Sentence:  [W]hen the bare boughs of a tree intervened between her and the rising bright but deep red sun, frosted as the twigs were, on them sat a merry flock of sparrows, the feathers on their breasts <WSD>bluffed</WSD> out, as if they had donned warm winter spencers to shield them from the biting blast. 
Meaning of the word marked with <WSD> tags:  To fluff, puff or swell up.