The British government called its research on a worst-case scenario in the event of a no-deal Brexit Operation Yellowhammer:
Ministers have published details of their Yellowhammer contingency plan, after MPs voted to force its release.
It outlines a series of "reasonable worst case assumptions" for the impact of a no-deal Brexit on 31 October.
The OED defines yellowhammer as:
a. A large bunting having (esp. in the male) a bright yellow head, throat, and underparts, Emberiza citrinella (family Emberizidae), native to Europe and Asia and introduced to New Zealand, Australia, and South Africa.Also called yellow bunting, yorling, yowlring.
In the United States, the state of Alabama is known as the Yellowhammer State.
There’s a book, of the genre chiller, called The Yellowhammer’s Cradle, by Sally Spedding, published in 2016, which turns up on Google’s NGram with the following tagline:
According to ancient folklore in Scotland and northern England, the yellowhammer bird is said to drink a drop of the Devil’s blood every May Day morning . . .
Was the choice of "Operation Yellowhammer" arbitrary, or is there an origin for this use that explains it? Are there previous uses, such as "yellowhammer paper," related to its use as a worst-case scenario?
The whole purpose of a code name is (or should be) to conceal the nature of the thing referred to. British government code names are invariably arbitrary, and usually taken from pre-prepared lists. A number of British Cold War weapon projects had names consisting of a colour and another word. Blue Steel was a missile, Green Grass was a radar system. These are now informally called the “rainbow codes”. Police and government civil-affairs operations have names like Yewtree, Crevice, Barkertown, Zoomania and Bagel. The names come from an approved list that has been decided in advance. They can be anything from exotic birds to towns on the south coast.
“You pick one off the list,” says Bob Cox, the recently retired head
of press with the Metropolitan Police.