...What a cheerful contrast is Bob White's clear, staccato whistle to thedrawing coo of the amorous dove! Character is often expressed in abird's voice as well as in ours... 
Neltje Blanchan 「Birds Every Child Should Know」
...A soft,crooning coo...
Frank M. Chapman 「Color Key to North American Birds」
...The natural voice ofthe pigeon is a soft, gurgling coo repeated over and over withmonotonous effect...
John H. Robinson 「Our Domestic Birds」
...The note of this dove is exceedinglyplaintive, and is softer, but much resembles the coo of the turtle-dove...
Charles Sturt 「Expedition into Central Australia」
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