I hadn't seen it before but apparently they're known for it. From Birdlife Australia: "To safeguard their eggs and nestlings, Diamond Firetails are often recorded building their nests into the base of the large stick-nest of a bird of prey such as a Whistling Kite, White-bellied Sea-Eagle, Wedge-tailed Eagle, Brown Falcon, Nankeen Kestrel or a Square-tailed Kite. One nest of a Whistling Kite contained nine Diamond Firetail nests!"