Cold weather often raises concerns among Bald Eagle watchers. Will their favorite eagles keep their eggs warm and dry in sub-zero temperatures and snow? Eagles don’t have central heating or electric blankets, but they have everything they need to incubate their eggs through the worst a Midwestern winter can throw at them: meticulously built nests that aid incubation and withstand winter’s chill, highly vascularized brood patches that transfer body heat directly to their eggs, and remarkably robust eggs. In short: trust the eagles! Their eggs will be fine.
The Nest: Cradle and Climate Control
Bald eagle DNF and her two eggs on a cold, snowy evening. The nesting materials, scrape, egg cup location, site preparation, and tree all create a microclimate that traps heat and shelters eggs and incubating parents from harsh winter weather.
Eagle couples usually begin building or replenishing their nests in autumn, piling them high and deep with materials that will insulate and cradle the eggs they lay months later. They churn, dig, and probe with their powerful legs and sensitive beaks, loosening the nest’s substrate to form a soft, shallow depression or scrape in the lee of a large trunk or limb. Layer by layer, they build an egg cup on top of it, moulding soft materials into the nest’s floor and carefully testing and refining the fit to ensure it is ready for eggs. The nesting materials, scrape, egg cup location, site preparation, and tree all create a microclimate that traps heat and shelters eggs and incubating parents from harsh winter weather.
The Brood Patch: Temperature Control
Mrs. North perches on a branch. An eagle’s brood patch transfers heat from its body to the eggs beneath it.
Eagles transfer heat to eggs via a special area of bare skin on their bellies called a brood patch. Shortly before laying, the hormones oestrogen, prolactin, and progesterone cause feathers on a bird’s belly to loosen and drop off, creating a patch of bare skin. Oestrogen stimulates the growth of additional blood vessels in and around the patch, increasing the flow of warm blood to the surface. It may also cause eagles to store fat in and around the patch, making it swell and increasing its pliability.
The eggs are warmed and protected by their parents’ bodies and the insulation beneath and around them.
The Eggs: Remarkably Robust
It’s hard not to worry about eggs freezing, especially when an incubating eagle is buried under inches of snow! But eggs don’t freeze easily or quickly. In 1969, researcher H. Lundy identified five temperature zones characterized by their effect on developing embryos. These studies were carried out on chicken eggs in an artificially controlled environment, but we can look to them for some idea of how eagle eggs might be affected by ambient temperature. The five zones are:
- The zone of heat injury or death: Above 104.9°F or 40.5°C.
- Optimal temperature, or 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 injuries and 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 about 29°F will freeze, which usually causes death. Some exceptions to this rule have been documented in Mallard ducks.
February 15, 2025: DNF’s eggs. Eagle eggs don’t freeze easily or quickly.
How warm are adult Bald Eagles? Eagle researcher Mark Stahlmaster found that wintering adult bald eagle body temperatures ranged from 102.2°F to 106.1°F depending on ambient temperature, wind, precipitation type, shelter, and time of day. Since the zone of heat injury is lower than an adult Bald Eagle’s highest body temperature, we might see our parents spend some time off their eggs, especially early in incubation. Trust the eagles – they know more than we do!
Egg Laying and Temperature Records
| Date |
Maximum |
Minimum |
Average |
Departure |
Eagle |
Rain |
New Snow |
Snow Depth |
| 2/11/2025 |
25 |
9 |
17 |
-4.9 |
DNF |
0.02 |
0.5 |
0 |
| 2/15/2024 |
45 |
25 |
35 |
15 |
DNF |
0.24 |
0.6 |
0 |
| 2/20/2023 |
43 |
18 |
30.5 |
8.6 |
DNF |
0 |
0 |
0 |
| 2/16/2022 |
44 |
32 |
38 |
14.4 |
DNF |
0 |
0 |
4 |
| 2/16/2021 |
17 |
-14 |
1.5 |
-22.1 |
DNF |
0 |
0 |
8 |
| 2/21/2020 |
35 |
-3 |
16 |
-9.6 |
DNF |
0 |
0 |
6 |
| 2/21/2019 |
30 |
6 |
18 |
-7.6 |
DNF |
0 |
0 |
12 |
| 2/21/2018 |
26 |
9 |
17.5 |
-8.1 |
Mom |
0 |
0 |
0 |
| 2/20/2017 |
60 |
42 |
51 |
25.8 |
Mom |
0.3 |
0 |
0 |
| 2/18/2016 |
40 |
22 |
31 |
6.6 |
Mom |
0 |
0 |
4 |
| 2/18/2015 |
10 |
-7 |
1.5 |
-22.9 |
Mom |
0 |
0 |
3 |
| 2/23/2014 |
16 |
-7 |
4.5 |
-21.9 |
Mom |
0 |
0 |
8 |
| 2/17/2012 |
44 |
20 |
32 |
8 |
Mom |
0 |
0 |
0 |
| 2/23/2011 |
32 |
24 |
28 |
1.6 |
Mom |
0 |
0 |
6 |
| 2/25/2010 |
27 |
-8 |
9.5 |
-17.8 |
Mom |
0 |
0 |
11 |
| 3/2/2009 |
25 |
6 |
15.5 |
-14 |
Mom |
0 |
0 |
0 |
Things can and do go wrong, but in sixteen years of watching, we’ve only seen complete egg failure twice: once in 2018, when Mrs. North’s first egg broke, and once in 2023, when DNF didn’t incubate. Even in the brutally cold years of 2014, 2015, and 2021, all of the eggs hatched.
Lundy’s research focused on artificial incubators, where temperature and humidity are meticulously controlled. Our eagles live in the real world, subject to unpredictable weather. But our nests have an impressive track record of successfully hatching eggs. Can we guarantee that the eggs won’t freeze or suffer cold damage? As much as we’d love to, we can’t. But our eagles are experienced and attentive parents with well-constructed nests, established territories, and steady food supplies. Unlike us, they don’t need instruction manuals for regulating temperature, turning eggs, or providing care. Talons crossed and hoping for the best. Go eagles!
Look for a difference in egg ‘line-ups’, especially in three egg nests. Larger female eagles tend to incubate eggs in a triangular pattern, while male eagles tend to incubate 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.
Resources
The Raptor Resource Project