(Cisticola exilis),
“The golden-headed cisticola, also known as the bright-capped cisticola, is a species of warbler in the family Cisticolidae, found in Australia and thirteen Asian countries.” – Wikipedia
“The Golden-headed Cisticola is a very small insectivorous bird growing to 9–11.5 cm (3.5–4.5 in) long, wingspan 15 cm.”
They really are small, even with a 600mm lens you need to be very close to get good detail in a photo.
I recently photographed this bird which I believe is an immature male – “Females resemble non-breeding males”