Blue Cranes Anthropoides paradiseus at Etosha Pan, Namibia: what is the origin of this isolated population?

There is an isolated breeding population of Blue Cranes Anthropoides paradiseus around Etosha Pan, in northern Namibia, despite a lack of regular reports of the species from adjoining regions of Botswana, southern Namibia or even north-western South Africa. A search for historical records of Blue Cranes north of South Africa suggests occasional vagrancy to southern Namibia, eastern Botswana and perhaps Zimbabwe, with consistent sightings of resident, breeding birds only from Etosha since 1918. It is apparently not a relict population. While the natural establishment of a breeding population by rare vagrants appears unlikely, there is no documented evidence for the alternative explanation that birds were deliberately introduced to this locality.