This bird is believed to be a second cycle bird based partly on less pointed, more truncate primary tips with a trace of "string-of-pearls" pattern and light iris visible here. Note heavily mottled head and chest and some dark matle feathers emerging. This is not a plumage I am familiar with, but expert opinion favors Slaty-backed.
Click here for more discussion and details and an additional photo showing the spread underwing.
Digiscoped with Panasonic DMC-LZ5 | Nikon FieldScope III | 30XWA | hand-held (no adapter)