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?