Before migrating, blue whales switch up the timing of their songs

D uring the summer feeding season in high latitudes, male blue whales tend to sing at night. But shortly before migrating south to their breeding grounds, the whales switch up the timing and sing during the day, new research suggests. This is not the first time that scientists have observed whales singing at a particular time of day. But the finding appears to be the first instance of changes in these daily singing patterns throughout the yearly feeding and mating cycle, says William Oestreich, a biological oceanographer at Stanford University.
In the North Pacific, blue whales (Balaenoptera musculus) spend summers off North America’s coast gorging on krill before traveling to the tropics to breed in winter. Data collected by an underwater microphone dropped into Monterey Bay in California to record the region’s soundscape for five years allowed Oestreich and his colleagues to eavesdrop on whales that visited the bay. When the team separated daytime and nighttime whale songs, it stumbled upon a surprising pattern: In the summer and early fall, most songs occurred at night, but as winter breeding season approached, singing switched mostly to the daytime. “This was a very striking signal to observe in such an enormous dataset,” says Oestreich. The instrument has been collecting audio since July 2015, relaying nearly 2 terabytes of data back to shore every month.
The researchers also tagged 15 blue whales with instruments and from 2017 to 2019, recorded the whales’ movements, diving and feeding behavior, as well as their singing — nearly 4,000 songs’ worth. Whales that were feeding and hadn’t yet started migrating to the breeding grounds sang primarily at night — crooning about 10 songs per hour on average at night compared with three songs per hour in the day, or roughly three times as often. But those that had begun their southward trip sang mostly in the day, with the day-night proportions roughly reversed, the team reports October 1 in Current Biology. Predicting whale movements via their songs may be particularly useful for this blue whale population, which is commonly struck by shipping vessels, Oestreich says. Advance notice of a regional influx of whales could help keep the endangered animals safe and singing far into the future.