The turkey on nearly every festive table belongs to a breed so heavily engineered for a large breast that the birds can no longer physically mate, which means every commercial turkey alive is the product of artificial insemination, and the entire commercial breed now depends on human hands to reproduce at all.

The turkey you’re going to buy for your Christmas dinner this year is almost certainly a Broad Breasted White. It’s the breed that accounts for something like 99 per cent of the commercial turkey market in Europe, North America, and most of the rest of the world. It weighs somewhere between fifteen and forty pounds depending on age and sex at slaughter, has almost no dark meat by volume, has been genetically selected across roughly seventy years of continuous breeding for exactly the qualities that a supermarket carcass photograph wants, and cannot, physically, mate with another turkey. This last fact is not widely advertised in the marketing material.

How the modern turkey got so big

According to the Livestock Conservancy’s own definitional history of the heritage turkey, compiled by breeder Frank Reese of Good Shepherd Farm, Marjorie Bender of the Livestock Conservancy, Dr Scott Beyer of Kansas State University, and other collaborators, the modern commercial turkey is the end result of an intentional breeding programme that started in the 1920s and accelerated through the middle of the twentieth century. The initial goal was straightforward. Breeders wanted turkeys with more breast meat, because breast meat was what customers were willing to pay the most for. The traits selected included wider chests, deeper keel bones, shorter legs, and faster growth. Over successive generations the birds got progressively larger and progressively more heavily muscled across the front of the body. By the late 1930s, a Mrs. H. P. Griffin had coined the commercial term “Broad Breasted Bronze” to describe the result.

Why it can’t mate

The problem became apparent in the 1950s. As the breeders continued selecting for wider and heavier breasts, the anatomy of the birds shifted in a way that made physical reproduction increasingly difficult. The tom, or male, has to mount the hen from behind and above in order to successfully deliver semen. The commercial toms had by that point become so top-heavy across the chest, and so short in the legs and keel bone relative to their overall mass, that they couldn’t reliably manage the manoeuvre. They fell off. They couldn’t balance. When they did manage to stay on, the specific angles of their bodies made successful contact with the hens’ reproductive tract effectively impossible. Fertility rates in commercial flocks collapsed. The Livestock Conservancy timeline notes plainly that “broad-breasted conformation brings with it a shorter keel bone and shorter shanks in addition to the bulkier muscle mass on the breast” and that this configuration “prevents males from effectively mounting females.”

How the reproduction actually happens now

The commercial turkey industry’s response, worked out through the 1950s and standardised by the 1960s, was to bypass natural mating entirely. According to a 2011 explainer for VICE by science writer Sam McDougle, drawing on standard poultry-science literature, every commercial turkey now alive is the product of artificial insemination performed by human technicians. Semen is collected from the toms by trained handlers who physically stimulate the birds until they produce, in a process the industry euphemistically calls “milking.” The collected semen is then loaded into a syringe-like applicator, and each hen is inseminated individually, by hand, on a schedule that maintains her egg production across the laying season. The hens’ fertility is monitored. The offspring are hatched in incubators. None of the birds involved in the process have ever mated naturally, and none of them ever will.

The heritage remainder

What has kept the wider species intact through all of this is the survival of the older, non-commercial turkey breeds, collectively known now as heritage breeds. These are the Bronze, Bourbon Red, Narragansett, Royal Palm, Slate, Black, Beltsville Small White, and White Holland varieties, all of which trace their bloodlines back to the pre-Broad-Breasted era of the early twentieth century. All of them can and do mate naturally. All of them can and do produce fertile eggs without human intervention. The Livestock Conservancy’s formal definition of a “heritage turkey” requires exactly this: naturally mating pairs of grandparent and parent stock, expected fertility rates of 70 to 80 per cent, breeding hens productive for five to seven years, and a growth rate slow enough to reach market weight in about 28 weeks rather than the sixteen or eighteen of a commercial bird. In 1997, when the Livestock Conservancy first surveyed the surviving population, there were only 1,335