Wednesday, February 26, 2014

Eggs and Cold Weather

Mom with eggs on February 26, 2014
Everyone is worried about the eggs in Decorah. Iowa is facing extremely cold temperatures in the weeks to come. While there have been other cold years – 2008, for example, had sub-zero temperatures into early March – this looks like one for the record books. We can’t guarantee that the eggs won’t freeze, but 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, we watched Mom and Dad build a wonderful nest bowl or cup of soft dry grass and shredded cornstalks. The grass conforms to the eggs, wrapping around to provide warm, dry insulation below and around them. Mom and Dad transfer heat from their bodies and regulate temperature by alternately sitting on or standing away from the eggs. Of course, this year has been cold enough that they’ve spent very little 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 Dad? 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 Dad might (in warmer weather) spend time away from their eggs. Although Dad has taken his share of egg duty this year, I wonder if larger Mom 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 Dad, 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. Mom and Dad live in the real world, subject to real weather events. Yet they 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 and Dad are diligent, experienced parents with a well-built nest cup, a long-term territory, and a good food supply. They are spending a lot of time clamped down on their eggs, applying heat from their very warm bodies directly to the eggs. 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 Dad!

PS: Sherri Elliott has mentioned the difference in the egg ‘line-ups’  - Mom incubates the eggs in a triangular pattern, while Dad incubates 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.


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!