By Rob Banks, England – Candling eggs is an age-old technique that has modern applications in incubating and hatching poultry. After studying the incubation of many species and breeds it became clear to me that almost all eggs follow a similar process during incubation and hatching. Once we understand the hatching process, we can then use applied artificial techniques and candling eggs to improve our hatch rate and save viable eggs of valuable breeds from the common problem of “dead in shell.”
This article is applicable to many breeds and species, and details the key stages of incubation and hatching. It explains methods of pin-pointing hatching time and when intervention is really necessary. I use my exhibition Dewlap Toulouse geese as an example breed and used pictures of a Macaw parrot to illustrate the hatching process. It cannot be stressed enough how important it is to be prepared before the incubation of any egg. It can also be broadly stated that any egg will do much better if left in the care of reliable parents for at least 66% of the incubation period.
The work to obtain viable eggs starts with sound husbandry and care of the breeding stock and the old saying of “you only get out what you put in” holds true in all aspects of a breeding program.
As part of a comprehensive incubation tool kit you should consider acquiring the following items:
• Reliable and accurate forced air incubators with adjustable vents and auto turn facilities. (Checked with at least two reliable thermometers).
• A reliable and accurate still air incubator with adjustable vents that can be used as a “hatcher incubator” (Checked with at least two reliable thermometers).
• Calibrated thermometers (I use two mercury rod, alcohol, and digital thermometers).
• At least two reliable humidity gauges.
• LED mains operated Candler for candling eggs.
• Weighing scales which measure in gram units (those used for cooking are ideal).
• A Hatching Tool Kit that should contain: surgical tape, surgical gauze, alcohol hand gel, Inadine Dry Powder spray, Q-tips, forceps, artery clamps, surgical scissors, bleeding control spray, magnifying glass, artificial skin spray (for damaged eggs), clean towels, pencils, plastic boxes for isolation of eggs or hatchlings.
The final thing is to house your incubators in a quiet cool room and test run them for accuracy every year before your eggs are due. This is also when all of the thermometers are used, after checking them for accuracy (calibration). These are placed in each incubator to check that all temperature readings are accurate.
Once you have collected the eggs they are washed (if required), weighed, marked and stored for a maximum of 14 days in cool conditions with a daily 180-degree turn. The egg is weighed and in pencil write on the egg the weight, a code to identify the parents, date laid and date set. Finally, place a + on one side and an x on the opposite side. During the breeding season, individual egg information is easy to forget and once written onto the egg no errors can be made regarding identity.
You should do your research on the chosen breed or species’ individual incubation requirements before setting eggs in the incubator. For example, it appears African and Chinese geese have eggs which lose moisture more readily than Sebastopol and Dewlap Toulouse (Ashton 1999). Therefore their humidity requirements will be higher, perhaps 45-55% humidity. Incubating chicken eggs and duck eggs requires a slightly higher optimum incubation temperature of 37.5C where geese benefit from being a little lower at 37.3C. A little research before incubation pays dividends later. However many owners have a mix of eggs from different breeds and will have to provide average conditions if only one incubator is available. A more flexible option is to have two machines so you can run one as a dry incubator and the other at average humidity to accommodate the needs of the eggs being incubated.
Overall eggs should lose approximately 14-17% of their fresh laid weight by external pipping to produce healthy hatchlings. For example, if a fresh laid Toulouse egg weighs 150 grams then it needs to lose 22.5 grams by approximately Day 28 to achieve a 15% weight loss. This would be a weekly weight loss of 5.6 grams. By checking weekly weights of the eggs the humidity can be adjusted accordingly so the target weight is achieved. Eggs may also be assessed for weight loss visually by checking the size of the developing air cells, but it’s not as precise as weighing. So for the example breed of Dewlap Toulouse eggs, the incubation requirements should be as follows:
Temperature 37.3°C/99.3°F, humidity 20-25% (dry incubation), vents fully open, hourly auto turning after 24 hours with a once daily hand turn of 180 degrees. After six days commence daily cooling and misting for 5-10 minutes increasing to 15 minutes daily from 14 days until internal pipping. Eggs should be weighed weekly to check they are losing adequate moisture.
The technique of cooling and misting of the eggs remains controversial though other experienced breeders have employed these techniques (Ashton 1999, Holderread 1981). There appears no clear rationale how this benefits the growing chick though some consider the cooling as beneficial to the chick’s stamina. In relation to moisture loss, it appears that as the egg cools to the room environment then heat is lost from the egg. It may be argued that rapidly escaping heat from the pores of the eggshell also carries water and gas molecules with it. Certainly, there is evidence that daily cooling appears to improve hatch rates in domestic geese. The misting of eggs with tepid water at first appears illogical in stimulating water loss but this may increase further heat loss by evaporation.
It is better to set eggs in batches of at least six which usually ensures there is very good chance there will be more than one hatchling. The eggs are incubated in a horizontal position and not turned for the first 24 hours, after that the auto turn mechanism is switched on. In the very early stages of the embryo’s development, it is crucial that optimum and stable conditions are maintained. During this time the embryo grows from a simple cluster of cells to a basic embryo with a supporting cardiovascular system.
Not only is this a period of major physiological change but also a time of rapid biochemical processes as cells divide and migrate to their pre-programed positions to form the basic structure of the embryo. The biochemical processes are complex and include converting iron stores to hemoglobin to establish a vascular system and also the conversion of nutrients to fuel this whole process. It is in this five-day period that the early embryo is so fragile and any mistakes in incubating chicken eggs and other poultry eggs can result in an early embryonic death. With this understanding, it can be clearly understood why stable incubation is required. Temperature swings only serve to slow down or speed up these complex processes and lead to major disruption. It is, therefore, vital that the incubator is “run in” for days before setting the eggs, as changes at this time should be avoided. Often an incubator will produce temperature spikes when eggs are introduced. To avoid this fill the incubators with infertile fresh eggs which are gradually replaced by fertile ones as more eggs are introduced. This solves the problem of fluctuations in temperature and provides the stable conditions required.
Candling Eggs Throughout the Incubation Period
So the eggs are now set and have been incubated in stable conditions. At 5-6 days the owner can begin candling eggs and determine which are fertile. The eggs can remain in the incubator and the candler is positioned over the air cell (blunt end) to illuminate the contents of the egg. If you look carefully at this stage, candling eggs should reveal a red “dot” about the size of a match head with faint blood vessels surrounding it. Those eggs without any indications of fertility should be re-candled at 10 days and thrown away if they’re infertile.
Once the basic embryo has developed then more complex cardiovascular structures grow which act as the embryo’s life support systems. Candling eggs at this stage will reveal a system of blood vessels grows out over the yolk sac to supply the nutritional needs of the growing chick while the body is encased in an amniotic sac filled with amniotic fluid. This sac serves to protect the fragile growing embryo and its delicate tissues by bathing it in amniotic fluid. A further sac develops from the naval area and rapidly grows as a vascular balloon which encases the chick, the yolk, and amniotic sac. This “balloon” is covered with an intricate and generous supply of blood vessels leading directly back to the chick.
Candling eggs over the next two weeks, you can observe how the chorioallantoic membrane grows to completely line the inner surface of the entire eggshell. As the membrane and its blood vessels lie adjacent to the shell it places the blood vessels in near contact with the pores of the eggshell. Therefore gas and moisture exchange can occur, ridding the embryo of carbon dioxide and excess water molecules and also absorbing oxygen for the growing chicks needs. This vital membrane meets the growing embryo’s internal respiration needs until it is mature enough to use its own lungs for pulmonary (lung) respiration. Research has shown that inadequate turning of the egg in the first two-thirds of incubation can lead to stunting in the development of the chorioallantoic membrane. This would reduce the membrane’s ability to provide adequate gas and water molecule exchange to meet the growing chick’s needs and lead to a late death at approximately the third week of incubation.
Once the basic form of the bird has been developed, the remaining part of incubation is simply about growth and maturing the chick until it is capable of independence free from the egg. Incubator conditions should remain stable and a regime of daily cooling and misting of the eggs maintained. There should be continued monitoring of the egg’s weight loss and therefore candling eggs at this stage will reveal a development of the air cell which provides a visual reference of moisture loss.
This appears to be one of the most controversial topics about incubation and yet although complex can be easily understood. The chick does not randomly hatch — there is nearly always a set sequence and process to follow. Once this is understood then hatching and management of hatching chicken eggs and other poultry eggs becomes clearer.
By the 24th to 27th day of incubation (depending on breed) the egg should have lost approximately 13% of its weight and the air cell should be of a good size. The air cell should be slightly slanting downwards. At this point, candling eggs daily is the best way to determine their progress. Within a 24-hour period, the air cell appears to suddenly dip downwards and appears to have grown noticeably in size. It often takes up a distinctive “dipped” shape and becomes easily recognizable.
The egg is now out of balance and no longer needs turning. If the egg is placed on a smooth surface it will always roll to the same position, which is the side with the greatest amount of air cell uppermost. This now becomes the top of the egg and a cross marked on the shell so the egg always remains in this position. The chick is now lying in its optimum position for hatching and will find maneuvering into its final hatching position easier to achieve. The sudden change in size and shape of the air cell is caused by the chick changing its position within the egg. During late incubation, the chick normally settles into a position with its head bent over and pointing towards its tail. To encourage correct positioning, incubate the eggs on their sides with the blunt end slightly elevated at a 20-30 degree angle. Again this mimics the position of many eggs in nature as they lie in the concave of the natural nest. At this point incubation settings remain unchanged for temperature and humidity, the only change is the eggs are now placed in their final position and turning is stopped.
Within another 12-24 hours of “dip down” of the air cell, small shadows become visible within the air cell when candling eggs. These shadows start at the back of the air cell and over a further 12-24 hours gradually extend down the sides and finally along the front of the air cell. Candling eggs at this stage often reveals visible movement of the shadows. This change is due to the chick gradually moving into its final hatching position. It gradually draws its head up from a position facing its tail and upwards towards the air cell.
When viewed from the air cell end of the egg the chick’s head is turned towards the right and under its right wing. With the head and beak lying adjacent to the air cell membrane, the chick is ready for internal pipping. As the chick is almost fully mature the chorioallantoic membrane is unable to fully meet the chick’s respiration requirements. Oxygen saturation levels fall slightly and carbon dioxide levels begin to rise. Often this change in the failing chorioallantoic membrane can be seen when candling eggs as previously red blood vessels appear to take on a darker red color. The change in blood gas levels is thought to provoke involuntary muscle contractions which have a direct effect on the chick.
The large hatching muscle located on the chick’s neck begins to contract with force and results in the chick’s bill piercing the inner membrane of the air cell. This is further helped by a tiny sharp harder area on the tip of the upper bill (the egg tooth). With a hole in the air cell membrane, the chick is finally in a position to commence breathing using its lungs. Starting with occasional breaths a regular pattern of pulmonary respiration soon becomes established. Internal pipping has now been achieved and a major physiological change has occurred. Internal pipping can be verified in two ways: candling eggs at this stage will often show visible shadows in the air cell that appear to pulse rhythmically, and if the blunt end of the egg is held to the ear a faint “click… click… click” sound can be heard.
It is in this phase of hatching that many chicks die resulting in late “dead in shell.” It is a time of great stress and physiological change within the chick’s body. The heart is pumping rapidly due to exertion and trying to compensate for changing blood gasses. It appears that inadequate moisture loss during incubation causes the chick and its supporting cardiovascular system to be overloaded with fluid (hypervolemia). With the heart having to pump faster and harder to compensate, the chick goes into acute heart failure. The tissues in the body become swollen with excess fluid (edema) and the chick weakens. The space for maneuvering into its hatching position becomes even tighter and the chick’s body is simply too weak to withstand the vital changes required. It is now clear why monitoring egg weight loss and candling eggs are so important!
In the incubation of rare breeds, every chick is vital. Therefore if I am concerned in any way about the chick or external pipping is delayed, I intervene. Using a sterilized tiny sharp drill bit I carefully enter the air cell at the center and very top of the egg. Candling eggs allows me to check that the chick is not directly below the proposed point of entry. By twisting the drill bit by hand the eggshell is gradually eroded away and a hole approximately 2-3mm diameter is made. This safety hole provides access to fresh air and should be no larger or premature drying of the membrane will occur. This is termed as artificial external pipping. This safety hole can save the lives of many healthy chicks. I can recall instances of rare chicks successfully external pipping then going into rotation within the egg until their body occluded the external pip area and then died!
With the chick successfully internally pipped it can breathe easily and rest for a while. However, the oxygen within the air cell is soon used up. After about 6-24 hours the chick’s bill begins to strike upwards against the eggshell. This repeated “jabbing” action results in the breaking of the eggshell over a small area and appears either as a small raised pyramid, a cracked area or even a hole. The chick has now externally pipped and has access to free air to meet its respiration needs. It is only at this point that you change incubation conditions. It is recommended to lower the temperature by about 0.5°C and increase the humidity to 65-75% (lockdown).
It is now that the chick enters its latent phase and it appears as though there is little progress. This phase can last from 6-72 hours depending on species or breed incubated. Gradually the chick becomes more vocal as the lungs finally mature. Apart from the constant “clicking” noise from breathing the chick will occasionally whistle or peep. It is vital to point out that the “clicking” or ’“tapping” noise is not the chick tapping against the shell trying to release itself. Many owners’ nerves are shredded at this stage and they misinterpret the noise and prematurely intervene with disastrous consequences! To reassure the reader I advise putting your chin on your chest and try forcibly breathing in and out. In this position, you can mimic the “clicking” noise which is actually caused by the chick’s head being bent and made in the pharynx as it breathes.
While the chick rests during this quiet phase it is preparing for its final hatching sequence. By changing the pressure in the thorax and abdominal contractions the yolk sac is drawn inside the abdominal cavity. Meanwhile, the lungs have finally matured and the job of the chorioallantoic membrane becomes redundant. The blood vessels start to gradually close down and recede into the chick’s navel. If you prematurely assist before this stage, you will usually cause hemorrhage from the still active blood vessels and find the yolk sac un-absorbed.
It is this stage that you find it so difficult to judge when intervention is both necessary and safe. I do not follow the school of thought that chicks that are unable to hatch are best left due to a weakness in the chick or their bloodline. This sweeping and erroneous statement does not account for healthy chicks previously hatched from the same parents. Hatching delays are often the result of slightly imperfect incubation techniques and this should be taken into account. Yes, sometimes chicks are weak and there is often mortality under the parents, nature does select for the strongest. However, if we are to employ artificial incubation techniques then we must accept that we are capable of making errors and at least give these chicks the opportunity of life before assessing their worth later. This is especially so in the incubation of endangered species or rare breeds when every egg counts.
The final phase of incubation is finally reached once the yolk sac and blood in the vessels have been absorbed into the chick’s abdomen. The egg and its structure have completed its purpose and the chick must now release itself from the shell. If viewed from the blunt end of the egg the chick suddenly starts chipping around the shell in an anti-clockwise direction. This is called rotation or unzipping and it is a relatively quick phase. I have seen chicks rotate around the entire shell in less than 10 minutes but usually, it is completed in 1-2 hours. By the actions of chipping at the shell and pushing of the feet the chick works around the circumference of the egg until it has gone almost 80% around. At that point, the egg weakens and with a pushing action the cap of the shell “hinges” open allowing the chick to scramble free from the egg. The chick is then taken and its navel area is sprayed with dry iodine powder then placed into a clean container to rest. This action dries any slight bleeding as the powder coagulates and helps to minimize the risk of navel infection. The chick is then left to recover, rest and dry out thoroughly before being transferred to its rearing unit.
Predicting when the chick is ready for final release and if assistance is required is fairly easy. The essential tool required is a good quality tool for candling eggs (and a dark room to view in). After external pipping the yolk sac and blood vessels are still to be absorbed. Candling eggs through the air cell and around its low point at the front will show very little visible detail. The dense yolk sac appears as a dark mass, though the major umbilical vessels may be seen. This is more easily achieved in white and thin-shelled eggs and incubating white chicken eggs is an excellent way to practice your techniques. As the yolk sac and blood is absorbed, a hollow void appears in the area below the lowest point of the air cell. The light visible while candling eggs will clearly illuminate this void area.
Now it is safe to assist and you should prepare by sterilizing your hands and instruments using alcohol hand gel. Working from the top of the air cell where an artificial external pip hole may have been made pieces of shell can be gradually removed. It is safe to work down to the demarcation line of the air cell which should be outlined in pencil to guide you. Once a hole has been enlarged sufficiently for you to work in, then the situation can be assessed. Do not remove any more shell than necessary. Using a Q-tip moistened with boiled cooled water (or sterile saline) the membrane over the chick can be moistened directly. Check the position of the beak and ease the membrane away by stretching rather than tearing if possible. If no bleeding occurs continue gradually easing the membrane away until the chick is exposed.
The aim here is a little progress at a time, then after about 5-10 minutes stop and replace the chick back into the brooder for another 30-60 minutes. This allows the chick to rest and warm through. It also allows the membrane to dry and shrivels any blood vessels a little further. Gradually the entire membrane is eased back and using the Q-tip the beak can be eased forward and over the right wing. At this stage, the chick may start pushing with renewed vigor or you can ease the head up and out, which will provide you with your first direct view down into the eggshell. Candling eggs will help you assess and check that the blood vessels have receded and the yolk sac is absorbed.
If you have assisted too early then allow the chick to curl up its head and re-cap the egg. Infertile eggs are excellent for this purpose. They are broken in two and the top half cleaned of its membranes. The top has a safety hole put in it and the eggshell soaked in boiled water. This action causes the shell to be pliable and it can be trimmed just below the widest point so it provides a snug fit. After soaking again in hot water remove the cap, allow to cool and simply place over the chick in the shell. If necessary use surgical tape to hold it in place. You are now committed to a fully assisted hatch.
After a few hours assess the situation again and repeat as necessary until you confirm absorption of the yolk sac and blood vessels. You should then free the head and chest leaving the chick’s abdomen in the remaining eggshell. Often the chick is exhausted but after being left in the hatcher for an hour or so they make final efforts to kick themselves free from the egg. In cases where they fail to do this, they will come to no harm and can be safely left to rest. They can be left this way overnight which allows the naval area to thoroughly dry and the chick may be safely removed from the shell.
I hope this article has demonstrated that all incubation and hatching follows a process that can be monitored by the owner, and the value that candling eggs has in monitoring these processes. It has shown how to identify when and how intervention should take place to assist hatchlings in difficulty. With improved skills in incubating and candling eggs, along with an understanding of the growth process, owners should be able to follow this fascinating process and improve their breeding success rates.
Ashton, Chris (1999). Domestic Geese, Crowood Press Ltd.
Holderread, Dave (1981). The Book of Geese. Hen House Publishing
Co-authors Rob and Peter Banks both work in a background of healthcare but have maintained a collection of birds for over 30 years. They initially specialized in artificial incubation techniques for parrots and endangered South American Macaws. Their theories learned from hatching parrots have been extended to other domesticated poultry, tortoises and reptile eggs that are also incubated artificially.
They specialize in breeding exhibition Dewlap Toulouse geese and found these incubation techniques resulted in a higher-than-average hatch rate.
This year they hope to hatch their first Buff Dewlap Toulouse directly descended from the USA bloodlines of Dave Holderread. They are also working with Vicky Thompson in Michigan to breed high-quality Sebastopols and introduce the more unusual colors of Lilac, Lavender and Cream to the breed and hope to import some of her Sebastopols to the U.K.
Originally published in the April/May 2012 issue of Backyard Poultry and regularly vetted for accuracy.