As this thread has been thus revived, it might not be out of place to revert to the interesting "Nicky-Tams' drift above —

to point out that one of their functions at least, in the eponymous song, appears indeed to be to prevent the incursion of vermin. Those who know the song will recall that, in the last two stanzas, the narrator is unwillingly dissuaded by his girlfriend from wearing his nicky-tams to church as being inappropriate for the Sabbath ~~

"And a rat crawled up my trouser-leg in the middle o' the Psalm ~
Heuch! Never again will I rig the kirk wi'oot my nicky-tams!"

~Michael~