...All that dark earth heaved in mighty travail with the bursting bolls of the cotton while black attendant earth spirits swarmed above, sweating and crooning to its birth pains... 
W. E. B. Du Bois 「The Quest of the Silver Fleece」
...In the tall beech woods its low, monotonous crooning note was heard all day long from all sides...
W. H. Hudson 「Birds in Town and Village」
...The king-crows are feeding theiryoung ones in the greenwood tree, and crooning softly to thempitchu-wee...
Douglas Dewar 「A Bird Calendar for Northern India」
...We could hear the birds crooning and cooing in alldirections as we rode, “like the moan of doves in immemorial elms,and the murmurings of innumerable bees...
Ernest Giles 「Australia Twice Traversed, The Romance of Exploration」
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