Thornbills are small, around 10 cm. I find the most common species in the area and my birdbath is the Brown Thornbill. They have a brown/rufous forehead and rump, with dark streaking on throat and breast. They have several calls including a buzzy noise (like a rasp on wood), various three-note calls and something I call a ‘squirt’.
Brown Thornbill
Striated Thornbills also like birdbaths and have dark streaking on throat and breast. The crucial difference is white streaking on the rufous forehead. They are almost always in groups of four plus while Brown Thornbills are generally in ones or twos. It’s tricky for ID but you can get one or two Brown Thornbills amongst a group of Striated Thornbills. I have only heard one call from Striated Thornbills: a continuously repeated and high-pitched zit noise. No Brown Thornbill call is similar.
Striated Thornbill
Good luck deciding which thornbills are in your birdbath and hopefully the photos will help.
James Orton