Blue whales start singing in the daytime rather than at night just before migrating, study finds 

US experts recorded both individual whales and their greater populations in the Northeast Pacific, using underwater microphones called 'hydrophones'. https://www.dailymail.co.uk/sciencetech/article-8794477/Blue-whales-start-singing-daytime-night-just-migrating-study-finds.html?ns_mchannel=rss&ns_campaign=1490&ito=1490