Coal miners used to carry a canary down into the pit, to act as early warning: if methane started seeping in, the canary would stop singing before the level of human hazard was reached.
The current controversy about chlorinated chickens has been dismissed by Liam Fox as a media distraction. In reality, it is the modern equivalent of the canary as we enter into the dark unknown territory of post-Brexit trade negotiations.
The issue, superficially, is just about whether we should accept US chickens that don't meet EU standards as part of a post-Brexit trade deal. We can debate the desirability of accepting the health hazards of factory farming and trying to fix them with chlorine. But the first fundamental point is this: if we open the British market to food that Europe considers sub-standard, we automatically close the European market to the same British products. Otherwise, Britain would become a convenient channel for US exporters to funnel their otherwise banned products into Europe.
This creates a double whammy for British farmers, simultaneously undercut on their home market by US imports and cut off from their main export market. In theory, Britain could negotiate a new trade deal with Europe creating an inspection and certification system to distinguish chicken produced within or outside the European standard. Good luck with that: it would take years to negotiate and set up and cost a fortune to operate.
This has implications far beyond the chicken industry and the similar issues relating to hormone-fed beef, and meat and milk from clones and GM animals. Indeed, when politicians talk blithely about Britain negotiating new trade deals, any change in any industry which means that products not accepted in Europe can enter Britain will introduce a "hard" Brexit for those products. And if there are multiple products of this kind, then we're going to see strict border controls for goods in general.
The second fundamental issue is what we want to get out of Brexit. We voted collectively to "take back control". But control to do what? It is possible to imagine a bargain-basement Britain, in which we welcome any goods with minimal quality controls so long as they're cheap: health, the environment and animal welfare would be less important than getting 10% off. Would that feel like taking back control? Probably not.
Or we could, as Michael Gove has argued in his ground-breaking recent speeches, use Brexit to make Britain take a lead in welfare and environmental standards. People would want to buy British because we had high standards. And having higher standards than other countries is no obstacle to exports. Germany did it after the war with their industrial products: nobody ever bought a BMW because they thought it would be cheaper than a Skoda, but because they thought it would be better.
Compassion in World Farming is against drenching chickens in chlorine because it masks rather than addresses the problems of factory farming. But even as individual citizens we should surely all want trade solutions that don't devastate domestic producers and which make Britain lead the world in standards.
Chlorinated chickens are the first of many such issues to come. As the insidious calls to weaken our standards emerge, we need to hear the canary faltering. Otherwise, Britain faces not just a hard Brexit but a dark Brexit which few - Leavers or Remainers - would want.