Poultry Fact Sheet No. 32
Cooperative Extension - University of California

Egg Candling and Break Out Analysis for Hatchery Quality Assurance and Analysis of Poor Hatches

R. A. Ernst, F.A. Bradley, U.K. Abbott and R.M. Craig
Animal Science Department, University of California, Davis, CA 95616.

Candling of hatching eggs during incubation and examination of the clear eggs or dead embryos is a useful tool for hatchery managers.  Undoubtedly managers will want to monitor many other parameters in the hatchery to assure optimum hatching success.   These might be parameters such as temperature of egg storage room, shell quality, incubator temperature, incubator humidity, chick quality, hatching percentage, bacterial counts in air or on hatchery surfaces, and egg moisture loss during incubation.  This publication illustrates the appearance of eggs during candling and how to identify infertile eggs and early dead embryos during break out.  Those who are inexperienced in incubation may find our attached "Definitions of Common Hatchery Terms" useful.  A video "Hatching Egg Breakout" is available from the University of California for $20.  To order contact: Cheryl Dempsey, Administrative Assistant, e-mail cadempsey@ucdavis.edu, (530) 757-8930, FAX (530) 757-8940  or point your browser to: http://anrcatalog.ucdavis.edu/merchant.ihtml?pid=834&lastcatid=102&step=4

Hatchery Quality Assurance and Diagnosis of Poor Hatches

In cases of poor hatches it is important to identify the cause of the trouble as quickly as possible. Poor yield may be caused by a failure in fertilization or by excessive mortality of the embryos do to a variety of factors.  A list of problems, causes and possible solutions may be useful in trouble shooting incubation problems (Poultry Fact Sheet No. 33 Incubation Problems -- Some Causes and Remedies).

A careful examination of a sample of eggs is useful to provide quality assurance or to diagnose hatching  problems. Such an examination must include not only candling but breakout analysis. Even during problem-free periods, chicken, pheasant, partridge or quail eggs should be candled after 5-7 days of incubation, and turkey, duck or goose eggs--because of their longer incubation period--should be candled after 8-10 days of incubation.  The sample should be candled again at transfer to the hatcher and dead germs examined.  If you wish to set up an organized program for hatchery quality control we suggest you consult "Quality Control Procedures for the Hatchery" J.M. Mauldin, University of Georgia.


If small groups of eggs are set, the most appropriate sample is the entire group.  If sets exceed 300 eggs, examination of a sample of 100 to 200 eggs is suggested.  In large hatcheries sampling procedures should be carefully planned with the assistance of a statistician or scientist to optimize the quality of the results and minimize costs.  During problem periods, more frequent candling and analysis may be advisable.  To determine true fertility, breakout analysis is necessary.

Suggestions for Using This Publication

The color photos included here will help you to distinguish normal, healthy embryos from infertile eggs and "early-deads." We suggest you refer to the "Table of Incubation Problems" which indicates their causes and possible remedies.

Loss of incubated eggs from modern, high-hatching chicken strains, stored under optimal conditions, should be no more than 10 percent at the first candling. Losses in turkey, waterfowl and game bird eggs may be slightly higher. The mortality measured by candling and breakout during the first period will normally represent one third of the total to be expected.  The mortality after the second candling should represent two thirds of the total with very little mortality during mid-incubation. Mortality during mid-incubation may indicate a dietary deficiency if infected embryos or developmental abnormalities are not seen.  However, the most common nutrient deficiencies seen by the authors result from marginal vitamin levels and usually cause weak chicks which have difficulty hatching without other symptoms.

When eggs are candled after the first mortality peak at 7 to 10 days, three distinct classes can be recognized:

  1. Living Normal embryos
  2. Blood Rings (definition)
  3. Clears

When eggs are candled at transfer clears are not expected unless they were missed on the first candling.  Dead embryos should be expected in small numbers.  Some of these may be associated with poor or damaged shells which were not removed during the first candling or which occurred after the first candling.  Break out may reveal infected eggs which can be detected by abnormal color and odor.

When eggs which fail to hatch are examined several types of abnormalities are likely.  The technician should look for malpositioned embryos (other than head under right wing and in the large end of the egg).  Excessively wet or dry embryos  indicate incorrect incubator humidity, extended or improper egg storage (dry) or poor shell quality.  Some genetically abnormal embryos are expected at this stage but if numbers are excessive a more detailed investigation is advisable.

Living Normal Embryos

(Click on photos to enlarge.)

They will show:

  1. Clearly defined blood vessels with no hemorrhagic areas evident

  2. Some body movement when stimulated by the candling light

  3. A generally "healthy" appearance

Four-day-old turkey embryo.

This photo is included to serve as a reference in determining the developmental age of normal embryos. Note the small, centrally located embryo and the developing blood vessels.

Four-day-old normally developing chicken - candled.

Note the more advanced circulatory system than in the turkey embryo of equal age. The dark area near the center of the egg is the embryo; the radiating lines are the blood vessels of the extra-embryonic membranes.

Twenty-four-day-old turkey - candled.

The light upper part is the enlarged air cell which is important for proper hatching. The dark, lower part contains the embryo. One of the keys to good poult hatchability is the proper amount of "dry down" at the end of the incubation period.

Twenty-seven-day-old turkey - pip candled.

The blood vessels at the lower left have not yet completely dried up and are still needed for respiration. Humid conditions must be maintained to prevent premature drying of these vessels and death of the poult.


Blood Rings and Dead Embryos

(Click on photos to enlarge.)

These eggs show a ring of blood outlined on the inner surface of the shell. They usually contain fairly advanced embryos which have died only recently.

Blastoderm without embryo (BWE) and infertile - chicken.

A comparison between two candle-outs. The infertile egg (right) shows no development, the 2-1/2 day BWE (left) shows the typical extra-embryonic membranes surrounded by a blood ring.

Blastoderm without embryo (BWE).

The blood ring indicates a dead and probably abnormal embryo from a fertile egg. The lack of any visible embryo structure indicates that it is a BWE.

Cystic embryo - candled.

Similar in appearance to BWE. Upon breakout, a cystic embryo often has heart tissues as the most recognizable normal structure.

Blood ring: chicken.

The presence of the blood ring is symptomatic of the early death of an embryo. Causal      diagnosis may be made by breakout analysis.

Turkey egg - candled after 10 days incubation.

The large dark, round spot and lack of visible blood vessels are the major indications that the embryo is dead.

Turkey embryo about 8 days old.

Abnormal embryo from an egg candled after 10 days incubation. The abnormally shaped head and mouth parts indicate developmental disturbances which led to its death. Decomposition of tissues as seen here, is typical of dead embryos.



(Click on photos to enlarge.)

  • Fertile Eggs
  • True Infertiles
  • Fertile, No Development (FND)
  • Positive Development

This term is used for eggs that show no development when candled. Under normal circumstances, more than half of the clears are true infertiles. The others comprise the dead-embryo categories listed and shown in the photos. These are not true fertility problems and, especially where they are abundant, must be identified correctly so that measures can be taken to remedy the specific problem.

Fertile Eggs

Fertile unincubated turkey.

Characteristic of this fertile egg is the regular, smooth blastodisc, showing a distinct, circular edge.

Unincubated chicken egg - candled.

Candling an unincubated egg will not show whether it is going to develop or not. Characteristic of the unincubated egg is the indistinct shadow of its yolk.

Chicken egg - 24 hours of incubation - candled.

Note that the yolk shadow is more distinct than in photo 7, and that it shows near the top, a raised round area where the embryo will develop.

Cracked chicken egg - candled.

Eggs with cracks - not always visible without candling - should be excluded because they are likely to break further and may be contaminated with bacteria or molds which will infect other eggs.

True Infertiles

Infertile incubated turkey.

The blastodisc is a small, light spot surrounded by a cloudlike fuzzy area of irregular shape.

Infertile (right) and blastoderm without embryo (BWE) - chicken.

A comparison between two candle-outs. The infertile egg (right) shows no development, the 2-1/2 day BWE (left) shows the typical extra-embryonic membranes surrounded by a blood ring.

Fertile, No Development (FND)

Infrequently seen. Causes, in addition to those mentioned in the list, include highly inbred stocks and insemination with irradiated semen.

Fertile, pre-oviposital death, turkey.  This is a type of FND.

A rarely diagnosed condition. The egg was incubated for 3 days before being broken out. The blastodisc has deteriorated but still has the characteristic shape and size of a typical fertile blastodisc.

Positive Development

Most frequent among fertile clears. Although under normal conditions only 1 to 2 percent of the clears are PD's the incidence will increase under the conditions mentioned. The embryo dies early but the growth of cells over the surface of the yolk may continue for several days.

Positive development (PD): turkey.

A fertile egg that began to develop and then stopped.  Candled, it appears to be clear. Note the absence of blood, as compared to the BWE illustrated above.

Positive development (PD): turkey.

This blastoderm has developed further than the one in the photo above. The absence of blood formation characterizes it as a PD.

Other Useful References:

PFS No. 22    Hatching Egg Production, Storage and Sanitation