What Makes Bald Eagles Incubate?

We wrote this blog to describe events at the North Nest back in 2023, when Mr. North and DNF were coping with a lot of intruders. Instead of perching near the nest, packing in food, and developing the reserves she needed to lay eggs, DNF guarded her nest, egg, and mate from potential rivals. After egg number one, her testosterone and progesterone should rapidly decrease, while prolactin, a hormone that induces incubation and brood patch swelling, should rapidly increase. But intruder-related stress kept her testosterone and progesterone levels high and inhibiting prolactin, which reduced incubatory behavior. 

Why are we talking about it this year? Although the North nest failed, Mr. North incubated his unhatched egg very regularly until April 14 and on and off until April 20. Hatch started on March 20 and would usually have been completed by March 25 or 26, so he went almost a month incubating an egg that was not going to hatch. The Canada Geese eggs on N2B should have started hatching nine days ago. At this point, they are almost certainly not going to hatch, but the female goose is still faithfully incubating them. What gives?

Hormones, egg production, and broodiness

As daylight length increases, birds’ gonads swell. Both sexes produce testosterone, although male birds produce more than female birds. Testosterone is associated with aggression, territoriality, courtship, nest-building, testicular development, and sperm production. Female birds also produce progesterone, aka the “pregnancy hormone”, which induces egg production. Most birds are louder and brighter in early spring, when bird song, plumage, and courting behavior are in full swing.

Mr. North and DNF on April 19. He incubated the couple's lone egg almost a month past the time we expected hatch.
Mr. North and DNF on April 19. He incubated the couple’s lone egg almost a month past the time we expected hatch.

So why do eagles get ‘broody’ after they start laying eggs? Another hormonal change! After egg number one, testosterone and progesterone rapidly decrease, while prolactin, a hormone that induces incubation and brood patch swelling, rapidly increases. Opioid peptides help stimulate prolactin production. Perhaps a sudden, sharp rise in opioid peptides helps normally active birds switch from moving around to sitting on eggs all day.

The chart below shows the relationship between daylight length, testosterone production, prolactin, and luteinizing hormone in male Svalbard Ptarmigans. Male Svalbard Ptarmigans don’t care for their young, but similar hormonal progressions have been observed in male birds that do. Testosterone peaks early, prolactin ramps up as testosterone falls, and prolactin production decreases rapidly after peak.

Changes in the concentrations of plasma testosterone, LH and prolactin in five captive, male Svalbard Ptarmigan exposed to seasonal changes in daylength
Changes in the concentrations of plasma testosterone, LH and prolactin in five captive, male Svalbard Ptarmigan exposed to seasonal changes in daylength
What happens when eggs don’t hatch?

I couldn’t find a similar chart for Bald Eagles, but I suspect that peak prolactin probably lasts 45 to 50 days when eggs hatch normally. Hatching triggers another cascade of metabolic changes: when eggs hatch, prolactin ebbs, metabolism speeds up, and parents become more active. But in the case of Mr. North and Mother Goose, the eggs didn’t hatch. Prolactin ebbed naturally, but didn’t drop quickly – there were no triggers for the next phase of nest life and no new hormonal regime to cue the behaviors necessary for attending hungry eaglets and active goslings. In short, there was reason for them to stop incubating.

February 11, 2025: DNF's first egg!
February 11, 2025: DNF’s first egg!

DNF laid her first egg on February 11. If we add 50 days, we land on April 2: the last day that DNF spent time incubating the couple’s egg. Although DNF changed her behavior fairly quickly, Mr. North hung for 18 more days. His incubatory behavior, right down to the length of time he stayed incubating, is pretty consistent with that of Peregrine Falcons whose eggs fail to hatch. I’ll be curious to see if it holds true for Mother Goose as well. I’d also love to know whether male eagles generally spend longer incubating unhatched eggs than female eagles since this is not the first time we’ve seen this behavior from Mr. North.

We’re still hoping to retrieve the Norths’ unhatched egg. In the meantime, we hope Mr. North and DNF enjoy their summer off. We look forward to seeing a resumption of nesting activities this fall!

References

Related posts:

  1. Do bald eagles delay incubation?
  2. Bald Eagles, Eggs and Cold Weather
  3. Why do bald eagles lay eggs in ice and snow?
  4. Bald Eagles, Menopause, and Ova
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