Observations of these two species have been few and far between in recent times. I’ve crossed paths with a handful of Spotted Harriers in the past year, but nary a single Black Falcon in that time.
Both are ‘plains country’ raptors with distinctive silhouettes that show off some useful diagnostic features.
The Spotted Harrier has deeply fingered wing-tips, more pronounced than the Swamp Harrier, while the general shape of the wings is squarer and broader. While hunting low over open country the wings are up-swept and the tail appears wedge-shaped, whereas that of the Swamp Harrier is squarer.
Spotted Harrier, Moolort Plains, 8th April 2024
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The Black Falcon in flight is hard to mistake for its close relative, the ‘pedestrian’ Brown Falcon. While not as rapid as the Peregrine Falcon, it is fast and powerful, especially in level flight in pursuit of small to mid-sized birds. In silhouette the Black Falcon has a long tail, typically ending in a point – the third image below shows a classic soaring profile – fanned tail with sharply pointed wings. The bird pictured below was seen hunting over recently burnt stubble, fending off a pair of Nankeen Kestrels upset at its presence.
Black Falcon
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Natural Newstead 
Thanks for all your beautiful and often stunning photos! The Black Falcon IV photo reminds me of the Swamp Harrier II image from https://geoffpark.wordpress.com/2023/02/09/circus-tricks/ in which the bird seems to have assessed you as a threat and is intent to take you out.
Your full definition camera files are far more detailed than in the images in which they are rendered in this WordPress site. I right click the image to show it in a new browser tab, make the browser window full-screen on my 40″ 4k monitor and, if necessary, click or zoom to fill the screen.
Cell-phone or mobile browsers enable direct zooming into the smaller image.
My wife Tina and I are keen to visit the two dams near the north end of the South German Track, 3.1km east of Welshmans Reef which is a site you sometimes mention as the location of your photos.