because Hatch 1 is usually delayed and Hatch 3 is not, the interval between Hatches 1 and 2 is considerably shorter than the interval between Hatches 2 and 3:

• The shortest hatch-to-hatch time on record was 3 hours 57 minutes between Hatch 1 and Hatch 2 in a 3-egg clutch.
• The longest hatch-to-hatch time on record was 99 hours 42 minutes between Hatch 2 and Hatch 3 in a 3-egg clutch.

Geography does not appear to play a predictable role in incubation behavior.

• One might expect that eggs in colder northern climates would be incubated more regularly than those in warmer southern climates. The data do not bear this out.
• Longer incubation times for first eggs, indicating partial parental attentiveness, occur in both colder climates including Pennsylvania, Maryland, Minnesota, Iowa, and British Columbia, and in warmer climates including Florida, Virginia, and the Channel Islands off the California coast.
• Conversely, shorter incubation periods for first eggs, indicating close to full parental attentiveness, occur both at nests in colder climates, including Wisconsin, Minnesota, Iowa, New Jersey, and British Columbia, and in warmer climates, including Florida, Virginia, and the Channel Islands.
• But local climate is not solely a function of latitude. Other conditions such as elevation above sea level, proximity to a large body of water like an ocean or lake, precipitation and humidity, rural or urban habitat, and changeable weather conditions from year to year certainly affect both the dates of egg-laying and the incubation behavior of parents.

The charts below show the information collected.

You can sort on a column by clicking its heading. Nest codes used here are listed in this chart. All times are local nest time and are given in 24-hour format without a colon (0000 = midnight, 1200 = noon).

Abbreviations:
h = hour
m = minute
d = day
< = before or by (not included in calculations)
~ = approximately (not included in calculations)
strikeout = unhatched eggs
italics = nestlings that died before fledge

2-Egg Clutches

3-Egg Clutches

© elfruler 2016, 2017