Hackle

[1] In the British Army and the armies of some Commonwealth countries, the hackle is worn by some infantry regiments, especially those designated as fusilier regiments and those with Scottish and Northern Irish origins.

The modern hackle has its origins in a much longer plume, originally referred to by its Scots name, heckle, which was commonly attached to the feather bonnet worn by Highland regiments (now usually only worn by drummers, pipers and bandsmen).

Alternatively, it may be a sympathetic gesture to a former Lowland regiment, the Cameronians (Scottish Rifles), who went into 'suspended animation' in 1968 (and later disbanded), who wore a black hackle in their rifle green dress Balmoral.

(It is, however, worthy of note that in the 19th century, all line regiments of the British Army used to designate their "light company" with a green hackle.

However, the Highland Band of the Royal Regiment of Scotland (Territorial Army) continues to wear the red hackle with the Tam o' Shanter.