Whistling kites (Haliastur sphenurus) are also common around Serendip Sanctuary and the nearby You Yangs. They tend to stick together but will often share thermals with black kites.
Slater & Slater describe them as "dingy-looking" but I'd take issue with that. In the right light, the head and breast feathers are as bright a shade of ... well, fawn ... as you will ever see. (Okay, one bird looks a bit dingy in these pictures.) Young birds also have a sprinkling of white spots on the wings, which give them a bit of a 70s look. They also give a whistling call that is simultaneously plaintive and melodic. What's not to like?
Whistling kites occur throughout mainland Australia, with occasional forays into Tasmania. Elsewhere, they are found in New Guinea and New Caledonia but are nowhere near as widespread as the black kites, which have taken over the eastern hemisphere, just about.