In the UK, we typically associate spring and summer with bird breeding season. Birds find a mate and lay a clutch of eggs, which they incubate before hatching. Once the brood of chicks has been raised successfully and flown the nest, birds will either finish breeding and wait to begin again the following year, or they will go on to lay another clutch or sometimes two.
Most British birds follow this pattern except for pigeons and doves which can breed all year round, as long as there is a plentiful food supply and the weather is warm enough.
Domestic chickens also lay eggs all year round, but the vast majority of these eggs don’t hatch and produce chicks. Instead, they are collected for our consumption.
It takes a lot of energy to produce the next generation of birds, from developing the eggs inside the female’s body, to incubating them, and keeping the chicks safe and fed.
Therefore, it seems to go against what we know about evolution that chickens waste so much energy to lay eggs that will never result in a new generation of birds.
It’s widely accepted that the chicken is descended from the red junglefowl with domestication occurring about 8,000 years ago.
The red junglefowl is a tropical bird that inhabits most of Southeast Asia and parts of South Asia. Although similar in appearance to some species of modern chickens, it is smaller in weight, and the male has a shorter crowing sound than domestic roosters.
Red junglefowl inhabit coastal scrubs, forests, and mangroves, as well as cultivated areas where humans have planted crops such as tea and palm oil plantations. They are sociable birds and live in small flocks with a dominant male and several females and sometimes subordinate males.
Most historians and archaeologists agree that chickens weren’t originally domesticated for food. Instead, their dispersal around the world was due to their instinct to fight. In the wild, dominant male junglefowls are very protective of their territory and flock, and they use the spurs on their lower legs as weapons to see off any potential competitors.
Cockfighting was a popular sport in ancient Eastern societies before spreading to Greece, Rome, and the rest of Europe, including the UK where it remained a favourite pastime of all social classes until it was banned in England and Wales in 1835 and Scotland in 1895. Despite this, illegal cock fights continued to be held, and it wasn’t until 1952 that an act was passed that made it an offence to possess any instruments associated with the sport.
Ancient civilisations also used chickens for rituals and religious ceremonies, and they were sometimes included in burials and sacrifices to the gods.
The earliest evidence that chickens were used for food was found by archaeologists at Maresha in Israel. A large number of chicken bones were found at the site, mostly belonging to adult females dating back to the Hellenistic period which covers the fourth to the second centuries BC, thousands of years after chickens were first domesticated. Analysis of nearby sites showed a sharp increase in chicken bones over the same period further supporting the theory that chickens were not used as a food source until relatively recently. Evidence suggests that they were not domesticated for food in Europe until 100 years later.
You may have read that red junglefowl lay eggs all year round, producing as many as 300 eggs annually. However, this is not true.
In most areas, red junglefowl breed during the drier months of the year, which is usually winter or spring, although year-round breeding has been observed in some locations including Nepal and Malaysia.
Red junglefowl lay around 10 eggs in a clutch which are incubated for 21 days. Chicks take about 4 to 5 weeks to fledge and after another 7 weeks, they are chased out of the group by their mother and move to a new flock.
This means that even in areas where red junglefowl breed all year round they couldn’t possibly lay an egg every day as a good proportion of the time is taken up with producing and bringing up the chicks.
To understand why domestic chickens can lay eggs every day we need to understand how eggs are formed.
An egg starts out as a tiny cell held in the ovary, and its development is triggered by hormonal changes in the female’s body. As the cell increases in size it forms a yolk and passes from the ovary to the mouth of the oviduct. It is at this point that the egg is fertilised if the female has mated with a male. But even if it isn’t fertilised it continues to move through the oviduct developing the albumen, or egg white, and membranes. Finally, the shell is added, and the egg is laid.
The female will produce an egg like this every day until she has laid the entire clutch. Only then will she then begin the process of incubation so that all her eggs hatch at the same time.
In the wild, female birds tend to only produce eggs when there is a plentiful food supply and the days are light and long which is why spring and early summer are the optimal times for breeding. In most species, their ovaries shrink when breeding season is over to conserve the energy needed for moulting, migration, or survival in cold weather, although there are some exceptions as we have seen.
With male birds desperate to pass on their genes, it’s unlikely that a female wild bird would be in a position to lay many unfertilised eggs. But if you removed all the males in a population of birds, the females would still produce eggs and attempt to incubate them, unsuccessfully of course.
And if you remove an egg, fertilised or not, after she has laid it, she will continue to lay an egg daily until the clutch size is complete.
After chickens were domesticated, humans used selective breeding to develop traits that were valuable to humans, such as showy feathers, reduced, or in the case of cockfighting birds, increased, aggression, and better meat and egg production.
In 2017, a study by an international team of scientists found that the gene responsible for egg-laying, the thyroid-stimulating hormone receptor (TSHR), started to be selected from around 920 AD which coincides with the time when chicken consumption increased throughout Europe. Humans then wouldn’t have known they were specifically targeting the gene, but only chickens that were observed laying more eggs over a longer period would have been allowed to breed increasing the chances of their offspring being good layers too.
Although selective breeding of chickens has been around for thousands of years, until the First World War keeping poultry was very much a cottage industry. However, after the war, thousands of ex-servicemen with no jobs to return home to were encouraged to go into agriculture with poultry farming a popular choice as capital outlay was low.
In the UK, Utility Poultry Societies were formed to help market eggs and improve technologies, and chickens that laid more than 200 eggs over 48 weeks were awarded with a copper ring.
By the 1930s, over 60 million layers were being raised on UK farms, and although production dropped during the Second World War with grain diverted to feeding people rather than livestock, in the 1950s it accelerated once again with new techniques developed to increase egg production.
Red junglefowl often live near bamboo, most species of which flower very infrequently sometimes at intervals of several decades. When the plants have finished flowering vast quantities of seeds are produced and red junglefowl take advantage of this abundance of food to boost their reproductive success.
Poultry farmers used the same technique feeding large amounts of food to promote egg-laying. To further increase production, chickens were moved into barns and battery cages which meant less space was needed and light could be regulated.
Meanwhile, in the US, hybrid strains of layer were bred suited to living in cages, and using most of their nutrients to produce eggs instead of meat. Most commercial layers in the US are derived from white leghorns, a breed of chicken originating in Tuscany, that can lay an average of 280 white eggs a year. In the US, white eggs are commonly sold in supermarkets but in the UK, they are mostly brown due to the mistaken idea that took off in the 1970s that brown eggs are healthier than white. In the UK, commercial hybrids such as Lohman Browns, Goldlines, Hylines or Isas are used which are all Rhode Island Red crosses.
Hens are in their prime laying period between the ages of 20 weeks and 78 weeks after which egg production will slow down. Commercial chickens will often be slaughtered once egg production drops although there are schemes in place to rehouse these chickens in gardens and smallholdings.
In commercial flocks of chickens, the males are removed so that the eggs produced are always infertile. Occasionally mis-sexing occurs which is why it’s possible to hatch an egg bought from a supermarket, although this usually happens with eggs from species other than chickens. Eggs produced by hens kept on smallholdings or farms where there is a rooster present are more likely to be fertilised.
Of course, chickens don’t know whether the eggs they lay are fertilised or not, and although the tendency to brood has been bred out of commercial chickens, purer breeds may still attempt to brood due to instinct, even if her eggs are taken away.
