...THE MOON SHONE down out of a cloudless sky—a huge, swollen moon thatseemed so close to earth that one might wonder that she did not brushthe crooning tree tops... 
Edgar Rice Burroughs 「Jungle Tales of Tarzan」
...Its call is a crooning coo...
Frank M. Chapman 「What Bird is That?」
...When she settles downto brood them she calls them with a peculiar crooning note...
John H. Robinson 「Our Domestic Birds」
...The crooning gradually took the form of words...
John Charles Beecham 「The Argus Pheasant」
...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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