Storey's Guide to Raising Chickens (75 page)

If the temperature runs slightly high or slightly low during incubation, the eggs will not hatch in exactly 21 days. Some pipped eggs may not hatch at all, and in general the hatch rate among fertile eggs will be lower than if the temperature had been just right.

At a temperature that is ½ to 1°F (0.3 to 0.5°C) low, chicks will take longer than the normal 21 days to hatch. They will tend to be big and soft with unhealed navels, crooked toes, and thin legs. They may develop slowly or may never learn to eat and drink and therefore will die.

If the temperature runs ½ to 1°F high, chicks will hatch before the allotted 21 days. They will tend to have splayed legs and can’t walk properly.

If the eggs hatch in exactly 21 days but not all pips hatch and the hatch rate of fertile eggs is generally poor, the problem is not the temperature. Rather, most likely the humidity is off.

For a successful hatch, moisture must evaporate from the eggs at just the right rate. Overly rapid evaporation can inhibit the chicks’ ability to get out of their shells at hatching time. Overly slow evaporation can lead to mushy chick disease (
omphalitis
), in which the yolk sac isn’t completely absorbed so the navel can’t heal properly; as a result, bacteria invade through the navel, causing chicks to die at hatching time and for up to 2 weeks afterward.

Evaporation is regulated by the amount of moisture in the air — the more moisture-laden the air, the slower moisture evaporates from the eggs and vice versa. To reduce the rate of evaporation from eggs, every incubator has some sort of device that gradually releases moisture into the air.

The water-holding device might be a simple pan, a divided pan, or grooves molded into the bottom of the incubator. Humidity is regulated by adjusting the surface area available for evaporation. Using a pan with a larger surface area increases humidity; a smaller pan decreases it. Filling more divisions or grooves with water increases humidity, and filling fewer decreases it. Increasing surface area with sponges or humidity pads increases humidity; partially covering the water pan with foil decreases humidity.

Some devices must be filled manually. Since cool water draws heat from the incubator, always use warm water. Some incubators may be fitted with an external water container that automatically feeds into the incubator, which must be checked occasionally and refilled as needed so it never runs dry.

Humidity control may be fine-tuned by adjusting the incubator’s vents. Some vents must remain open at all times for good oxygen flow; others have either removable plugs or sliding covers that allow you to adjust the size of the openings. Closing vents increases humidity by trapping more moisture-laden air within the incubator. Opening vents decreases humidity by allowing more moisture-laden air to escape.

An accumulation of moisture on the incubator’s observation window during the hatch is an obvious indication of excess humidity. Low humidity tends to be the greater problem with small incubators and those that have to be opened to turn the eggs manually. If humidity drops inexplicably during a hatch, the water pan may need to be cleaned. Fluff released by newly hatched chicks will coat the surface of the water, preventing evaporation and causing the humidity to plummet.

Because the eggs themselves contribute to humidity by evaporating through the shell, an incubator that is not filled to capacity may have more difficulty maintaining proper humidity than a full incubator. The larger the incubator, the greater the problem. At the end of the season, when my cabinet incubator has few eggs left in it, I boost the humidity and ensure a good final hatch by adding a cake pan full of water during the hatch.

Most incubators call for about 60 percent relative humidity, except during the last 3 days prior to the hatch, when it should be increased to 70 percent. An electronic
hygrometer
, or an incubator with an electronic control system, shows the humidity level in a digital display.

Measuring Humidity.
Humidity in a nonelectronic incubator is measured by a thermometer in what’s known as
wet-bulb degrees
, in contrast to
dry-bulb degrees
, in which the same thermometer measures heat. A wet-bulb reading is more accurate in a forced-air incubator than in a still-air incubator.

To obtain a wet-bulb reading, slip a piece of cotton, called a wick or sock, over the bulb or stem end of the thermometer with the tail end of the wick hanging
in water. You can buy wicks or make them by cutting up a white cotton tennis shoelace. You’ll need spare wicks to replace those that get crusty with mineral solids and lose their absorbency. If your tap water tends to be hard, use distilled water, and your wicks will last longer.

You can make a temporary hygrometer by wrapping a piece of cheesecloth or gauze bandage around the end of the thermometer and wetting the gauze with warm water. Place the thermometer in your incubator where you can see it when the cover is on or the door is shut.

As water evaporates from the cotton, the thermometer gives a lower reading than it would without the wick. After a few minutes the wet-bulb temperature will stabilize, and you can take a humidity reading. A typical wet-bulb reading is 86 to 88°F (30 to 31°C) during incubation and 88 to 91°F (30 to 33°C) during the hatch. The table {below/opposite} shows the relationship between wet-bulb degrees and percent relative humidity at common incubation temperatures.

In place of a hygrometer, or in conjunction with one, a good indication of humidity is the changing air-cell size inside the developing eggs, as determined by candling. Moisture evaporating from an egg causes its contents to shrink, which increases the size of the air cell. If air cells are proportionately larger than the one shown in the sketch below, increase humidity; if they’re smaller, decrease humidity.

As moisture from within an egg evaporates during incubation, the egg’s weight decreases. Under proper humidity, an egg will lose 12 to 14 percent of its weight during the 21 days of incubation. So another way to measure humidity is to monitor weight loss as incubation progresses. At the start of incubation, weigh a sample of eggs — say, half a dozen — and weigh the same sample every 5 days, then compare their actual weight to their expected weight based on average loss. If the actual weight is less than the expected weight, increase the incubator’s humidity; if the actual weight is greater than expected, decrease the humidity.

Relative air cell sizes on the 7th, 14th, and 18th days of incubation.

The “Expected Weight Loss during Incubation” table that follows shows the expected drops in weight during incubation of a sample of six eggs of different sizes having typical starting weights for their size. Because of the small numbers involved, a scale that measures in the metric unit of grams is more accurate than a scale measuring ounces.

HUMIDITY AT COMMON INCUBATION TEMPERATURES

EXPECTED WEIGHT LOSS DURING INCUBATION

Maintaining adequate incubation humidity is more difficult in really dry weather than in humid weather. Those of us who live where the weather is extremely dry one half of the year and extremely humid the other half of the year must compensate accordingly.

The heat and humidity within an incubator offer ideal conditions for germs to flourish, and hatching produces plenty of organic wastes for the germs to thrive on. An incubator must therefore be cleaned of debris after every hatch and periodically disinfected. No incubator is truly easy to clean and sanitize, but some are easier than others.

A Styrofoam incubator is the most difficult type to clean. My experience, and that of others, is that after a few consecutive hatches the success rate drops sharply. Although a Styrofoam sanitizer is on the market, some incubator manufacturers recommend cleaning Styrofoam only with plain water. Easily sanitized plastic liners are available for some models. An alternative is to press aluminum foil against the bottom, shiny side down, and poke holes for the necessary air vents. If you opt for a Styrofoam incubator, plan on one hatch, or only two or three consecutive hatches, then thoroughly clean the incubator and let it sit for several months before using it again.

Illness in freshly hatched chicks comes from one of two sources: poor sanitation in the incubator or around the brooder and disease in the breeder flock that’s transmitted through eggs. Some disease organisms are inside the egg; others get on the shell when it is laid or when it is improperly washed.

Diseases that may be transmitted via the egg spread from infected chicks to healthy chicks in the incubator (often inhaled in fluff) or in the brooder (usually through ingested droppings in feed or water). The most common of these diseases is omphalitis caused by the incubation of dirty eggs, operation of an unsanitary incubator, or improper temperature or humidity during the incubation period.

Good incubator sanitation not only improves hatching success and gives chicks a healthy start in life but also helps break the natural disease cycle in a flock. Because hatching itself is a major source of contamination, take time to clean your incubator thoroughly after each hatch.

The best sanitary measure is to incubate one setting of eggs, then clean and disinfect the incubator before starting another setting. Some incubators are designed for continuous hatching. They typically have three trays to hold incubating eggs and a fourth to hold the hatch, allowing you to fill one tray with eggs every week and hatch a tray every week. Continuous hatching is a significant source of contamination.

To minimize contamination move the hatch to the lowest tray, where no eggs are below for hatching debris to fall on. If the hatching tray has a screen bottom, place a layer of paper toweling or foil under the tray to simplify cleanup. After each hatch remove the hatching tray, and scrub and disinfect it as well as the area below it. Clean and disinfect the water pan as well.

A better sanitary measure for continuous hatching is to use a separate incubator as a hatcher. Ideally, it will be a forced-air incubator, but it doesn’t need a turning device. After heating up the hatcher, move the about-to-hatch eggs from the incubator to the hatcher. After the hatch is complete, scrub and disinfect the hatcher in preparation for the next hatch. Not only does this plan improve sanitation, but it allows you to leave the incubator at the optimum temperature and humidity for incubation, while adjusting the hatcher for optimum hatching temperature (½ to 1°F cooler [0.3 to 0.5°C]) and humidity (6 to 10 percent higher) for an even better rate of hatch.