Great Eggs-pectations

Bird eggs vary in shape, size and color but not in their stages of embryonic development.

As a bird curator, I frequently find myself being asked, “What is your favorite bird?” People are always surprised by my answer: “The one that is still in an egg!” So at this time of year, I have a lot to be eggcited about. I am fascinated by the structure and function of eggs. The idea that a bird can appear out of one in as few as 11 days (or as many as 85!) still astounds me. Despite this broad range of incubation times for different species of bird, every stage of embryonic development occurs in consistent order across all 10,000-plus species, resulting in an amazingly specific choreography of development and hatching. Each species, however, requires its own particular temperature, humidity and other nest conditions to result in a successful hatch. Take a crack at these fun facts about eggs to see how many you know:

  • The largest single cell in the animal world is the yolk of an ostrich egg.
  • Duck eggs are best for baking because their yolks are larger than those of chickens, making baked goods extra moist. The more independent a chick is at hatch, the larger their yolk is relative to the egg.
  • It takes about 24 hours for a chicken to produce an egg, and she can lay more than 300 per year.
  • It takes 3 to 4 days for a cinereous vulture to produce an egg, and typically only one will be laid each year.
  • A kiwi egg is the largest egg relative to mom’s body size. Laying one is equivalent to a woman giving birth to a 24-pound baby.
  • A hummingbird egg weighs less than 1 gram, about the weight of a U.S. dime.
  • Small eggs require higher incubation temperatures, but shorter incubation times, than large eggs.
  • Nothing but oxygen passes through the 15,000 to 20,000 pores in the eggshell from the time the egg is laid until it hatches. Carbon dioxide and water vapor are expelled, however, so eggs actually lose weight as the chick develops.
  • The yolk of the egg is the ovum. It is fertilized at the top of the fallopian tube. The white of the egg is deposited as the yolk continues to pass down the fallopian tube. The shell is created in the uterus. The fertile egg is in stasis when laid, and the chick will not begin to develop until the egg is warmed by an incubating parent.
  • Brush turkeys, a species native to Australia, don’t sit on their eggs but bury them in mounds of earth to be warmed by the natural composting of leaves and other organic materials.
  • Most eggs at the zoo are incubated by parents in nests, but some are artificially incubated in machines. These are tended by keepers and are “candled” with a bright light and weighed twice weekly to document development and adjust the conditions in the machines to ensure healthy hatches.

Already in 2012, crested wood partridge, Bourke’s parrots, white stork, fairy bluebird, sunbittern, swan goose, Northern pintail, Micronesian kingfisher, blue-grey tanager and pheasant pigeon eggs have successfully hatched at Lincoln Park Zoo. The next time you crack open an egg, take a minute to think about how amazing it really is! Colleen Lynch Colleen Lynch is Lincoln Park Zoo’s Hope B. McCormick Curator of Birds.


Wonderful article! I had no idea that eggs were so varied and phenomenal! I enjoy all the articles published by your bird curator! She knows her stuff!!!

I agree this is a wonderful article. Please keep me posted on all the articles.

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