They make this noise even before hatching
doi.org/10.1016/j.cub.2008.04.011
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( , Sat 11 Jul 2020, 7:14, Share, Reply)
doi.org/10.1016/j.cub.2008.04.011
(scroll down for audio file)
( , Sat 11 Jul 2020, 7:14, Share, Reply)