When will our eagles lay eggs? Bald eagle breeding in Iowa and Florida

This blog was first published on Tuesday, November 07, 2017. It has been updated to reflect new information. TLDR: We’ll most likely see eggs in Decorah in mid to late February.

We’re getting some questions about why the Decorah Eagles aren’t laying eggs yet. Are they going to lay eggs? Will the nest be productive? The short answer is ‘Yes’ – the Decorah eagles historically lay eggs in February, while the Decorah North Eagles laid eggs in March in 2015, and in February in 2016. The longer answer is more complicated. If you don’t want to read it, skip down to the bottom of the blog for breeding season information.

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 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.

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. This might free the gonads of Florida eagles from the circannual light-based cycle of 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.

Daylight Length Graph from Fort Myers Florida and Decorah Iowa

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 chart below.

Weather: A year in fort St. Myers, FL

Daylight length isn’t a great predictor of seasons in Fort Myers, but rain sure is! 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 SW 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 cloud cover chart from Fort Myers…
Two things. Firstly, the cloud cover chart for Fort Myers, Florida, somewhat resembles the daylight length chart for Decorah, Iowa. What does that look like?

Decorah Daylight Length Chart

Like the daylight chart, it has a steep-sided trough that shows rapidly changing light availability, even though we are measuring intensity instead of length. Secondly, the breeding cycle of eagles in SW Florida appears to be tightly coupled with light intensity. Their overall cycle is quite similar to that of northern eagles, but the external cue that fires breeding behavior appears to 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. 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.

Important events in Decorah, Decorah North, and Fort St. Vrain

Interested in key events at the Decorah, Decorah North, and Fort St. Vrain nests? Mark your calendars as follows!

  • In the nests we watch in Iowa and Colorado, bonding and copulating behaviors become more pronounced and frequent after the winter solstice. Female eagles begin laying eggs 5-10 days after productive copulation begins. This usually happens in mid-February at all of our nests, but can change if an eagle takes a new mate. New mates often seem to push nesting chronology a little later, especially in the first year.
  • Each egg is laid about 3-5 days apart, and incubation starts with the laying of the first egg. However, eagles may spend more time off their eggs in warmer weather.
  • Eagle eggs begin hatching roughly 35 to 39 days after they are laid. This usually begins in late March at all of our nests. Hatch can take more than 24 hours for any given egg.
  • Eaglets spend 75-80 days in the nest before fledging. This usually happens in mid to late June at all of our nests.

Did you know?
Fledglings appear to leave the SW Florida nest 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: