We assumed this bird was standing on its own nest until we saw it eat a whitish downy chick from out of the nest. We speculated that perhaps this was an unguarded active nest belonging to somebody else. This species is known to be a nest robber. Another possibility is that the youngest chick was either close to death or already dead when the adult swallowed it. Secretary-birds are reportedly facultatively fratricidal with the youngest of three chicks in the nest typically dying of starvation.
A more likely explanation was offered by Simon Thomsett is that this is a replacement adult. He documented similar behavior of a replacement adult towards a much larger chick at Soysambu Conservancy Kenya. There he photographed a Secretary-bird trying to kill a single large chick in the nest on 28 Feb 2012. "Only after observing the nest for a few days and noticing only one adult attended did we see odd behaviour of the interloper. I then checked a nearby watertank to see the dead adult....The chick survived only because we rescued it. The new adult was persistent in its effort to kill the chick. The single parent was never in a position to defend the chick. I never found out the sex of the adults."
The species was upgraded to "vulnerable" by the IUCN in 2011 citing sudden decline across its range, but especially in Tanzania and South Africa. This odd-looking long-legged raptor is placed in its own family, the Sagittariidae. They have the longest legs of any raptor. The spelling of the English name varies depending on the authority. African Bird Club spells it with two words "Secretary Bird" while IOC compresses it into the one word "Secretarybird." Clements (which I prefer) hyphenates its name.
Nikon P510 Coolpix Point-and-shoot