EGG-LAYING TIMINGS - Bald Eagles

The Bald Eagle nest cams from 2007-2016 are giving us priceless new information about breeding in the wild (as opposed to captivity). We now have some real numbers to crunch. Data about egg-laying collected from JudyB’s charts, the Hancock Wildlife Forum, the Channel Islands EagleCAM Forum, nest cam websites, and my own observations give reliable dates and times for 61 breeding seasons at 29 nests with 2-egg clutches, 38 seasons at 21 nests with 3-egg clutches, and 1 nest with a 4-egg clutch. Only nests with precise timings are included in this report.

Three 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)

These data yield some general findings:

• Most first eggs of a clutch are laid between about noon and midnight.
• The eggs come at roughly 3-day or 4-day intervals. This is consistent with afternoon-evening layings: if another egg is not laid by the end of the third day, it is likely to come about a day later.
• The shortest interval between one egg and the next was 68h40m or just under 3d (BC dl2 in 2016). The longest interval was 116h37m or nearly 5d (IA dec in 2010).
• The interval from one egg to the next almost always increases with subsequent layings.
• A particular female generally has a consistent pattern of timings. Some tend to lay eggs at 3-day intervals, others at 4-day intervals. A particular female also tends to lay at roughly the same time of day from year to year, although there are exceptions. If you sort on the Nest column in each chart you can observe the patterns and inconsistencies. See comments below the charts for further details.

2-egg clutches

• About 82% (50 of 61 breedings) had a 3-day interval from the first to the second egg, with a mean average time of 73h21m.
• About 18% (11 of 61 breedings) had a 4-day interval from the first egg to the second, with a mean average time of 94h34m.
• Most females are consistent 3-day or 4-day layers from year to year (sort on the Nest column).

3-egg clutches

• About 92% (35 of 38 breedings) had a 3-day interval from the first to the second egg, with a mean average time of 72h16m.
• About 8% (3 of 37 breedings) had a 4-day interval from the first egg to the second, with a mean average time of 93h57m. (Note the contrast with 2-egg clutches, where a larger percentage (18%) had a 4-day interval.)
• The proportion between 3-day intervals and 4-day intervals from the second to the third egg is more balanced than with 2-egg clutches:
  • About 55% (16 of 29 nests with precise timings) had a 3-day interval, with a mean average time of 74h12m.
  • About 45% (13 of 29 nests) had a 4-day interval, with a mean average time of 96h46m.
• As with 2-egg clutches, eagles at particular nests usually had a consistent pattern of laying from year to year (sort on the Nest column), either 3 days both between first and second eggs and between second and third eggs, or 3 days between first and second egg and 4 days between second and third eggs. Where the pattern is not consistent, possible reasons may be a change of mate (as at CA trt in 2014), inclement weather, intruders, etc.
• The 2010 season at IA dec was unusual in that the interval between the second and third eggs, 116h37m, was  more than 20 hours longer than the next longest interval, at CA trt in 2013 (97h58m).  If the third egg-laying at IA dec in 2010 is excluded from the calculations, about 57% (16 of 28 breedings) had a 3-day interval, with a mean average time of 74h12m, and 43% (12 of 28 breedings) had a 4-day interval, with a mean average time of 94h55m.