The heat radiating from up to 70,000 birds in tiers of cages up to eight stories high, all in a single shed, is tremendous. Which is just as well, because when they have spent some time in the cages they will have lost most of their feathers from stress and being pecked by other chickens and scraping against the wire.
To eat, the battery hens have to peck through the cage at food running past on a conveyer belt. Each hen produces about six eggs a week. Each one rolls down a sloping ramp on to another conveyer belt as soon as the bird produces it.
After a year in the cage they have reached the end of their working lives, and they have one more use. The scrawny, near-naked birds are pulled out of their cages and stuffed into crates to be taken off for slaughter. The small quantities of meat on their bodies are turned into the sort of chicken you don't ever get to see in large pieces. It is used in pies, soups, other processed products and pet food.
A recent study found that 30 per cent of the birds arriving at the slaughterhouse have freshly broken bones. Their meagre skeletons cannot take the punishment of being crammed and roughly handled, because battery chickens suffer from osteoporosis - brittle bone disease. This is the result of being squashed in their cages, unable to walk or stretch.
Yesterday Compassion in World Farming, a British pressure group campaigning to outlaw the battery cage, published a report which pulled together the scientific evidence on this disease. One Edinburgh University study found that every chicken was suffering osteoporosis after a year in confinement. Osteoporosis can cause paralysis and starvation; a third or more of the deaths inside cages are probably due to brittle bones, as ribs snap and puncture the heart.
The cages have, deservedly, become the chief symbol of the cruelty of factory farming in this country. There is no other system which seems so determined to treat a living creature as a machine. But the pathetic figure of the battery hen may be a thing of the past.
Tomorrow, the European Parliament will vote on an EC proposal to improve the battery cage. The result will give an important indication of the way the wind is blowing across the Continent on farm animal welfare. In Britain, about six million shoppers have switched to buying free-range or barn eggs over the past 20 years. With a Euro-election coming up this summer, British MEPs will be particularly sensitive to their voters' views on the subject. It is likely that any improvement to the cage will be only a prelude to a complete legislative ban across Europe. That may take a decade or more to accomplish, but consumer pressure is tearing down battery cages across much of the Continent.
The European Commission's proposal, published last spring, is to give each battery chicken 800 square centimetres of space, instead of the 450 square centimetres allowed by the current EU directive implemented a decade ago.
They would also get more head room, with the minimum height in the cage raised from 35 centimetres to 50. They will be given a pole on which to perch, allowing them to step up off the cage's wire-mesh floor. They will also get what is called a "claw-shortening device". This is a bit of sandpaper at the front wall of the cage on which they can scratch their claws, and keep them short.
Mark Watts, Labour MEP for East Kent, says these improvements don't go nearly far enough. He wants the cages completely banned across Europe by 2009, and has put down an amendment to that effect for the vote in Brussels.
``To be brutally honest, the odds are stacked against us because the battery cage lobby is very strong,'' he said yesterday. The likeliest outcome is that the European Parliament will come down in favour of the Commission's proposals, which do not come into effect until 2009. Even if they do vote to adopt the improvements, there is no guarantee that the Commission's plans will become law, for the proposal has to be passed by the farm ministers of each member state.
There will be the usual north-south spilt, with countries including Britain, the Netherlands, Germany and Austria pressing for big improvements to the battery cage, while Spain, France and Greece will want to hold back. In Britain, the National Farmers' Union has grave doubts about the proposal. It estimates that egg producers will have to invest pounds 500m in installing the new, larger battery cages, and that production costs would rise by a quarter.
Farmers are objecting to the high costs of converting to much larger battery cages, but animal welfare groups see this as a plus point: they think the cost is likely to make many egg producers abandon the battery system altogether and switch to free-range chickens.
The farmers of Britain and Europe argue that the battery cage should only be banned once the entire world has agreed to its demise - a clever move when one thinks of kittens in China kept in battery-size cages. They are hardly likely to get sentimental about chickens' rights.
But why shouldn't Britain act alone and ban the battery cages now? After all, we are renowned for loving animals more than our fellow human beings. We led the the way in banning the veal crate - 15 years before the rest of Europe will finally end it. The UK was also first to outlaw sow tethers and stalls - cruel, intensive production systems which confine pregnant pigs on very short lengths of chain or cram them into cages so tiny they cannot turn around.
The Labour Government says it will do all it can to ban battery cages across Europe, but it will not act alone. Battery hens are big money savers - alternative systems cost at least a third more for each egg produced. If Britain's farmers are forced to throw out their battery cages and raise their birds in free-range systems, the small trickle of imported eggs could turn into a flood of cheap product from mainland Europe.
The other way forward is for more consumers to buy only free-range eggs, or so-called barn eggs, in which the chickens never go outdoors but still have freedom to behave like chickens - to move, scratch, flap and perch. Marks & Spencer only sell free-range eggs, while every major supermarket chain posts at least a fifth of its sales as free-range or barn eggs.
Our compassion comes with a price tag attached, for the chicken-friendly eggs can cost as much as 30p per half dozen more than the cheapest battery variety. That's almost two-thirds extra - you have to care a lot to pay that much money.
Compassion in World Farming is pleased with the expanding market, but it condemns several supermarkets for selling eggs from battery-caged hens with cheerful labels describing them as "farm fresh" ; labels which make no reference to the fact that the farm in question is most definitely of the factory variety.
Overall, campaigners feel they are on a roll. A Europe-wide battery cage ban may indeed be thousands of days off but, when it comes, the outlawing of the most hated symbol of factory farming will be their greatest victory.