I thought of Keats. Was he the first to talk about swallows twittering? Probably not, but then again he was a magician with words and I wouldn't put it past him. Time and again he arranged them in combinations that hadn't been tried before but were absolutely right.
Charmed magic casements opening on the foam
Of perilous seas in fairylands forlorn.
Kipling once wrote that these two lines from the Ode to a Nightingale- and a passage from Kubla Khan- were the "true magic", compared to which every thing else was "just poetry". I'm not sure about the passage from Kubla Khan- which I suspect of straining just a little too hard- but I'm with him absolutely on the Keats. Every word in that progression is inevitable; change a single one and you kill the effect. Keats makes it seem easy- and perhaps it was- perhaps it just "came" to him- the way the tune for "Yesterday" came to Paul McCartney- without any conscious exercise of craft. These things do happen.
Back to the swallows. They were amazing. Once in a while they'd cant to one side and there'd be a flash as the low-lying sun caught the white of their breasts and underwings.