Eggs and cold weather

This blog was first published on Wednesday, February 26, 2014. It has been updated to reflect Mom’s new mate and includes some information about the Fort St. Vrain eagles as well.

Mom Decorah with her second egg in 2014

Mom with eggs on February 26, 2014

Everyone was worried about the eggs in Decorah back in 2014. Iowa was facing a polar vortex while Mom was laying and incubating eggs. While there have been other cold years, 2014 was one for the record books, with windchills of -50F when Mom laid egg #2. Given that 2019 looks to be a rerun, I thought I’d talk a little bit about eggs and cold damage.

Much of what we know about eggs and temperature comes from incubationists, or people who incubate eggs. Temperature, humidity, and regular turning are all important to insuring an embryo’s survival and eventual hatch. In 1969, H. Lundy identified five temperature zones characterized by their effect on the developing embryo. These studies were carried out on chicken eggs in an artificially controlled environment, but we can look to them for some idea of how the eggs in Decorah and elsewhere might respond to ambient temperature. The five zones are:

  • The zone of heat injury or death: Above 104.9°F or 40.5°C.
  • Optimal temperature, aka the zone of hatching potential: 84.5° to 104.9°F or 35 to 40.5°C. The American Eagle Foundation lists this temperature as 99° for bald eagle eggs incubated in an artificially controlled environment.
  • The zone of disproportionate development: 80.6 to 95°F or 27 to 35°C. Embryos in eggs that spend too much time in this zone can develop unevenly, leading to crippling injuries or death. Successful hatching is greatly reduced.
  • The zone of suspended development: 28.4 to 80.6°F or -2°C to 27°C. Eggs at this temperature don’t develop at all. Freshly laid eggs can spend a lot of time at this temperature with no harm to the egg or embryo.
  • The zone of cold injury or death: Below 29°F or -2°C. Although eggs contain a great deal of water, they can get colder than 32°F/0°C without freezing. However, eggs that reach 29°F will freeze, which usually causes death. Some exceptions to this rule have been recorded in Mallard ducks.

So what does it all mean?
In the last couple of days before egg-laying, Mom and DM2 will build a wonderful nest or egg cup of soft dry grass and shredded cornstalks. They’ve been carefully preparing the spot for months, but they won’t finish it until shortly before Mom lays eggs. The soft materials conform to the eggs, wrapping around to provide warm, dry insulation below and around them. Mom and DM2 transfer heat from their bodies and regulate temperature by alternately sitting on or standing away from the eggs. Of course, this year may be cold enough that they don’t spent very much time standing. Sitting low in the nest helps keep the eggs and incubating parent warm. One of our facebook fans compared it to Tupperware – a great analogy, since the eggs are cupped below and sealed above. We’ve also watched the eagles bring in extra cornhusky insulation this year, building their pile as the cold deepens. The eagles may be on to something, since in 2009 the US Department of Energy awarded $200,000 to Husk Insulation, a company that makes insulation from cornhusks. The company’s panels are 10 times more effective than conventional insulators like foam.

So how warm are Mom and DM2 ? In a study of overwintering bald eagles, Mark Stahlmaster found that adult bald eagle average body temperatures ranged from 102.2°F, or 38.9°C, to 106.1°F or 41.2°C. Ambient temperature, wind, precipitation (snow can be insulative, but rain is not), shelter, and time of day all had an impact on body temperature. Since the zone of heat injury can be lower than the body temperature of an incubating adult, it’s easy to see why Mom and DM2 might spend time off their eggs in warmer weather, especially early in incubation. The eagles at the Fort St. Vrain nest lay eggs in a warmer, drier climate, and it is very common to see them spending time off the first and second eggs when weather permits.

Although males take their share of egg duty, I wonder if larger females might spend more time incubating in extremely cold weather or at night, when eagles can drop their body temperature to reduce energy loss. Larger animals have an easier time conserving heat, since they have lower area-to volume ratios. While Mom isn’t that much larger than DM2, it might make all the difference in a cold weather event.

Lundy’s work was based on artificial incubators, where temperature and humidity can be tightly controlled. Our eagles live in the real world, subject to real weather events. But our nests have an excellent record of hatching eggs. Can we guarantee that the eggs won’t freeze or suffer cold damage? As much as we’d like to, we can’t. But Mom is a diligent, experienced parent with a well-built nest cup, a long-term territory, and a good food supply. Unlike us, they don’t need an instruction manual to provide information about temperature regulation, egg-turning, or care. We are watching and hoping for the very best. Go Decorah Mom and DM2!

PS: We used to see a difference in the egg ‘line-ups’ – Mom incubated the eggs in a triangular pattern, while Dad incubated them in a straight line. This almost certainly has to do with the size difference between the two (and the overall area of their brood patches), although ‘shapes’ are not uncommon. Like sitting and standing, shapes and positioning can help regulate temperature and keep eggs alive. We’ll be curious to see if DM2 does the same thing.


Stalmaster, Mark V., and James A. Gessaman. “Ecological energetics and foraging behavior of overwintering bald eagles.” Ecological Monographs (1984): 407-428.

Incubation zones: Brinsea Incubation. Check them out for incubators, hatchers, brooders, and information:

Cornhusk Insulation:

Did you know?

Some birds incubate eggs by burying them in mounds. Meet the Megapodes!

Do bald eagles delay incubation? It’s more complicated than we used to think!