Kookaburras

With great difficulty I would think, John, especially if your kookaburras are laughing kookaburras rather than blue-winged kookaburras.

According to my early edition of Graham Pizzey's Guide to the Birds of Australia male laughing kookaburras often have "some blue-green in the centre of rump, in females much-reduced or absent." Immature birds have "shorter dark bill, base of tail tinged blue, fine brownish margins to white feathers." So careful observation would be needed to distinguish one from the other.

Female blue-winged kookaburras have red or red-brown tails, males blue. Females are a stronger buff colour on the front than males. Immature birds have lighter streaking on the head than adults.

I hope this helps.

P.S. A bird field guide is a very useful addition to your library if you're interested in birds.