After initially identifying this as juvenile Cinnamon Bittern I was this week corrected by Des Allen, who pointed out it was a classic female Schrenk's Bittern. Chocolate brown upperparts with large white spots, and creamy white underparts.
A female Cinnamon Bittern. Differing from the Schrenk's in having a buffier base colour to the throat, and the spots on the back edged buff.
Another Cinnamon Bittern, male
A male Cinnamon Bittern, looking magnificent