Mystery bird quiz - a confusing late summer warbler?

This was one of three very recently fledged birdlings begging for food at my campsite at White Lake Provincial Park yesterday, Aug 22, 2017. 'Quite surprising to see baby warblers at this late date - there's a frost warning tonight! Any guesses as to the ID of this smudgy specimen? Answer to follow.

...and the answer

Feeder watch, August (high water) edition.