Leghorn roosters and hens diverge in nearly every measurable trait, from comb size and feather shape to bone composition, muscle force, immune response, and vocal behavior. The rooster is larger, louder, and built for competition. The hen’s body is engineered around egg production, including a specialized bone type that males never develop. What makes these differences especially interesting is the underlying genetic system: chickens use a sex-determination mechanism distinct from mammals, and some of the sexual differences in their brains appear before the gonads even form.
Physical Differences You Can See
White Leghorn roosters and hens are both covered in white plumage, so color alone will not help you tell them apart. The differences show up in body size, feather shape, and head ornamentation. Adult roosters typically weigh around 2.5 to 3 kg, while hens come in closer to 1.7 to 2.2 kg. The rooster’s comb is noticeably larger, upright, and deeply serrated, with long, pendulous wattles. Hens have a smaller comb that often flops to one side after it grows out, and their wattles are modest by comparison.
Feather structure tells the sexes apart even when the color is identical. Roosters grow long, pointed hackle feathers on the neck and saddle feathers draping over the back near the tail. Their main tail feathers include curved sickle feathers that arch dramatically. Hens have rounder, shorter feathers throughout, with no sickle feathers and no pronounced hackle. Roosters also develop spurs on the backs of their legs, bony projections that grow longer with age and serve as weapons. Hens may show small spur buds, but they rarely develop into anything substantial.
Telling Chicks Apart at Hatch
Because White Leghorn chicks all hatch as fluffy yellow balls regardless of sex, sorting them early is a genuine challenge. The traditional method is vent sexing, where a trained technician examines the cloaca of a day-old chick and identifies subtle differences in the genital structures. This requires significant skill and is stressful for the chicks, but it remains the fastest option in commercial settings.
Researchers have explored genetic tricks to make sexing easier. One approach uses the sex-linked barring gene, which is carried on the Z chromosome and produces a distinctive white head spot in chicks that inherit it. A recent study isolated this gene from White Leghorn chickens and crossed barred hens with black-feathered roosters. The idea is that male chicks (ZZ) inherit the barring gene and display the spot, while female chicks (ZW) do not. Accuracy reached about 90%, and when the researchers excluded chicks whose mottled feather patterns masked the barring, it climbed to roughly 94%. They found that focusing on the brightest white spot along the midline of the head improved identification further.1PubMed Central. Isolation and fixation of the sex-linked barring feather gene from white leghorn for chick sexing This kind of approach is most practical in crossbreeding programs, because purebred White Leghorns lack the color contrast needed to see barring in day-old down.
Crowing and Other Behavioral Differences
Crowing is the most unmistakable behavioral marker. In domestic fowl, crowing is a male-specific vocal behavior; hens do not crow under normal circumstances. Research has shown that this sex difference has roots in embryonic development: estrogen during a specific window of incubation organizes the neural circuits that control crowing. When researchers blocked estrogen production in developing female embryos using an aromatase inhibitor, roughly half of the treated hens eventually produced crow-like calls as adults, though these calls were shorter and carried less acoustic energy than a rooster’s full crow.2PubMed. Effects of estrogens during embryonal development on crowing in the domestic fowl The frequency characteristics stayed similar, suggesting the vocal apparatus itself is not radically different between the sexes. The brain’s wiring, shaped by early hormone exposure, determines whether the bird crows.
Social structure also differs between the sexes. Roosters and hens form separate dominance hierarchies maintained through pecking order and perch location. Males dominate all hens, but this dominance is so stable that overt aggression from a rooster toward a hen is uncommon in natural-like conditions. Once the hierarchy settles in, overt fighting gives way to threat displays and submissive postures.3Brazilian Journal of Poultry Science. Aggressive behavior in the genus Gallus sp
When faced with a predator threat, male and female Leghorns also respond differently from each other and from their wild ancestor, the red junglefowl. In a study that simulated a predator attack, Leghorn chickens tended to freeze in a standing-alert posture, while junglefowl were more likely to walk alertly and vocalize alarm calls.4Applied Animal Behaviour Science. Domestication effects on foraging strategy, social behaviour and different fear responses: a comparison between the red junglefowl (Gallus gallus) and a modern layer strain Domestication has apparently damped down the active-escape instincts in Leghorns broadly, but within the breed, roosters still show more vigilance and alarm-calling behavior than hens, consistent with the ancestral pattern where the male acts as a sentinel while the flock forages.
Growth, Muscle, and Bone
Male Leghorn chicks grow faster than females from the first week of life. This was documented as far back as the 1930s, when researchers separated day-old White Leghorn pullets and cockerels to study their growth rates and feed efficiency independently.5Poultry Science. Sexual Differences in Growth and Utilization of Feed in White Leghorn Chicks The gap in body weight widens with age and is one reason nutritional experiments historically struggled with accuracy when researchers mixed both sexes in the same pen without accounting for the difference.
By adulthood, the muscle architecture of males and females diverges substantially. Mature Leghorn roosters have greater physiological cross-sectional areas in their pelvic limb muscles compared to hens, meaning their legs can generate more force. Their pennate muscles also tend to have longer fascicles, allowing greater range of contraction. These structural advantages translate directly into performance: roosters achieve higher maximum sustainable running speeds than hens. Interestingly, immature Leghorns of both sexes do not show these same dimorphisms, indicating the differences emerge during sexual maturation rather than being present from the start.6Journal of Anatomy. Variety, sex and ontogenetic differences in the pelvic limb muscle architectural properties of leghorn chickens (Gallus gallus domesticus) and their links with locomotor performance
The most striking skeletal difference is medullary bone, a specialized bone tissue that exists only in sexually mature females. As a hen approaches egg-laying age, her body begins remodeling the interior of long bones like the femur, depositing a calcium-rich matrix that serves as a mineral reservoir for eggshell formation. In Lohmann Selected Leghorn hens, researchers tracked this process week by week and found that markers of bone matrix breakdown began rising around week 17 of age, estrogen levels climbed around week 20, and measurable increases in medullary bone calcium appeared by week 23.7Poultry Science. Transcriptional and endocrine orchestration of medullary bone formation and mineral turn-over in female chickens Roosters never form medullary bone under normal conditions because they lack the sustained estrogen levels that trigger it. This is one of the few truly binary physiological differences between the sexes.
How Chicken Sex Is Determined
Chickens use a ZW sex-determination system, the reverse of the mammalian XX/XY pattern. Males carry two Z chromosomes (ZZ), and females carry one Z and one W (ZW). The key gene on the Z chromosome is called DMRT1, and sex appears to depend on how many copies of it are active. Males, with two Z chromosomes, get a double dose of DMRT1, which drives testis development. Females, with only one Z, get a single dose, and their gonads develop as ovaries.
This was demonstrated directly in a landmark study where researchers disrupted one copy of DMRT1 in chromosomally male (ZZ) chicken embryos. The resulting birds developed ovaries in place of testes, confirming that the avian sex-determining switch is based on DMRT1 dosage.8PubMed Central. Primary sex determination in birds depends on DMRT1 dosage, but gonadal sex does not determine adult secondary sex characteristics A separate study found that disrupting DMRT1 in early male embryos induced gonad feminization with substantial physiological and molecular changes, though the feminized gonads could not achieve functional female reproduction because hormone synthesis was disturbed.9PubMed Central. DMRT1 gene disruption alone induces incomplete gonad feminization in chicken
One of the more surprising findings from this line of research is that some sex differences in the chicken brain emerge before the gonads have even differentiated. Gene expression studies on chicken embryos at very early developmental stages found over 200 genes that were already expressed differently between male and female brains, long before any gonadal hormones could be influencing them.10PubMed. Sexually dimorphic gene expression in the chick brain before gonadal differentiation This suggests that in birds, the brain is not simply a passive target of gonadal hormones the way older models assumed. The Z and W chromosomes themselves appear to drive some neural sex differences directly, independent of whether the bird develops testes or ovaries.
Immune System Differences
Males and females do not respond identically to immune challenges, and this has practical implications for poultry management and research. A study exposing White Leghorn embryos to a single low dose of lead found clear sex-based differences in immune response. Male chicks showed significantly increased antibody production at the highest exposure level compared to controls, while female antibody levels remained unchanged. Males that were normally unable to produce certain autoantibodies were induced to produce them after lead exposure, while females were unaffected. Total white blood cell counts also varied by treatment group in males but not in females.11PubMed. Gender differences in developmental immunotoxicity to lead in the chicken: analysis following a single early low-level exposure in ovo
The takeaway from this work is that the male immune system appears to be more reactive to at least some environmental insults during development, while the female system is more buffered. For backyard flock owners, this is mostly a curiosity. For researchers using Leghorns as model animals in toxicology or immunology, failing to account for the sex of the birds could easily skew results.
Heat stress, on the other hand, appears to affect both sexes equally. When young White Leghorn chickens were exposed to acute heat and their body temperature response was measured, sex did not influence the outcome, and neither did supplementation with ascorbic acid.12Poultry Science. Effect of ascorbic acid and acute heat exposure on heat shock protein 70 expression by young white Leghorn chickens So while males and females differ immunologically, their basic thermoregulatory machinery handles heat in the same way.
Why the Egg Industry Has a Male Problem
The White Leghorn hen is one of the most productive egg-laying animals ever developed. Decades of genetic selection have pushed commercial Leghorn strains to lay upward of 300 eggs per year, with improvements in peak production and laying persistence across generations.13PubMed. Egg production curve fitting using nonlinear models for selected and nonselected lines of White Leghorn hens Modern commercial lines produce more egg mass per day and maintain higher hen-day production rates than unselected control strains kept in the same conditions.14PubMed. The effects of genetic selection on production parameters of single comb White Leghorn hens
This extreme specialization for laying creates an uncomfortable commercial reality: male Leghorn chicks cannot lay eggs, and they are not economically viable for meat production either. Leghorn males are small, lean, and slow-growing compared to broiler breeds. Research comparing breast meat across genetic groups found that inbred Leghorn birds had higher protein and lower fat content in their breast meat than commercial broilers, and their meat was a more intense red color. But the sheer lack of body mass makes raising Leghorn cockerels for meat commercially uncompetitive.15Poultry Science. Breast meat quality and composition in unique chicken populations
As a result, billions of male layer chicks worldwide are culled at the hatchery within their first day of life, a practice that raises significant animal welfare and ethical concerns.16PubMed. Current approaches to avoid the culling of day-old male chicks in the layer industry, with special reference to spectroscopic methods Three broad alternatives are being pursued: in-ovo sex determination (identifying sex inside the egg before hatching so male eggs never develop into chicks), dual-purpose breeds that lay reasonably well while also producing enough meat to make rearing males worthwhile, and raising male layer chicks for niche meat markets.17PubMed Central. The Need for an Alternative to Culling Day-Old Male Layer Chicks: A Survey on Awareness, Alternatives, and the Willingness to Pay for Alternatives in a Selected Population of Dutch Citizens
In-Ovo Sexing and the Push to Identify Sex Before Hatch
Of the three alternatives to male chick culling, in-ovo sexing has received the most research attention and commercial investment. The goal is to determine the sex of the embryo early enough in incubation that male eggs can be diverted before the embryo develops pain sensitivity, which current evidence places at some point after day seven or eight of incubation.
One recent study tested a low-cost approach using smartphone imaging and machine learning on White Leghorn eggs at day 10 of incubation. Researchers photographed 143 fertilized eggs using a simple light-box setup, then opened them at day 20 to verify sex through dissection of the chicks’ reproductive organs.18PubMed Central. Morphology-Based In-Ovo Sexing of Chick Embryos Utilizing a Low-Cost Imaging Apparatus and Machine Learning The appeal of imaging-based methods is that they are non-invasive and could theoretically scale to commercial hatcheries without expensive laboratory equipment. Other approaches under development include spectroscopic methods that detect hormonal or genetic markers through the eggshell, and genetic tests performed on fluid sampled through a tiny hole in the shell.
Several European countries, including Germany and France, have already banned or restricted the culling of day-old male chicks, accelerating adoption of these technologies. The industry is moving fastest toward spectroscopic systems that can sort eggs on commercial lines, though accuracy, speed, and cost remain active challenges. For Leghorn-based production specifically, the stakes are high because every hatch of a commercial Leghorn flock produces roughly equal numbers of males and females, and the males currently have no commercial destination.
Leghorn Meat Quality in Males
If the culling problem has a partial solution in rearing male layer chicks for meat, the question becomes whether anyone wants to eat them. Leghorn cockerels grow slowly and carry little breast meat compared to a modern broiler. But the meat they do produce has distinct qualities. Leghorn breast meat has a higher percentage of protein and a lower percentage of fat than broiler breast meat, and it registers lower shear force values, meaning it can actually be more tender.19Poultry Science. Breast meat quality and composition in unique chicken populations
In some European markets, niche producers are already selling male layer chicken as a premium product, marketed on welfare grounds and the appeal of leaner, more flavorful meat. It is a small market, and it will likely stay that way unless consumer willingness to pay a significant premium grows. Research in the Netherlands found that surveyed citizens were broadly willing to pay more for eggs and chicken produced without male chick culling, but the actual premium they would accept varied widely, and awareness of the issue among the general public was uneven.20PubMed Central. The Need for an Alternative to Culling Day-Old Male Layer Chicks: A Survey on Awareness, Alternatives, and the Willingness to Pay for Alternatives in a Selected Population of Dutch Citizens The economics of rearing Leghorn males remain challenging: they take longer to reach market weight, consume more feed per kilogram of meat gained, and yield a carcass that does not fit the profile most processors and retailers are set up for.
Egg Production Genetics and Why Hens Keep Getting Better
The performance gap between Leghorn hens and roosters is not just anatomical; it is the product of decades of intense artificial selection applied almost exclusively to the female side of productivity. Researchers studying a White Leghorn population selected for feed efficiency found that the heritability of egg production varied across a hen’s laying cycle. It was highest during the first month and the final months of lay, and lowest during peak production, when nearly all hens are performing near their ceiling anyway.21Journal of Animal Breeding and Genetics. Genetic and phenotypic parameters for monthly egg production in White Leghorn hens Cumulative production over the full laying cycle showed higher heritability than any single month, meaning breeders get the most genetic progress by selecting hens based on total lifetime output rather than short-term peaks.
This selection pressure has reshaped the Leghorn hen’s physiology in ways that have no male counterpart. The medullary bone system described earlier, the massive oviduct that can weigh over 60 grams in a laying hen, the finely tuned calcium metabolism that shuttles minerals from bone to eggshell on a roughly 25-hour cycle: all of this exists because generations of breeding have favored hens that could sustain relentless production. The rooster’s contribution to this system is entirely genetic. He passes on the genes for laying performance to his daughters, but he never expresses them himself. This is why the genetic evaluation of roosters in a layer-breeding program depends entirely on the performance records of their female relatives.