The 15 Most Common Egg Defects And How They Happen

A type of irregularity you may encounter is an egg with what looks like a water spot — a darkened roundish area surrounded by a white band. This odd marking is a sign that two eggs have passed through the hen's oviducts into the shell gland pouch (where the shell material is secreted around the rest of the egg) at the same time. These marks form when the two eggs and their partially formed shells come into contact with each other; the white ring is excess calcium that builds up on the first egg that enters the shell gland pouch.

This issue points to another abnormality: two eggs being laid at once. Normally, hens lay one egg at a time at roughly 24-hour intervals (they may skip days when they're molting or lay less frequently as they age). Egg production is also linked to hens' exposure to sunlight, so they tend to lay less in the winter when days are shorter. Conversely, artificial light in henhouses, sometimes used to stimulate laying during the winter, can trigger the production of multiple eggs, along with white-banded eggs. Stress and certain conditions, such as bronchitis, can also cause this defect.

Another common issue chicken breeders may encounter is blood stains on eggs. While disturbing to see, they're not always a sign of a serious problem and the eggs are still safe to eat (however, they cannot be sold). Still, for the health of your flock, it's important that you understand some of the reasons why this happens so you can prevent more serious problems from emerging.

A common cause is developmental – young hens just starting to lay may strain hard when laying, causing blood vessels in their bodies to tear and thus stain the egg as it passes through. At the other end of the lifespan, older hens may also experience bleeding when laying eggs. But blood-stained eggs may indicate more serious problems, such as vent pecking, a form of bullying in which one bird attacks the vent (the orifice from which eggs emerge) of another, causing bleeding. Mite infections around the vent can also cause bleeding and stained eggs. To prevent these issues, separate any hens with injured or inflamed vents from the rest of the flock until they've completely healed, and ensure your hens have enough space and enrichment opportunities to keep them from becoming stressed and aggressive.

Mottled eggs are those with a subtly spotted shell surface that may appear marbled. This defect is most noticeable when the eggs are candled, which will reveal a shell of uneven thickness (with the spots being abnormally thin areas of the shell). While normal eggshells have even pores that facilitate the proper flow of air and moisture for fertilized eggs to develop, the abnormally large and uneven openings on mottled eggs can prevent these eggs from developing and hatching.

Mottling results when an egg's shell fails to dry out quickly after hatching. Humid or overcrowded henhouses are one cause of this, but other factors can contribute to mottling as well. Stress (especially heat stress) can induce hens to lay mottled eggs, as can diseases impacting the shell gland, such as mycoplasma and infectious bronchitis. Digestive and dietary problems, such as manganese deficiencies and insufficient absorption of calcium, are other causes of mottling. Proper vaccination and careful monitoring of heat and humidity levels in henhouses, along with nutritional supplements, can ease the problem.

Chicken eggs come in a range of colors, from white to brown to even blue, green, and pink. But because egg color is correlated with chicken breed, any individual hen will lay eggs of the same color over the course of her life. At times, however, you might notice some eggs with conspicuously irregular coloration — the absence of the expected color on all or part of the eggs.

Abnormal coloration can be traced to a number of possible causes. Older hens, along with those who've been laying intensively for a prolonged period, may produce paler-than-normal eggs. Poor nutrition can also be a factor — zinc, manganese, and copper are all critical for the development of proper shell pigmentation, and diets with insufficient quantities of these minerals can result in discolored eggs. Viral and parasite infections can also interfere with the oviduct's ability to produce normally colored eggs, as can stress. Finally, the drug Nicarbazin, sometimes found in chicken feed, can interfere with pigment production.

One of the most startling defects you may encounter is an egg that emerges with no shell — only the white and yolk within a thin membrane. The very fact that shell-less eggs can exist is a reminder of how complex egg-laying is. While healthy hens lay eggs nearly every day, seemingly without effort, eggs are in fact produced in a multi-step process in which the yolk forms first, followed by the shell membranes, the white, and finally the shell.

All of this means hens have to be both physically and emotionally healthy to produce good-quality eggs. If any of the organs involved in egg production fail to function properly — whether due to poor nutrition, stress, or other factors — defective eggs can result. Eggs can emerge without shells, for example, if the gland responsible for producing the shell misfunctions. This can happen in immature hens whose shell glands haven't yet fully matured, but can also result from illnesses such as Newcastle disease, which can damage the gland. Poor nutrition can hinder shell development as well. Because egg shells are mostly calcium carbonate, diets with insufficient calcium can result in shells failing to form, as can shortfalls of phosphorus, vitamin D3, or manganese.

Natural organisms, from plants to people, all come with mechanisms for surviving and recovering from injuries and stress. We take it for granted that small cuts and bruises will disappear within a matter of days, and slightly larger ones will eventually heal, but may leave scarring. Even chicken eggs can recover from damage sustained during their development — but often do so bearing visible scars, which look like what they are: healed-over breaks or cracks.

Yes, the eggs can actually get cracked or broken before they're even laid. These cracks occur while the egg is still within the shell gland and the shell is still undergoing development. Fortunately, cracked eggs in the shell gland have the opportunity to heal (the egg will emerge with a thick layer or shell on the area of the crack), but the eggs that result, while safe to eat, aren't attractive. Moreover, they can be a sign of other problems with your hens' living conditions that need to be addressed. Eggs with healed breaks are more likely to occur when hens become startled or agitated in the late afternoon when shell formation typically takes place. Common stressors that can interfere with normal egg production are fights and overcrowding — so be sure your hens have enough space to lay their eggs and go about their business in peace.

Occasionally, you may encounter an egg with a conspicuously flat area on one of its surfaces. While glass-half-full types may appreciate an egg that's less likely to roll off the kitchen counter unbidden, these flattened eggs — called slab-sided eggs – are less than ideal for a number of reasons. For one, the flat surface is abnormally thin, meaning the overall quality of the egg is lower. More importantly, however, they're an indicator of disruption in the hen's reproductive cycle or living conditions, both issues that need to be addressed.

Normally, hens lay one egg at a time, so only one egg should normally occupy the shell gland at any given time. Slab-sided eggs, however, result when two eggs enter in quick succession, pressing against each other as their shells develop. This traffic jam results in the second egg coming out with a flat side. Slab-sided eggs tend to result from various stressors such as sudden scares, overcrowding, or sudden increases in light exposure (for instance, the introduction of artificial lighting to increase egg production during the winter). Diseases that impact the reproductive tract, such as infectious bronchitis, can also contribute to this defect.

One of the strangest and most surprising egg defects — if you can even call it that — is technically known as a counter-peristalsis contraction. In non-technical terms, it's a whole egg encased in another egg, and the only way you'll know for sure you've got one is if you break an egg open and discover yet another egg inside (another hint, however, maybe that you have an unusually large egg).  And while it's obviously not the way an egg is supposed to form, it's so cool and uncanny it's hard to get upset about encountering one.

This is just as well since exactly what triggers their formation remains a mystery (but genetics are thought to play a role). We do know, however, how these nesting-doll eggs come to be: They occur when a partially formed egg somehow travels back up the reproductive channel, where another shell and layer of white form around it. Alternately, the backward-moving egg may encounter another egg in progress, and a shell will subsequently form around both. It's a relatively rare phenomenon, and is considered harmless – chickens have been known to lay multiple embedded eggs and still stay healthy.