Count your chickens: a new ageing technique for domesticated fowl

The team from the University of Exeter applied their formula to 1,366 domestic fowl skeletons found across Britain, ranging in date from the Iron Age through to the modern day.

For most animals, including humans, osteoarchaeologists are able to determine age at death through a series of techniques, such as analysing bone fusion and tooth-eruption and -wear. For birds, however, ageing their skeletons can be problematic, as they do not have teeth and their skeletons have a low number of fusion points compared with mammals. These characteristics have historically hindered our ability to conduct robust zooarchaeological analyses on birds, prohibiting us from identifying husbandry and hunting practices in the past, as well as from assessing general human–bird interactions. In this month’s ‘Science Notes’, we explore a significant advance in this area: a new technique, developed by a group led by researchers from the University of Exeter, for ageing domestic fowl.

An Iron Age cockerel from Houghton Down, Hampshire, recently radiocarbon dated to the 4th-3rd century BC. Analysis of the spurs suggests the bird lived to at least two years old. Image: J Best and G Clark

As traditional ageing methods cannot be used on birds, the research team had to look at other characteristics more specific to avian species. They turned to the bony spur found on the lower legs of many male fowl (and sometimes on females as well), as it has a known development: starting as just a keratin sheath at birth, it begins to ossify around seven months, and then fuses to the leg bone around nine months. The spur continues to increase in length and breadth as the bird ages. To test whether this growth is predictable enough to be used to accurately age birds, the team assessed the leg and foot bones of 71 modern domestic fowl and red junglefowl of known age and sex from multiple UK and Ethiopian collections.

By taking standardised measurements of the leg bones and spur, it was determined that the length of the spur alone was not enough to determine age accurately, due to a significant degree of variability between species. They did find, however, that the rate of spur growth in relation to that of the leg bones provides a more accurate assessment, and they were able to create a simple equation to account for this difference (spur length/greatest length of the tarsometatarsal × 100). The researchers determined that this equation was able to be used for all species, no matter their size, and they also found that it worked for the small number of females who developed a spur as well.

The next step for the team was to determine if this technique could be applied archaeologically. To do this, they applied their formula to 1,366 domestic fowl skeletons found across Britain, ranging in date from the Iron Age through to the modern day. They found that fowl from the Iron Age were relatively rare and that the few archaeological specimens that have been identified do not display any signs of butchery, having mostly been recovered from individual burials rather than amid food waste. Previously, it has been suggested that this was because fowl were more highly prized during this time, either as a sacred animal, used for leisure activities such as cockfighting, or as a display of status. The new age-estimate results support this theory, showing that most of the skeletons from this period had lived for at least a year before they died, with many having lived at least two to three years.

This trend continued into the Romano-British period. Domesticated fowl, especially cockerels, appear to have become more prevalent during this time, and continued to live for at least two, three, or sometimes even four years, based on the team’s estimates. In particular, there appear to have been many more male fowl represented among the archaeological collections than females, possibly suggesting that the birds were being used for cockfighting. This idea is supported by the discovery of artificial cockspurs from several Roman settlements, including at the legionary fortress at Exeter.

This changed during the Anglo-Saxon period, though, when cockerels only accounted for roughly a quarter of the analysed skeletons. This shift has previously been used to suggest that poultry and egg consumption increased during this period, perhaps in response to the rise of Christianity and its religious fasting practices. Still, the cockerels that were identified from this period continued to live to advanced ages, perhaps indicating that the practice of cockfighting continued.

During the medieval period, the ratio of male-to-female fowl remained about the same, but their age of death decreased significantly, suggesting a sharp increase in poultry consumption. This trend largely continued into the modern period, although the number of cockerels again increased during this time, which the team suggests is in line with the widespread popularity of cockfighting during the 19th century.

Overall, the team was successful in creating a technique for ageing fowl archaeologically, which it is hoped can be employed in future research and revolutionise our understanding of human–bird relationships over time.