Given a category and a set of five words, find the word from the set that does not belong (i.e. is the least relevant) with the other words in the category. Words are separated by commas.

Category: bird 
Words: digger, heron, crane, mallet, sandpiper
digger

Category: corporate 
Words: leverage, solid, firm, amalgamated, concerted
solid

Category: ocean 
Words: current, flow, signal, dastards, sea

dastards