Green’s Dictionary of Slang

(Aus.) a kookaburra or laughing jackass.

1879		‘Old Hand’ Experiences of a Colonist 24: The first sound heard in the wood was the peculiar note of the laughing jackass (dacelo gigantea), a bird so celebrated in New South Wales for his particular virtue of punctuality, that the colonists gave it the name of the ‘shepherd’s clock’ [AND].
1941		Baker Popular Dict. Aus. Sl.
1955		N. Pulliam I Travelled a Lonely Land 381: Laughing jackass – one nickname for the kookaburra, the lovable mischievous and favorite bird of Australia. Other names are ... alarm bird, bushman’s clock, clock-bird, settler’s clock or shepherd’s clock [etc.] [AND].