(n.) a small brown songbird, with dun-coloured plumage
Words like dunnock, hillock and bullock all have a common etymological ancestor in the shape of an Old English suffix, –oc, that was used to form diminutive words.
So a dunnock is a small dun-coloured bird. A hillock is a small hill. And although the word tends to be used more generally today, a bullock was originally a young bull.
Other familiar words bearing this suffix include tussock (which derives from tusk, an old word for a tuft of hair or grass) and even buttock (which is literally a little butt, in the sense of the dulled end of something).
At the less familiar end of the scale, you’ll find the likes of rillock, meaning a small rill or woodland stream. A lassock is a little lass, and a bittock is a little bit. A wretchock is a little wretch—or more specifically, the small creature in a brood or litter. And when not being used to refer to an enclosed field for livestock, a paddock is a frog—or quite literally, a little ‘pad’ or toad.