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Inside an Eagle Egg

In about 35 days, a disc a few millimetres (about a tenth of an inch) wide on the yolk becomes a bald eagle chick. Turn the egg, dive inside, and watch its temperature, heartbeat, breathing and water loss, day by day.

Day 30of about 35

Vital signs of the egg

Egg temperature37.5 °C 100 °F

stops <27 °C
<81 °Fnormal 35–38.5 °C
95–101 °F>40.5 °C
>105 °F

Kept warm by the parent's brood patch

Heartbeat ♥–

Egg weight122.7 g (4.3 oz)

0 g of water lost through the shell

Air cell1.5 mL

Oxygen use0 mL/h

through about 10,000 pores (model)

Turned by the parents0×

Parent on the eggsyes, 98% of the time

Day 0

Inside an
Eagle Egg

35 days in 3D, with real footage from my nest cameras

Narration Speed

The numbers that keep an egg alive

What	Value	Why it matters
Temperature	About 37.5 °C (99.5 °F), normal 35–38.5 °C (95–101 °F)	The parent's bare brood patch keeps the egg warm. Below about 27 °C (81 °F) development stops; above 40.5 °C (105 °F) for hours the embryo dies. Cold mostly pauses development, heat kills.
Time covered	About 98% of the time	In a studied pair the eggs were uncovered only about half a minute per hour in cold weather. Both parents sit; the female did about 72%.
Cooling when uncovered	Time constant about 55 min in still air, 18 min in a 5 m/s (11 mph) wind (model)	At 0 °C (32 °F) and still air, an uncovered egg drops below 27 °C (81 °F) in about 18 minutes; in wind, about 6 minutes.
Turning	About once an hour	Keeps the embryo from sticking to the shell membranes. It stops about a day before hatching.
Water loss	About 15% of the egg's weight, 18 g (0.6 oz)	Water vapour leaves through the pores. The air cell at the round end grows to take its place.
Oxygen	Rises to about 30 mL (1 fl oz) per hour before pipping (model)	Three quarters of all the oxygen is used in the last third of incubation.
Shell	About 0.5 mm (1/50 inch) thick; egg 71 × 55 mm (2.8 × 2.2 inches), 123 g (4.3 oz)	DDT made shells about 10% thinner and they broke under the parents. After the US ban in 1972, breeding pairs went from 417 (1963) to over 71,000 nests (2019).

Eagle or chicken?

A chicken egg hatches after 21 days, an eagle egg after about 35. The stages are the same, only slower: as a rule of thumb, an eaglet on day 10 looks like a chick on day 6. Biologists number the stages 1 to 46 (Hamburger and Hamilton, 1951); the card above shows the stage for each day. There is a bigger difference at the end: chickens hatch ready to run and peck, while eagles hatch helpless and are fed by their parents for weeks.

Quiz: what do you remember?

Sources and notes

Stage timing: P. Sharpe, Institute for Wildlife Studies, Guide to Bald Eagle Egg Incubation and Chick-Rearing, Table 1 (Hamburger and Hamilton stages on a 35-day bald eagle incubation).
Hamburger, V. and Hamilton, H. L. (1951). A series of normal stages in the development of the chick embryo. J. Morphology 88.
Egg size and mass: Jenkins et al. (2002), Florida Bald Eagle egg characteristics, J. Raptor Research 36.
Attentiveness and turning: Gerrard, Wiemeyer and Gerrard (1979), Raptor Research 13; Wiemeyer (1981), Raptor Research 15; USFWS NCTC eagle updates.
Temperature limits: Webb (1987), Thermal tolerance of avian embryos, Condor 89; Lundy (1969).
Heart rate, oxygen and air-cell gases: Mueller, Burggren and Tazawa (2015) in Sturkie's Avian Physiology; Rahn et al. (1974). Curves for the eagle are scaled from chicken data (model).
DDT and eggshells: Hickey and Anderson (1968), Science 162; Wiemeyer et al. (1984, 1993); USFWS (2021) population report.
No one has filmed an eagle embryo inside its egg; the visuals are original 3D illustrations based on these stages.