CHAPTER TWO
The Science of Egg Formation
Egg-laying (oviparity) is a reproductive strategy in many animals. Of the mammals, only the platypus and four known species of spiny anteater (echidna) are classed as monotremes (egg-laying mammals), but among the reptiles, amphibians, and fish, many species are egg-layers. The eggs of all of these non-avian egg-laying species have a soft shell, which is either leathery or coated in jelly. Only bird eggs have a hard shell. The question many people ask is why do chickens lay eggs, or, more appropriately, what is the stimulus that causes them to lay eggs?
Eggs are the natural response of birds—in this case chickens—to reproduce. In general, pullets (young hens less than one year old) begin laying eggs at about 20 weeks of age and will continue to lay well for a year or so. As they age, their egg production begins to subside, increasing in the spring and summer and reducing in the fall and winter. But why do hens begin laying at 20 weeks?
As with all animals, age is a critical factor for reproduction. Maturity of the reproductive system, as well as of other organ systems and even a minimum body weight, are essential for successful reproduction. In mammals, puberty is attained at a certain age, which differs depending on the species. When puberty is reached, the brain signals the hypothalamus (the “master gland”) and pituitary to release certain hormones directly into the bloodstream. When these hormones reach the gonads (the ovary in females and testis in males), they respond by maturing into their functional reproductive state.
Days of the estrous cycle
Once a pullet reaches sexual maturity, ovulation begins. Females can be encouraged into lay by environmental stimuli such as light levels.
In female mammals, the ova that migrated to the ovary during embryonic development begin to complete their maturation, a process that occurs either one or a few at a time, depending on the species. In addition, the ovary releases hormones of its own, which target other sexual organs and trigger morphological changes to assure a successful reproductive event. After a few weeks, an ovum is released to meet with sperm from the male, which have undergone a similar maturation event. If fertilization is unsuccessful, the hormone levels subside for a period of about three to four weeks, when the process repeats. This is a simplistic review of the estrous or heat cycle in mammals.
THE STIMULUS: LIGHT
In chickens, and most birds in general, the process is similar but with a few dramatic differences. The first and most obvious difference is that chickens do not undergo puberty. Instead, it is a specific stimulus—the photoperiod, or light/dark cycle—that begins the process. The hen’s brain remains the controlling organ, but in this case the pineal gland is key. The pineal is directly light sensitive (it is sometimes called “the third eye”), responding to changes in day length and controlling the circadian rhythm by releasing the hormones melatonin and serotonin. The hen will begin laying eggs when the day length reaches the threshold level of 12 hours.
Commercial egg production has begun to move from cage production to the use of aviary systems in which hens are confined for sleep and egg laying and allowed access to the floor during most of the daylight hours.
When this level is reached and the hen is receiving at least 12 hours of light per day, its pineal gland is stimulated to release hormones that begin the cascade of events through the hypothalamus and pituitary and ending in egg production. For young pullets, the first egg will be laid after about two to three weeks, while older hens that have laid in the past will respond more quickly.
A second obvious difference with mammals is that rather than having an estrous cycle lasting three to four weeks, the hen will continue to lay eggs essentially daily for months while the stimulus of long day length remains. The pause in chickens occurs during fall each year, when the days become shorter. When day length drops below 12 hours, pineal hormones change and egg production slows and often ceases. During this lull in egg production, the hen will also molt, a process of losing and regrowing feathers. This natural process is much more severe in wild birds, which completely cease egg production in late summer and replace all their feathers by mid-fall. Some waterfowl find themselves flightless during the molt. Following centuries of breeding, the domesticated chicken often continues to lay during the molt and replaces only a portion of its feathers each year.
By taking advantage of the hen’s biological response to light, farmers are able to increase substantially the number of eggs laid. Placing 18–20-week-old pullets under 14–16 hours of light per day will bring them into production in about two weeks, no matter what the season of the year. In addition, keeping the lights on a long-day schedule will keep them laying through the fall and winter months. Eventually the birds will need to molt, which can easily be accomplished during their second winter. This is done simply by providing them natural short-day photoperiods, starting in about mid-November and continuing through the end of December. This period of short days (less than 12 hours) will give the hens plenty of time to molt and rejuvenate their egg production. Restoring long days of 16 hours of light (and even a little more, as they are older now) from January onward will restore their egg production.
What about the rooster?
The question often arises: What about the rooster? What is his role in egg production? Unlike mammalian males that breed with the females only when they are in heat, the rooster, like the hen, responds to photoperiod and will breed with hens daily as long as the stimulus of long day length is present. So, while the rooster is obviously important for reproduction, he has no role in egg production.
Biologically, eggs are the female’s contribution to the reproduction process. Chickens, like all birds, produce a hard-shelled egg in which development of their offspring occurs without any outside contribution throughout the 21-day process.
The egg begins as a germ cell that has resided in the hen’s ovary since the time the hen was an embryo developing in its own egg. Once a pullet reaches sexual maturity, ovulation begins. As previously explained, birds do not experience puberty but are instead stimulated into sexual maturity by an environmental stimulus—photoperiod in temperate regions. Long days, with more than 12 hours of light, stimulate the brain and pituitary gland to release hormones that in kind stimulate the ovary to begin the process of ovum (yolk) formation. From the ovary, the formation of the egg passes through several stages, described as follows.
Reproductive system of the hen
The reproductive system of the hen comprises two organs, ovary and oviduct, which are responsible for producing the ovum (egg yolk) and packaging into the egg (oviduct).
Chickens have only one active ovary, always on the left. This contains as many as 3,000–10,000 germ cells, only a few of which develop at any one time. The germ cells that will eventually become yolks develop in order, so that only one is mature enough for ovulation (release). Occasionally, more than one yolk ovulates at a time, resulting in a double-yolk or multiple-yolk egg. Once the yolk is released from the ovary, the process of egg formation begins in the hen’s oviduct.
OVIDUCT
As with the hen’s ovary, only the left oviduct is active. Occasionally a “right oviduct” develops, but it is nearly always vestigial and non-functioning. The oviduct’s function is essentially to package the yolk into what we know of as a chicken egg. The oviduct has five distinct sections, each with a function in the formation of the completed egg: the infundibulum, magnum, isthmus, shell gland, and vagina.
Infundibulum
The released yolk enters the oviduct through the infundibulum, the funnel-shaped end that actively engulfs the yolk. If the hen has mated in recent days, sperm cells that have been stored in “sperm nests” found in the infundibulum are released so that fertilization can occur. The yolk moves through the infundibulum in about 20 minutes.
Magnum
From the infundibulum, the yolk enters the magnum, passing through it in about three hours. It is in the magnum that albumen (egg white) is secreted around the yolk. Albumen is a combination of about 13 known proteins and is deposited around the yolk in distinct layers (see here).
Isthmus
The yolk, which is now surrounded with albumen, then reaches the isthmus, a distinctly thinner and shorter part of the oviduct compared to the magnum. It is here that with the shell membranes, inner and outer, are deposited onto the albumen. During the nearly 90 minutes that the egg spends in the isthmus, water begins to diffuse into the albumen, plumping the overall package and beginning the formation of the ultimate egg shape. The final process in the isthmus occurs as the egg begins to move to the shell gland, and involves the placement of small “seeds” of calcium carbonate (calcite) onto the outer surface of the outer shell membrane. These calcite “seeds” provide the starting point for shell deposition in the shell gland.
Shell Gland
Nearly five hours after ovulation, the nearly formed egg passes from the isthmus into the shell gland. At this stage its overall shape and size have already been partially determined. The egg is still somewhat flaccid, soft, and pliable, yet completely covered by membranes, so if it were to be laid at this time, it would be deemed a “soft shell.”
As the flaccid egg enters the shell gland, the plumping that started in the isthmus continues until the membranes are distended and the egg no longer appears flaccid. During the next 20 hours or so, the rather thick lining of the shell gland secretes about 0.3 oz. (10 g) of calcium carbonate onto the membrane surface, “growing” calcite crystals to a thickness of about 0.007–0.011 in. (0.2–0.3 mm). The crystals are deposited in columns with occasional spaces left vacant, resulting in a porous shell. It is estimated that the pores make up about 0.003 in.2 (2 mm2) of the total surface area of the shell of an average chicken egg. These pores allow exchange of oxygen, carbon dioxide, and water vapor.
Uterus versus shell gland
Many books continue to label the shell gland the “uterus.” The use of this name has, unfortunately, continued for decades, with seemingly only avian purists having difficulty with its connection to an avian system. The shell gland section of the avian oviduct is not a uterus. A uterus is a mostly mammalian organ, where mammalian embryos implant, receive nourishment by way of a placenta, and develop from a few cells to a fully formed baby mammal, until “forced” out during parturition (birth). In a bird, the section erroneously called the uterus but more properly termed the shell gland has the biological function of secreting mainly calcium carbonate to create the hard eggshell, a distinctly avian characteristic.
Most breeds of chicken lay eggs with brown shells, a few lay white-shelled eggs, and only a couple lay eggs with blue or green shells. The brown color is deposited on the shell surface as one of the last events in the shell gland. The production of white-shelled eggs is due to a mutation in the biochemical pathway that creates the brown color, disrupting this so that the color is not deposited. The blue or green color of eggs from the Araucana and related breeds comes from the deposition of a different pigment that is “mixed” with the calcium carbonate shell so that the color is throughout the shell material. The difference is evident by examining the inside of the eggs after breaking the shell: Brown eggs are white on the inside surface, while blue and green eggs are blue or green all the way through. The entire process of shell formation takes about 18–22 hours.
Vagina
Once the shell is complete, egg-laying (oviposition) begins. Just prior to laying, the egg, which has been moving through the oviduct pointed-end first, turns so that the egg will be laid blunt end first, a movement that probably relates to ease of laying.
During the process of egg-laying, the egg moves to the vagina at the end of the oviduct, which is connected to the urodeum section of the cloaca, the end of the digestive tract. In a move that is thought to be a way of keeping the eggshell surface clean, the hen’s oviduct everts from the cloaca, essentially turning “inside out.” As the egg moves through the vagina, a coating of mucus is laid down that serves to protect the eggshell pores from bacterial contamination and to reduce water loss from the egg after laying, which preserves egg quality.
The sound of success
In most hens, hormones are mobilized within 20 minutes of laying that stimulate the hen to cackle, telling the world she has just produced an egg, as well as ovulating the yolk for tomorrow’s egg when she does it all over again.
Eggs are a common sight in most kitchens, and while they may seem common, nature has taken great pains to create the avian egg into a powerhouse of nutrition, packaging it in a virtually sterile, single-serving container that makes most food scientists envious.
Avidin and biotin deficiency
One protein, avidin, which is found in small amounts in albumen, has the ability to “bind” the essential vitamin biotin. While unlikely, eating many raw eggs could result in a biotin deficiency. Cooking denatures the avidin slightly, so its ability to bind this important vitamin is eliminated.
ALBUMEN
Albumen, or egg white, nearly exclusively comprises protein and water. Water makes up about 85 percent of the total albumen volume, with the remaining portion being mostly protein—about half of the total protein in the egg. The final 2–3 percent of the albumen comprises some minerals and a small amount of carbohydrates in the form of glucose and glycoprotein.
No fewer than 13 different proteins are found in the albumen, the main one (making up more than half of the total albumen protein) being ovalbumin, with the other major types including conalbumin (ovotransferrin), ovomucoid, ovomucin, and lysozyme. These proteins have various roles in nutrition as well as maintaining the thickness of the albumen, binding metals and other compounds, and keeping potential bacterial pathogens in check.
The combination of these 13 proteins and their constituent amino acids has been found to be ideal for human consumption. When measuring the human requirement for each of these amino acids, egg albumen and yolk are considered the best source for the human diet.
Albumen is deposited around the yolk in four distinct layers: the inner and outer thick, and the inner and outer thin. Directly surrounding the yolk and the first layer to be deposited is the inner thick, also called the chalaziferous layer. This layer constitutes about 3 percent of the total and is observed as the nearly opaque, ropey structures (the chalazae, singular chalaza) on opposite sides of the yolk when the egg is broken open. When the egg is intact, this thick layer provides stability to the yolk, restricting its movement and preventing it from floating around the albumen. As the egg ages and the albumen becomes thinner, the chalazae lose their ability to hold the yolk in the center of the egg. Prominent chalazae are indicative of a fresh egg.
The inner thin layer, which could be described as watery, comprises nearly 25 percent of the total albumen but is not detectable and has no obvious function. The outer thick layer comprises over half of the total albumen. It is this layer that is observed during egg grading for firmness. In fresh eggs, the outer thick layer provides support to the yolk to make it stand tall. As the firmness is lost over time, it is this layer that is obviously responsible for downgrading. The final layer, the outer thin, is produced during the plumping stage of egg formation: As water from the oviduct diffuses into the as-yetunformed egg during and following membrane formation, it mixes with the protein, creating this watery “slurry.”
Typically, albumen is clear or slightly cloudy. Fresh eggs contain higher levels of dissolved carbon dioxide, resulting in a cloudy albumen. As the egg ages, the carbon dioxide is released through the porous shell, and the albumen becomes clear. Also noticeable is a slight yellow hue to the albumen, indicative of the vitamin B2 (riboflavin), which is at fairly high levels in eggs.
YOLK
Egg yolk, unlike the albumen, is fairly low in water content—it contains about 50 percent water when the egg is laid. However, because of the high water content of the albumen and the characteristic that water at high concentration will diffuse toward a lower concentration, the yolks slowly increase in water content during storage.
About half of the material in yolk is solids, primarily made up of proteins and lipids (fats). About half of the total protein of an egg is found in the yolk. The yolk proteins differ from those in the albumen and are found in combination with other compounds, yielding glycoproteins and high-density lipoproteins. Some of the proteins make up a protein solution that acts as a suspension material for other yolk particles of various structures and sizes. The larger of these particles are generally suspended in the protein solution, while the smaller particles are more or less free-floating. When observing yolk under magnification, tightly packed granules give it a yellow appearance, and when they are more loosely distributed, the yolk looks white.
Interestingly, during yolk formation in the ovary, over the course of about 10 days or more, the yolk material is deposited in layers—white and yellow yolk—that remain after laying. In fact, when egg yolks are stained under certain conditions in the laboratory, distinctive layers can be identified, and if fat-soluble dyes of various colors are fed to the chicken sequentially, a different color each day, each layer will reflect the color of the day. In addition, extending from the yolk’s core (the point just under the undeveloped embryo) is a loose structure of white yolk called the latebra, which because of its lower density causes the yolk to turn so that the embryo is always on the top.
The lipid fraction of yolk makes up about 35 percent of the total volume. These lipids comprise about 66 percent triglycerides, 29 percent phospholipids, and 5 percent cholesterol. The fatty acids that make up these lipids vary according to the feed intake of the hen. It is this fraction of the egg that can be greatly influenced by the hen’s diet. For example, feeding high levels of omega-3 fats, as found in flax, can increased the content of omega-3 fatty acids in the egg. Overall, about 66 percent of the lipids are unsaturated fats, with the remaining 34 percent saturated fats. In recent years, research has shown that the fat and cholesterol content of eggs in the human diet has little effect on blood cholesterol levels.
Unlike albumen, whose pH increases after the carbon dioxide it contains diminishes after laying, there is little change in the pH levels of yolk. Yolk pH rises from about 6.0 to about 6.3 during storage (in comparison, albumen pH increases from around 7.5 to 9.0 or higher), and there is no apparent effect on the yolk.
Typically, egg yolks range in color from pale yellow to bright orange. These colors are the result of the hen’s consumption of a group of carotenoid pigments called xanthophylls. The variation of the color is heavily based on the hen’s diet (see box).
MEMBRANES & SHELL
The shell membranes are a minor part of eggs and not particularly remarkable, but they do play an important role in the egg as the base on which the shell can form. The inner and outer shell membranes, composed of protein fibers, are distinct microscopically in their appearance and texture. The inner membrane, which is in direct contact with the albumen, is composed of thin, fine fibers, while the fibers making up the outer membrane are comparatively thick and coarse.
The shell is composed of nearly 95 percent calcium carbonate, along with small amounts of calcium phosphate, magnesium carbonate, and protein. It is thought that the minor compounds impart hardness to the shell material and the protein provides a matrix to hold the calcium-carbonate crystals together as they “grow” into a ridged structure.
The most obvious characteristic of the shells of chicken eggs is that they can be found in various shades of brown, blue, green, and white. The natural color of chicken eggs, and the color produced by most breeds, is brown, a remnant of their jungle-fowl heritage.
Diet & yolk color
When hens consume feeds such as corn and alfalfa, or grasses and insects that are also high in xanthophylls, the eggs they produce have dark orange yolks. When highly pigmented feed sources are not available, dark orange yolks will soon fade to pale yellow. In those cases, processed feed additives like extract of marigold petals can be fed to darken the yolk once again.
The brown color of eggshells is caused by a group of pigments called ooporphyrins, which are synthesized using hemin and biliverdin, remnants of old blood cells. This color is deposited just prior to egg-laying so is on the outer surface only—the inside surface of brown eggs is white. The brown color can be removed with sandpaper to reveal the white shell underneath, and when the egg is still wet from laying, the color can sometimes literally be rubbed off.
White eggs are the result of a mutation in the chemical pathway synthesizing the ooporphyrin pigments. Most white-egg breeds, including the Leghorn, can be traced back to the Mediterranean region.
Green and blue eggshell colors are the result of another genetic mutation. It is thought that many centuries ago, a mutation took place in chickens that were centered on the west coast of South America, resulting in the modern Araucana breed, which lays green or blue eggs. When the gene responsible for the eggshell color is bred into chickens that normally lay brown eggs by crossing individuals of the two breeds, the resulting offspring will lay green eggs. However, if it is bred into chickens that lay white eggs, the eggshell color of the resulting offspring is pastel blue.
It must be noted here that eggshell color has nothing to do with the nutritional value, taste, or quality of eggs. It is simply that different breeds of hens lay eggs with different-colored shells based on their genetics.
Color research
University of Nottingham researchers, publishing in 2013, suggested that the blue/green eggshell gene sequence is not found in jungle fowl. So it is actually a new gene, possibly introduced into the chicken genome by a retrovirus. More research is needed to determine the validity of this work.
While the presence or absence of a rooster in the chicken flock has nothing to do with whether the hens lay eggs or not, he is essential for fertile eggs. As in all higher animals, reproduction in chickens is accomplished by the contribution of genetic material in the form of sperm and egg from both males and females, respectively. Sexual reproduction, compared with the asexual reproduction of many lower forms of life, provides for enhanced genetic variation within and between generations. Each offspring has a unique genetic “fingerprint,” making it just a little bit different from all others. The result of this genetic variation is obvious when considering the multitude of breeds and varieties of the chicken.
Egg-laying is the process that birds, including chickens, use for reproduction. Eggs that hatch into chicks are formed in the same way as the eggs we eat, with just one minor difference: the introduction of sperm cells into the process that leads to a chick. During mating, the rooster deposits sperm cells into the hen’s oviduct in a process called a “cloacal kiss.” Roosters have a rudimentary copulatory organ, the papilla, which is used to transfer the sperm cells into the hen’s system. Upon their arrival in the oviduct, the sperm cells reside for a brief time in “sperm nests,” folds of tissue at the junction of the shell gland and vagina. Once the current day’s egg is laid, these “stored sperm” move, with help from the oviduct, to the junction between the infundibulum and magnum, where more sperm nests are present. Once here, sperm are available for up to two weeks to join with any and all ovum (yolks) that pass, often resulting in fertilization—the union of sperm and egg.
Development of the chick begins immediately, with the first cell divisions (cleavages) occurring about four hours after fertilization and continuing until the egg is laid. At oviposition, the embryo has between 20,000 and 60,000 cells, giving the characteristic ⅛–3/16-in. (3–4-mm) doughnut or bull’s-eye appearance to the germ, the developing embryo on the surface of the yolk. Once laid, the egg begins to cool. When the temperature reaches about 80°F (27°C), known as “physiological zero,” cell division in the young embryo ceases and the embryo enters a period of quiescence.
Under natural conditions, chickens are indeterminate layers, producing 8–12 eggs in a clutch, at which time they become broody and “set”—nature’s hormonal message for the hen to incubate her eggs. Hens begin to incubate the clutch after the last egg is laid, thereby hatching all of their young within a 24-hour period or so.
When artificially incubating eggs, we can take advantage of this natural pause in development to schedule setting and subsequent hatching. Following many years of research, the proper storage conditions for incubating eggs to maximize their hatching success have been determined (see here).
SETTABLE EGGS
When collecting eggs for hatching, note that some eggs should not be set. Eggs that are misshapen; have obviously poor-quality shells, thin spots, or a rough sandpaper-type surface; or are dramatically larger or smaller than the average in the group, will have poor hatching success and should be consider unsettable.
Dirty eggs should be cleaned or discarded. While washing by soaking or rubbing with a damp cloth is unnecessary, a quick rub with some fine sandpaper to remove any dried dirt or feces will reduce the possibility of contamination, which can result in infected eggs and problems later during incubation. Washing removes the cuticle, which is the hen’s protective barrier to contamination. Eggs that are excessively dirty should not be set.
Eggs ready for setting in a commercial incubator. Incubators provide a substitute for the hen by providing the proper environmental conditions of temperature, relative humidity, movement, and ventilation.
It must be stressed here that eggs should not be set immediately upon laying. Setting eggs that are still warm from the hen tends to result in reduced hatching success. The biological reason for this phenomenon is not fully understood but the results are well documented. Therefore, setting should take place no earlier than after about two days of storage. The maximum storage period that still results in a high hatchability rate depends on the conditions in which the eggs are kept, including temperature, humidity, position, and whether the eggs are turned.
Temperature during Storage
As with many factors affecting biological development, time and temperature are closely related. Therefore, when it is considered how long hatching eggs can be stored, the next question is immediate: What is the temperature of that storage?
Research tells us that hatchability of stored chicken eggs is best when the eggs are kept at cool, not cold, temperatures and for a maximum period of between 10 and 14 days. If eggs are to be stored for the full 14 days, the temperature should be about 55°F (about 13°C). As storage time is reduced, the temperature can be raised slightly. For one week of storage, temperatures of 60–65°F (16–18°C) are adequate, while if eggs are stored for only two or three days, storage at room temperature, or 70–75°F (21–24°C) will suffice.
When exceeding the time factor for any temperature level, hatchability will decline at a relatively constant rate, with a warmer temperature hastening the loss of hatchability. If chicken eggs are stored at 55°F (13°C), virtually none will hatch after about five weeks.
Other related species respond to storage conditions differently. Of the common galliform-type production birds, ring-necked pheasant eggs will generally store for a maximum of about nine days, turkey eggs for about 20 days, and the champions, chukar partridge eggs, for 28 days before hatchability begins to decline.
Humidity during Storage
In addition to time and temperature, relative humidity of the storage environment is also important. Because of the porous nature of the shell, eggs constantly lose moisture to the environment. Eggs that lose too much moisture during storage and incubation have poor hatching success. To slow the water loss from eggs, high humidity—about 70 percent—during storage is necessary. The longer eggs are stored, the more important it is to ensure that the storage humidity is at the correct level.
Egg Position & Turning during Storage
During storage, eggs should be positioned with the blunt end up. Once again, it is not well understood why, but eggs stored blunt end down—or upside down—do not tend to hatch as well. Turning eggs during storage is unnecessary if they are to be set in seven days or less. If they are scheduled to remain in storage for longer than seven days, however, tilting them once or twice a day, from the first day of storage, will help to improve hatching success by 3–5 percent.
There is a wide variety of incubators available on the market for small producers. While they come in many shapes, sizes, and models, there are essentially two types: still-air and forced-draft incubators.
Still-air versus forced-draft incubator
A forced-air incubator uses a fan to circulate air around the incubator. This helps remove the risk of hot or cold spots that can occur with still-air systems.
Still-Air Incubators
Still-Air incubators are quite popular among small-scale poultry-keepers. They are generally tabletop models, holding from 3 to nearly 100 eggs. They are generally inexpensive, constructed of plastic or Styrofoam, and can incubate eggs in only one layer. They are available with upgrades such as automatic turning and conversion to a forced-draft type.
In all still-air incubators, air movement is accomplished by the natural process of convection, in which warm air rises and cold air sinks. The implication here is that there is a temperature gradient in the incubator, with air at the top warmer than that at the bottom, so eggs are kept in one layer only where the correct temperature is found.
Forced-Draft Incubators
Forced-Draft incubators contain a fan that moves the air, so the temperature is consistent in all areas of the incubator. With consistent temperature, eggs can be stacked throughout the incubator.
As with storing eggs before setting, the environmental conditions in which the eggs are kept during incubation, and the management techniques used, have a huge influence on hatching success.
Temperature during Incubation
Temperature is the primary environmental condition that controls the biological process of avian development. The increase in temperature that occurs when eggs are moved from storage to an incubator stimulates cell division, and embryonic development resumes. Note that it is important to have an accurate thermometer to measure the temperature inside your incubator precisely. Inexpensive card or dial thermometers are not accurate enough to check that the correct temperature for incubation is maintained.
Development is a very precise process, and cells and tissues must be formed at the proper rate to produce a normal offspring—21 days in chickens. In addition, temperature must remain constant, with only minor variations of less than 1°F (roughly 0.5°C), for proper development to occur. Research on artificial incubation over many years has determined the correct temperature for forced-draft incubation is 99.5–100°F (37.5–37.8°C), while still-air incubation should be at 102–103°F (38.9–39.4°C). Because of the virtual lack of air movement in still-air incubation, the temperature should be measured at the top of the eggs. Temperatures just 1–2°F (roughly 0.5–1°C) higher or lower will change the development rate, so that chicks will hatch either one or two days early or late, respectively. In addition, hatch success will be lower due to various potential abnormalities or other problems caused by improper temperature.
Humidity during Incubation
As is the case during storage, moisture continues to be lost from the egg throughout the incubation process, some of which is essential. On average during incubation, eggs will lose about 12 percent of their weight due to water loss. While eggs may hatch after a weight loss of anywhere between 8 percent and 24 percent, those that lose closer to the average of 12 percent will successfully hatch at a much higher rate. To control water loss, humidity in the incubation environment must be kept at 56–60 percent. Once again, when incubating outside of these humidity levels, eggs will either retain too much water during hatching, resulting in chicks that are edematous and “drown,” or chicks that become dehydrated, dry out, and stick to the shell during hatching.
Many newer incubators have direct digital measurement of humidity, while older incubators usually use “wet-bulb” temperature. To accomplish this measurement, a moistened wick is attached to the bulb of a thermometer—due to evaporative cooling, the wet-bulb temperature will be lower than the dry-bulb temperature. For a humidity of about 56 percent, the wet-bulb temperature should be about 86°F (30°C).
Controlling humidity is accomplished by adding water to the incubator, usually in a small reservoir or pan placed in the bottom of the incubator. The humidity can be altered either by increasing or decreasing the surface area of the water reservoir, or by adjusting the ventilation to the incubator. Care must be taken if vents are closed to increase humidity not to allow the concentration of carbon dioxide to build up to lethal levels inside the incubator (see Ventilation during Incubation).
During incubation, developing embryos require oxygen and release carbon dioxide in the same way as in hatched birds, only in much lower amounts, these amounts increasing as the embryos approach hatching. All incubators have vent holes of some sort or another that can usually be opened and closed for humidity control and to adjust the ventilation rate. The primary consideration here is to keep the carbon dioxide concentration inside the incubator below about 0.5 percent. Follow the incubator manufacturer’s instructions on how to adjust ventilation, as they are all different.
Egg Position & Turning during Incubation
Egg position in most small incubators is horizontal, with the eggs laid on their sides. In larger incubators, eggs are generally set vertically with the blunt end up. Hatching success is generally about 5–7 percent better when eggs are set vertically, but horizontal setting is quite adequate.
Regardless of position, eggs need to be turned at least three times per day for proper hatching. If kept in the horizontal position, eggs are usually rolled to the other side, and if kept in the vertical position, they are generally tilted at a 45-degree angle, first to one side and then to the other. This daily movement prevents the growing embryo from sticking, thus increasing the hatching success. In addition, when eggs are set in the wrong position and/or not turned, the percentage of chicks that are in an incorrect position for hatching increases, again reducing hatching success. Turning should begin with the first day of incubation and continue through at least 14 days.
Candling
Although it is not required, candling eggs at around 7–10 days of incubation is good management practice. (Removing the eggs from the incubator for a few minutes as you do so will not disturb their development.) During candling, the egg is taken into a darkened room and a bright light is shone through it to observe the process of development.
Brown eggs are more difficult to candle due to their dark shells, so a brighter light and completely dark room are often necessary. White-shelled eggs are easy to candle. To make an inexpensive candler, cut a small hole (1 in. [2.5 cm] in diameter) in a piece of cardboard. Place a flashlight so that all the light is shining through the hole in the cardboard. Hold the blunt end of the egg against the hole so that all the light is aimed at the egg, which will be illuminated like a lightbulb.
Egg candling is a way to observe the internal contents of an egg without breaking the shell. Candling is used to determine the developmental status of hatching eggs and the quality of eggs that are to be consumed.
In a normally developing egg at 7–10 days, candling will reveal a fairly large air cell at the blunt end, a somewhat dark area in the middle, and an obvious series of blood vessels. Movement of the embryo may also be noted. Following these quick and easy observations, the egg can be returned to the incubator so that the embryo can continue on its way to becoming a hatched chick.
Abnormal observations include clear eggs, which look like unincubated eggs. These are either infertile or the embryo died before completing the first day of incubation. A blood ring forms when an embryo has developed to the point of some blood formation (three to five days) but then died, resulting in pooling of the blood and giving the blood-ring appearance. A small dark spot (the dead embryo) may be noticed as well.
Occasionally an egg will appear to have dark fluid oozing from some of the pores, it may be slightly cracked, and it will emanate a noxious odor. If candled at this point, it will appear nearly completely dark. An egg with these characteristics is infected and has become home to one of the several bacteria strains associated with chicken eggs. These eggs should be immediately and carefully discarded. Occasionally an infected egg will explode in the incubator, or in your hand as you attempt to lift it from the incubator, spreading the infection to other eggs and requiring the complete and thorough cleaning of the incubator before the next setting.
HATCHING
For best hatching results, eggs should be transferred at about 18 days to a different incubator. Hatching is quite a dirty process, with the broken eggshells, chick down, and meconium (embryo feces) making cleaning difficult, so a dedicated “hatcher” is best.
At transfer, the eggs should be laid on their sides with enough space so they can move slightly. They will not roll around during hatching, and at this stage should no longer be turned. Turning at this stage can serve to disorient the chicks and reduce the possibility of hatching. The temperature should be reduced by about 1.5–2°F (about 1°C), and the relative humidity should be increased to about 65 percent or about 88–90°F (31–32°C) on a wet-bulb thermometer.
If the incubation conditions have been correct, the first chicks should pip (break the shell) and begin hatching at about 20½ days, with most out by the end of the 21st day—although there may be a few stragglers. It is good practice to allow the hatch to continue through day 22, after which any chicks that have not yet hatched probably will never do so. Even during good hatches, some will pip and not hatch, pip and die, or simply remain in the shell and not pip—this should be considered normal. You may want to “help” the chicks hatch, but this urge should be suppressed. Nature has made the hatching process difficult so that only the strong succeed. Late chicks often have physical or developmental problems that make them unable to hatch, and even if they are helped and removed from the shell, their chances of surviving the first week are poor. Any unhatched or partially hatched eggs can be humanely dispatched by placing them in a refrigerator for a few hours.
