March 3, 2024: Mr. North rolls the eggs
When will the Decorah North, Trempealeau, and Fort St. Vrain eagles begin laying eggs? Mark your calendars as follows!
- In the nests we watch in Iowa and Colorado, eagles begin laying eggs 5-10 days after productive copulation begins.
- In 2024, DNF laid her first egg on February 15, Mrs. T laid her first egg on February 25, and Ma FSV laid her first egg on February 29. Mrs. T could float a little earlier this year given that Mr. T appears to have been new last year. We’ll see what happens! But my money is on mid-February for the Norths. If it stays warm and relatively moist, we should see eggs on the earlier side. If it gets cold and dry, they’ll be a little later.
- Each egg is laid three to five days apart and incubation begins as soon as the first egg is laid. However, our eagles will spend some time off their eggs if the weather is (relatively) warm. Embryos don’t develop between 28.4F to 80.6°F (-2°C to 27°C), which means that freshly laid eggs can spend a lot of time at this temperature with no harm to the egg or embryo.
- Eagle eggs begin hatching roughly 35 to 37 days after they are laid. This will begin in late March if the eagles lay eggs in mid-February. Hatch can take more than 24 hours for any given egg.
It looks like the lead-up to eggs has started at the North nest. Mr. North and DNF have copulated almost every day since January 24 and DNF was very interested on the one day they didn’t. This is very similar to 2024, which again, puts egg number one around February 15. We’re also seeing solicitation, footing, and duetting. More on eagle vocalizations here: https://www.raptorresource.org/2022/12/09/eagle-vocalizations-hd-hm-and-morning-song/.
Bald eagle breeding season: not the same everywhere
We know that Bald Eagle breeding season varies by latitude. While breeding chronologies differ from nest to nest and state to state, in general:
- In the SE United States, bald eagles may begin laying eggs in November.
- In the SW United States, bald eagles may begin laying eggs in December.
- In the northern United States, bald eagles may begin laying eggs in mid-January.
- In Alaska, bald eagles may begin laying eggs in late March or early April.
Bald eagles lay eggs earlier than many other diurnal raptors, or daytime birds of prey, and eagles in the southeastern US lay eggs earlier on average than their counterparts in other places. This is more unusual than it looks. Roughly speaking, a non-tropical bird’s year can divided into two big parts: the photosensitive period and the photorefractory period. In the northern hemisphere, the photosensitive period starts when daylight length begins increasing after the winter solstice in late December. Birds’ gonads begin to swell and produce sex hormones, leading to productive mating and egg-laying. During the photo-refractory period, which starts after eggs are laid, birds’ gonads shrink and mating becomes less frequent or stops altogether. Bald eagles in Florida are an interesting exception to this rule, since they begin laying eggs as early as November, when daylight length is still decreasing and their gonads should be (but obviously aren’t) senescent.
Light intensity versus daylight length
So why do Florida bald eagles lay eggs so early? One possibility: daylight length, especially in south Florida, is a poorer predictor of the season than it is in Decorah, Iowa, which might change the timing of gonadal swelling and shrinking that regulates northern bird breeding. In Fort Myers and elsewhere in south Florida, the difference between the longest day and the shortest day is just 3 hours and 21 minutes, compared to 7 hours and 26 minutes in Decorah.
Decorah, IA has a much steeper light curve than Fort Myers, FL
Birds that live in habitats where environmental cues such as photoperiod are poor predictors of seasons must rely on cues other than daylight length to regulate their circannual clocks. So if daylight length isn’t especially important in SW Florida, what is? Check out the rainfall chart below.
Seasonal rainfall chart, Fort Myers, Florida
Daylight length doesn’t change that much in southern Florida, but precipitation sure does! And what comes with rain? Clouds! The Max Planck Institute for Ornithology suggests that equatorial (and possibly near-equatorial) birds use daytime light intensity instead of length to regulate their internal clocks and breeding cycles. Under this scenario, the rapid increase in daylight intensity that begins in early September swells southern Florida eagle gonads and begins their annual breeding season. How tightly coupled is the increase in light intensity with the onset of the breeding season? Check out this annual cloud cover chart from Fort Myers.
Note that the cloud cover chart for Fort Myers, Florida, somewhat resembles the daylight length chart for Decorah, Iowa. Both charts have steep-sided troughs that show rapidly changing light availability, although one shows light intensity and the other shows photoperiod length.
Daylight length over one year in Decorah, December 1 to December 1
The annual reproductive cycle of eagles in the Upper Midwest and Plains is closely linked to daylight length, whereas in central and southern Florida, it is more closely tied to light intensity. While eagle cycles and behaviors are generally similar, the external cues that fire breeding behavior in Florida begin in September or October instead late December. Of course, laying eggs in November and December also allows nesting eagles along the Gulf coast to avoid the hurricane/rainy season, which runs from June 1st through November 30th, and takes advantage of seasonally available flushes of food when eaglets are at their most vulnerable stage. Fledglings will have two to three months to gain their wings and hone their hunting skills before light intensity drops and the rainy/hurricane season starts up again.
This results in a chicken/egg quandary for me. Gulf coast eagles appear to be responding to a change in light intensity instead of length. Given the timing of the hurricane/rainy season, it also seems very likely that laying eggs in November and December results in higher offspring survival rates among eagles along the Gulf coast. Is the difference in southwest Florida/Gulf coast breeding cycles the result of an evolutionary process over generations, or does the ability to change breeding cycles and circannular clocks quickly lie dormant in bald eagles and other organisms? This might be important to know given the weather and climate changes facing us.
Did you know?
Fledglings usually leave southern Florida nests in May, just as cloud cover begins increasing and light density drops. Fledglings in Decorah leave the nest area between August and mid-October, just as daylight length goes into a steep decrease. It is interesting to speculate that these behaviors are driven by the same circannual clock that drives eagle breeding biology.
Circannual clocks drive circannual rhythms, or biological rhythms that occur on an annual basis – in northern birds, think nesting and migration. But we are also driven by circadian clocks, which keep us in sync with Earth’s day/night cycle. Clock genes are extremely influential, affecting the activity of most other genes in the body in one way or another. To read more about our internal clocks, try starting here: http://www.sciencemag.org/news/2017/10/timing-everything-us-trio-earns-nobel-work-body-s-biological-clock and here: https://www.scientificamerican.com/article/molecular-clocks-scattered-throughout-your-body-not-just-in-the-brain-keep-your-tissues-humming/.
Things that helped me learn and write about this topic: