See Part 1 in this series to learn the history of Polish chickens and the genetics behind crests.

When people think of Polish chickens, they think of their elaborate, beautiful crests. Most people don't realize that Polish chickens also have “vaulted skulls.” Is it a weird coincidence that most crested chickens have vaulted skulls? Or is there a genetic connection between these two traits? When I set out to write this series, I didn’t know the answer to these questions. But as I researched the topic, I came to realize that even though people have been wondering about the crest/vault connection for hundreds of years, the answer has been elusive. But recent research has brought the answer almost within our grasp.

Before I launch into a discussion of the genetics of vaults and crests, I offer this caveat: To discuss genetics, it’s necessary for me to use some genetics terminology. I used up some paragraphs in the first installment of this series explaining that terminology. If you haven’t looked at Part 1 and you run into some terms or concepts here that you find baffling, you should head back to the first part and take a look.

Crested Chickens and Vaulted Skulls

People have been talking and writing about vaulted skulls in chickens for centuries. Vaults have been observed in most crested breeds, especially Polish, Houdan, and Paduan chickens, but never in non-crested breeds. Vaulted skulls have a “bump” protruding from their top. Vaulted skulls also typically have one or more holes through the bone. If a skull’s job is to protect the brain, it’s pretty obvious from their appearance that vaulted skulls are not meeting their performance expectations.

Vaults are most obvious in baby chicks—before their skull shape becomes obscured by their crest. That bulb at the top of a baby chick’s head is not just fluff—that’s the actual shape of the chick’s skull!

The pictures on the left and right show Angie and Angitou the Polish hens as babies. Their vaulted skulls are obvious. In the middle is a baby picture of Moe the Salmon Faverolles, a noncrested, nonvaulted breed.

To best view this abnormality, looking at an actual skull is better than trying to visualize the skull while looking at a chicken’s head. I reached out to taxidermy artist Sarina Brewer for permission to use an image of a vaulted chicken skull in her collection. She graciously granted permission and also sent me some additional high-resolution shots of that skull. For more information about Sarina and the art she creates, see the endnote at the bottom of the page.

These misshapen skulls hold equally misshapen brains. In 1998, scientists with the Vogt Institute of Brain Research at the University of Düsseldorf analyzed and compared the brains of white crested Polish chickens with brown leghorns and other noncrested breeds. They found that, “the brain has expanded to fill [the] larger endocranial space” created by “the protuberance of the head.” And, “as a consequence, the external morphology of the brain in [white crested Polish chickens] is drastically different from that in other breeds.”

Some of the differences: The whole brain shows “pronounced elongation.” The telencephalon (the part of the brain responsible for fine control of movement, and interpreting touch, vision and hearing) is oddly shaped. It is described as “nearly round” compared to a structure that is “broader than long” in a brown leghorn. The meninges (the membranes that enclose the brain) and the telencephalon are so firmly fixed together “it was almost impossible to separate” them during the necropsy procedure.

In the brown leghorn, the cerebellum (a part of the brain that controls walking posture balance, coordination, and eye movements), the tectum (responsible for auditory and visual reflexes) and the telencephalon are all in close contact with each other. But in Polish chickens, these brain structures have all relocated, so that “there is no contact between them.” And the optic chiasm (the junction of the optic nerves from each eye; important in transmitting information from the eyes to the brain) has also relocated. It has moved backwards, and instead of pointing forward toward the beak, as it does in brown leghorns, it points in exactly the opposite direction.

How does this bizarrely-shaped brain affect the behavior and intellectual capacity of the chicken? I’m not aware of any definitive conclusions in the scientific literature. But there are tons of anecdotes that span over a hundred years. In an 1868 book, Charles Darwin (yes, that Charles Darwin!) referred to some comments in an older source that described crested chickens as “extremely stupid.” He added his own observation of a crested chicken he owned that couldn’t find its way back to its feeding area when it was moved 100 yards away. Based on her research on learning behavior in chickens in the 1950’s – 1960’s, J. Schulze-Scholz, agreed with Darwin. Crested chickens, she reported, were apathetic, bad learners.

But let us not be too hasty to clap a dunce cap on top of these birds’ crested heads. There’s also the research reported by Horst Requate in 1959. He studied white crested black Polish chickens for three years and didn’t find even a suggestion of stupidity. As a matter of fact, he found them to be “full of psychic energy” with no signs of disturbed behavior. He made only one modification to these birds for them achieve the same reaction to optic and acoustic stimuli as noncrested chickens. He cut away the crest feathers covering their ears and eyes.

Vaulted Skulls and Brain Injury

The jury may be out regarding vaulted skulls causing altered behavior or reduced intelligence. But common sense suggests that the holes and open areas in vaulted skulls offer less brain protection. And there are a bazillion stories in chicken forums about crested chickens suffering neurological damage or sudden death. Often after an observed peck or blow to their head.

I’ve had three Polish hens in the ten years that I’ve been keeping chickens. I told Jennifer’s story in the opening paragraphs of Part 1 of this series. And there was Angie. Like Jennifer, Angie was in the group of chicks that made up my original flock. And like Jennifer, Angie was a white crested black Polish hen. She was the first chicken I ever lost. One winter morning I opened the coop door and saw Angie lying dead on the floor. There was a board on the floor next to her. I’d left a few loose boards in the coop about four feet off the floor. I think one of the other chickens had tried roosting on the board and knocked it loose. I think it struck Angie as it fell. It’s all conjecture based on evidence on hand when I found my dead hen. At the time, her fragility surprised me. A board fell a few feet and killed her. I didn’t know about vaulted skulls then. It makes a lot more sense now.

The following spring, when I put in my chick order, I included a golden laced Polish chick. I named her Angitou; “Angie Two.” Angitou grew into a beautiful chicken. She lived in a coop with the silkies and they were a big happy family. She was personable and loved for me to hold her. I found her dead on the coop floor when she was three years old. I have no explanation. A peck to the head? Maybe.

I’ve loved my Polish girls but I’ve not had good luck with this breed. When Jennifer passes on, I’ll be done with Polish chickens.

Jennifer’s brain damage is mild in the sense that she is still functional enough to eat and roost. Not every blow or peck to the head is fatal to vaulted-skull chickens. The severity of the brain damage that results from nonfatal injuries depends on the amount of brain swelling that occurs as a result of the injury. If the increased pressure causes their brain to swell through the holes in the skull, the chicken can suffer severe and probably fatal brain damage.

One result of brain swelling is the painful and often fatal condition called wry neck. The technical term is torticollis. Wry neck occurs with the cramping and contraction of the neck muscles. Flock-keepers find chickens suffering from wry neck with their head between their legs, twisted upside down, crooked sideways, or facing backwards. Wry neck is a symptom, not a disease. While the condition is a result of nerve damage, the cause of the nerve damage can range from Marek’s disease to vitamin deficiency to toxins. But if the chicken suffering wry neck is a crested bird, you should consider brain trauma.

In such cases, reducing brain swelling with an anti-inflammatory drug may save the hen. Gail Damerow in “The Chicken Health Handbook” suggests that aspirin may be helpful. She suggests 25 mg per pound, thus 125 mg for a five-pound bird. She also offers the caveat that prolonged exposure to aspirin can lead to kidney damage, and that you should never give asprin to a bleeding chicken (externally or internally), since it is an anticoagulant. A more focused, albeit expensive drug is the steroid prednisone, which requires a veterinary prescription.

The Genetics of Vaulted Skulls and Crests

Various observers, as early as the 19th century, have noted the weirdly shaped brains and incomplete skulls in certain lines of chickens. While these oddly-shaped skulls and brains have been associated with crests, nobody has been able to prove a genetic connection. If there are two separate genes controlling these two traits, then there is a potential that they could be separated. Imagine a future with beautifully crested chickens that have normal, healthy skulls and brains. If one gene controls both traits then, sadly, it is what it is.

Why hasn’t anybody figured it out yet? It’s harder than you might think. It would be so easy if scientists could focus their microscopes inside chicken’s cells and see little Punnett’s Squares with handy codes denoting the genetic makeup of that chicken. But nope. What scientists actually can observe with their microscopes are chromosomes at the center of each cell. Each cell has 78 chromosomes. A hen’s egg has 39 chromosomes and a rooster’s sperm contains another 39. They combine to form 78 in the developing chick.

Each chromosome is made up of tangled strands of DNA. If you could untangle the DNA in one cell and stretch it out, it would be about six feet long. The DNA strand is built out of four bases: adenine (A), cytosine (C), guanine (G), and thymine (T). A DNA molecule is made of two strands spiraling around each other, with each strand held together by bonds between the bases. Adenine hooks with thymine, and cytosine hooks with guanine. A gene is a chunk of the DNA strand. The order of the bases is the code that contains the the information a gene stores. All the information comes from the order of the bases! The order of those four bases in the six feet DNA in each chicken cell gives a chicken a crest, or a vaulted skull, or black or red feathers. And determines that a chicken is a chicken and not a petunia or a whale.

Want to see what it looks like? Here’s a specific combination of bases that make up part of a gene called HOXC10, located on chicken chromosome 33. This gene is present in both birds and mammals and is involved in the production of hair and feathers.

Sometimes, as I’m writing, I make mistakes. Sometimes I accidently add a letter that doesn’t belonhg in a word. Sometimes I leave out a leter. Sometimes I hit the wrong key amd wind up with a wrong letter. And sometimes I’ll repeat a sentence. And sometimes I’ll repeat a sentence. (See what I did there?)

Chromosomes can make mistakes like that during cell division. They can add in one or more bases, take a few out, or substitute one base for another one. That’s a mutation. One type of mutation, called “duplication”, happens when a huge chunk of bases gets repeated. Here’s an example of that – it’s the same section of the HOXC10 gene. Only in this one, 195 bases (green) repeat (red).

After years of research by a plethora of scientists all over the world, practically everybody has concluded that the 195 base duplication on the HOXC10 gene, illustrated above, is the mutation responsible for crests. Zhiwei Liu, Jinyu Chu, and associates at the Huazhong Agricultural University in Wuhan, China conducted a study in 2022. These Chinese scientists studied a group of 986 crossbred chickens with large crests, small crests and no crests, as well as “unsealed skulls” (skulls with holes) and cerebral hernias (vaulted skulls containing abnormally shaped brains). They documented the phenotype (physical traits) of each chicken. Then they performed genetic analysis to find out which chickens were homozygous or heterozygous for a 195-base duplication on the HOXC10 gene.

The results: The 195-base duplication was not detected in any noncrested chickens. But 100% of the chickens homozygous for the 195-base duplication had crests, as did 98.8% of the heterozygous chickens. The scientists concluded that “the 195- bp tandem duplication was closely associated with crest phenotype.” This matches results and conclusions of other scientists at other institutions.

What about vaulted skulls? Chickens with normal skulls were in all three genotypic groups – those homozygous for the 195-base duplication, those homozygous without the 195-base duplication, and the heterozygous group. Further, ‘’Only 94.4 % of the individuals with the [homozygous 195-base duplication] genotype and 19.0 % of those with the [heterozygous] genotype showed cerebral hernia and unsealed skull.” Thus, “Our data suggested that the 195-bp duplication was not entirely responsible for the unsealed skull and cerebral hernia.” While “the 195-bp duplication appears to be necessary for the occurrence of cerebral hernia and unsealed skull phenotypes,…there may be additional mutation sites/genes correlated with the brain and skull phenotypes.”

These results and conclusions correlate with other recent studies. It is good news. I am imagining a future where the standard of perfection for all the crested breeds continues to include all those beautiful crests, but the skull and brain malformations have been bred out of existence.

Silkies with Vaulted Skulls

The Chinese scientific paper that I quoted from in the previous section contained some interesting comments about Silkie chickens in its concluding comments. “In previous studies, the 195-bp duplication was detected in Crested Silkie chickens without cerebral hernia (my emphasis) from Asia. These Asian breeds may not have the additional mutation sites/genes related to cerebral hernia and unsealed skull, whereas Polish chickens with crest and cerebral hernia potentially developed the additional mutation sites/genes related to brain and skull phenotypes after their introduction to Europe ~ 2,000 years ago.”

So, according to these authors, there exists chickens—Silkies, developed long ago in China, that have those beautiful fluffy crests, but do not have vaulted skulls! That highlights the true fact that crested chickens without vaulted skulls can and do exist! Can chickens have elaborate crests, such as those on Polish chickens and also not have vaults? Somebody needs to work that out!

My other reaction to the statement about nonvaulted, crested Chinese Silkies is this. Here in the US, Silkies certainly do have vaulted skulls. Often their skulls are much more deformed than Polish chickens. While Polish chickens have holes in their skulls, Silkies are sometimes missing the entire top of their skulls, making them extremely prone to brain injury! Chinese Silkies don't have vaults?! What happened to the American Silkie?

European Silkie enthusiast Carina Abrahamsen (check out Carina’s delightful FB page, “Fluffy Silkies - Sedosas Fofas”) told me that American Silkies “have been bred to Polish chickens to introduce the vault, which people believe enlarges the Silkie’s crest. All vaulted breeds are prone to head injuries but in the American Silkie - which originally was a nonvaulted breed - there isn't enough bone material on top of the vault - only thin skin layers.”

An American Silkie breeder agreed that “many years ago, they did introduce Polish and probably other breeds as well to get or enhance certain traits on this breed.” She added that she felt that “vaulted skulls do not have anything to do with crest size.” And in fact, “from a conformational standpoint, the vaulted skulls give what I call a trilobed mushroom shaped crest instead of the nice round even crest we are looking for.”

The American breeder pointed out that not only does she not breed for vaults. But that vaulted birds tended to eliminate themselves from the gene pool through their “own natural selection.” “The big vaults on chicks prevent them from turning in the shell and pipping out, thus you see a lot of fully developed eggs that just don't hatch.” And because of the open skulls, “they are very prone to a condition called wry neck or crook neck. Basically, all it takes is a rough breeding attempt, a good peck or blow to that area, nutritional deficiencies, or heck even environmental pressure changes.” The result: “Inflammation on the brain and…anywhere from a head twitch to…tucking the head back between the legs and losing all control of their nervous system.”

Carina Abrahamsen suggested that, “more and more breeders in the US are trying to breed away from this. In Europe it was banned over 20 years ago. But it's not an easy task…”

I have so many questions. Apparently, at some point in the past somebody decided it would be a good idea to breed the deformed skull trait into Silkies in a lame attempt to improve their crests. I’d like to know when this happened. Who were the people who did this and what were they thinking?

And now that this Pandora’s box has been opened and somebody has inserted these malicious genes into the Silkie gene pool, everybody is at work to get rid of them. European breeders seem to be further along in this attempt than breeders in the US.

I need to roll up my sleeves and start talking to breeders in the US and Europe to find out more about the efforts to get back to nonvaulted Silkies. I would also like to find out if anybody, anywhere is attempting to create nonvaulted versions of the other crested breeds. I will report back. Stay tuned.

The author with Silkie friends, Snowball and Courtney

Thanks!

A special thanks to Sarina Brewer, a fellow Minnesotan internationally recognized for her avant-garde taxidermy sculpture. I perused the internet for photos of vaulted skulls to illustrate this article and the best photo I found was a shot by Sarina of a skull in her collection. When I reached out to ask for her permission to use the photo, she not only agreed, but also generously sent me some additional shots. In our exchange of emails, Sarina suggested that as an artist, she gives animals a second life through her sculptures. In the entirety of her 30-year career, Sarina has used only humanely acquired animals. She informed me that she is always seeking ethically-sourced chickens. She uses a lot of chickens in her art, and would gladly “accept donations of deceased animals, but also will pay people for fancier more exotic ones.” To find out more about Sarina, her art, and Rogue Taxidermy, you can visit her website here.