Firstly, I like the idea of a ‘snow’ bird being the colour of slate. And being obsessed with colours and the names we give them, I notice how many hues and shades can only be described in reference to something of that colour. Elsewhere Thoreau describes slate as the color of water.
Perhaps we could set up an International Commission for the Naming of Colours, giving thousands of nuances their own names.
‘Ditches and pools are fast skimming over, and a few slate-colored snowbirds, with thick, shuffling twitter, and fine-chipping tree sparrows flit from bush to bush.’ 25 Nov 1857
Source: Henry David Thoreau, The Journal 1837-1861, Damion Searls (ed.), preface by John R. Stilgoe (New York: New York Review Books, 2009), p. 480