Written by: Dr. Jacquie Jacob, University of Kentucky

Chicken lifecycle
Chicken lifecycle (Image by: Jakinnboaz on Shutterstock.com)

While we consider eggs an economical, healthy food item, for the chicken they are a means of reproduction.`It is important to remember, however, that hens do not need a rooster to lay eggs and commercial laying hens are housed without roosters. As a result, commercial table eggs are not potential chicks, even if incubated. Only fertile eggs have the potential to produce chicks if given the correction incubation conditions.

The study of the development of embryos (referred to as embryology) isa fun and interesting project that can be done by al ages. When doing incubation projects with young children, make sure that they handle the chicks carefully so that they do not hurt the chick. Also make sure that the children do not kiss the chicks or touch their face with their hands after handling incubating eggs or hatched chicks. Proper hand washing is a must.

child holding newly hatched chick
Child holding newly hatched chick (Image by Elena Sherengovskaya from Shutterstock.com)

The parts of the egg are explained in detail in the article ‘How does a hen make an egg.’ Everything that an embryo needs to develop, grow and hatch must be provided in the egg when the egg is laid. If a hen receives sub-optimal feed, there may be a lot of developmental problems with the embryos when the eggs are incubated.

Despite what some people claim, it is not possible to tell if a fresh egg is fertile or not. To tell if it is fertile you will need to either break it open and look closely at the surface of the yolk, or incubate the egg a few days and candle it. When broken open, the infertile egg has a small white spot on the surface of the yolk. For a fertile egg, the spot is slightly larger and has a minute clear spot in the middle. This is the microscopic embryo.

Many birds, including all poultry species, have what is called ‘Physiological Zero.’ Fertilization occurs in the infundibulum in beginning of the female reproductive tract. It then takes about 26 hours for a hen to create a complete egg to be laid. So, when an egg is laid, it has a 26-hour old embryo. This microscopic embryo then goes into status. It does not die, but does not develop either. It waits for the correct incubation conditions. This allows a hen to lay eggs over several days and then have them all the chicks hatch out at the same time. Physiological zero also allows us to store hatching eggs prior to putting them in the incubator. The eggs should be stored at around 55F (12.8C) or slightly below. Refrigerators are typically too cold and storing eggs in your home refrigerator may result in a poor hatch. Hatchability is reduced when eggs are stored for longer than ten days.

Physiological zero for chickens is 68F (20C). The ideal incubation temperature for chickens is 99-100F (37-38C). If the egg is heated to above physiological zero but below optimal incubating temperature, some embryonic development can occur but will result in weaker embryos and higher embryo mortality.

Yolks of fertile and infertile eggs
Yolks of fertile and infertile eggs (Image by Dr. Jacquie Jacob, University of Kentucky)

The amount of time required for incubation and hatching of poultry eggs depends on the species and are shown in table below.

Duck (except Muscovy)28
Muscovy duck35
Goose (except Canada and Egyptian)28-32
Canada and Egyptian Geese35
Quail - Bobwhite24
Quail - Japanese (Cortunix)16-18
Guinea fowl28
Chukar partridge24
Pigeon (For comparison)17

During embryo several ‘extra-embryonic’ membranes are developed. These include the yolk sac, amnion, chorion and allantois. The yolk sac is the membrane that spreads over the yolk and transports nutrients from the yolk to the embryo. The amnion is the fluid-filled sac that covers the embryo to protect it from physical shocks and injury. The chorion and allantois membranes combine to form the choir-allantoic membrane that performs four functions. It is a respiratory organ that provides oxygen to the embryo. It is a storage area for the waste products that the embryo develops. It provides nutrients from the albumen to the embryo. It also brings calcium from the eggshell to the embryo.

Graphic of developing chick embryo
Graphic of developing chick embryo (Image by Designua on Shutterstock.com)

Incubating eggs should be turned regularly from day 2 to 18. If not using an automatic turner, a minimum number of turns is recommended. For chicken eggs, the minimum is three times per day. Typically an X and O are marked on either side of each egg and the eggs rotated back and forth three times daily.

Chicken eggs in incubator without an automatic turner
Chicken eggs in an incubator without an automatic turner (Image by on By Mariya Pererodina Shutterstock.com)

When using automatic turners, chicken eggs are typically turned every hour. They are placed in the incubator on a 45-degree angle and turned 90 degrees back and forth every hour.

Incubating chicken eggs
Incubating chicken eggs (Image by branislavpudar from Shutterstock.com)

Turning the eggs is stopped at 18 days of age. This is the time when the eggs are transferred from the incubator to the hatcher. During this time the embryo is getting into position to break out of the shell. It must be in the correct position to hatch. Its beak is under its right wing with the beak pointing towards the air space. To break out of the shell, the embryo breaks through the shell membrane and enters the air space. This is referred to as internal pipping. At this time the chick goes from embryo to chick as it starts to breathe with its lungs. The level of carbon dioxide in the air space builds up as the chick continues breathing. This stimulates the chick to break out of the shell. It breaks through the shell (called pipping) using its ‘egg tooth’ (a hardened material on the tip of the chicks beak and lost shortly after the chick hatches) and cuts around the surface of the shell. It then pushes itself out of the shell. Of course, this is hard work and the first thing the newly hatched chick wants to do is rest. They soon dry and are able to move about.

Hatching chicks still in the incubator
Hatching chicks still in the incubator (Image by Malgorzata Surawska on Shutterstock.com)

Chicks that are able to get up and move around shortly after hatch are referred to as precocial birds. Those that require further development after hatch and are dependent on their parents to bring them food are referred to as altricial. An example of an altricial poultry species is the pigeon.

During incubation, the embryo develops in a predictable manner with specific effects occurring at specific times. The times for different poultry species will vary depending on the overall incubation time. The timing for the development of the chick embryo is shown below:

Before the egg is laidUnion of the ovum and sperm (fertilization)
From lay to incubationEmbryo is status with no development
Day 1 of incubationHead begins to form
Eyes begin to form
Vertebral column (spine) begins to form
Day 2Blood vessels form
Heart begins to beat
Ears form
Day 3Limb buds are visible
Extra-embryonic membranes begin to form
Day 4Eye pigmentation begins
Tongue begins to form
Day 5Reproductive organs begin to form
Day 6Beak begins to form
Day 7Egg tooth is easily seen
Segments of wings and legs are distinct
Feather tracts are present on the back
Day 8Feather tracts more visible
Day 9Toes are formed
Day 10Beak begins to harden
Day 12Down present on body
Eyes nearly closed
Scales begin on shanks
Day 14Eyes closed
Embryo turns to point head at air cell
Day 17Head of embryo under right wing
Day 19Yolk sac enters body
Day 20Yolk completely in body cavity
Chick pips air cell and then eggshell
Day 21Chick hatches

When using incubators, they can be still-air or force-air. As the names would indicate, still-air incubators do not have a fan for movement of air inside the incubator, while force-air incubators do. For chickens, the temperature should be 100-101F (37.8-38.3C) while in a force-air incubator it should be 99-100F (37.2-37.8C). Below optimal temperatures will result in abnormal embryo develop and possibly death. Overheating will also reduce the number of chicks that hatch.

The relative humidity inside the incubator is important to regulate moisture loss from the incubating eggs. For chicken eggs, the relative humidity should be 70 percent. It will be a little higher for waterfowl. Pans of water can be placed in the bottom of incubators to help increase relative humidity inside the incubator.

Each incubator is equipped with ventilation holes which should be opened to allow fresh air to enter the incubator.

Embryo development is a fragile process that can be easily disrupted. Many problems are related to improper temperature, humidity, ventilation and turning during the incubation process. A list of common incubation problems and their causes are given in the table below:

Many eggs with no embryoNutritional problems with parents
Eggs stored too long
Eggs stored above 55F (12.8C)
Blood ringsImproper incubation temperature
Improper care of eggs
Dead embryosTemperature too high or too low
Improper turning of eggs
Poor ventilation
Chicks breaking through shell (pipping) but not hatchingLow humidity in the hatcher
Chicks hatching too earlyTemperature too high
Late pipping of eggsTemperature too low
Eggs pipped by chick but take a long time to hatchTemperature too high
Short down on chicksHigh temperature
Low humidity
Rough navelsHigh temperature
Low humidity
Shell sticking to chicksLow humidity in the hatcher
Mushy, bad-smelling chicksBacteria in the incubator
Crippled and deformed chicksHeredity
Nutrition problems with parents
Hatched chick with empty eggshell
Hatched chick with empty eggshell (Image by Tsekhmister from Shutterstock.com)


Animation of chick embryo development. Source: PoultryHub.org

Hatching eggs in the classroom: A teacher’s guide (Texas A&M University)

Chickscope (University of Illinois)

Eggcellent adventures in classroom embryology (University of Florida)

Hatching my peeps embryology countdown calendar (University of Florida) – for purchase