Heading south? You’ve changed your tune

Changes in whale song may predict planned migration, a new US study suggests.

Researchers from Stanford University and the Monterey Bay Aquarium Research Institute (MBARI) have identified patterns in the trills and bellows of blue whales that seem to indicate when they are leaving North America’s west coast and heading south to breed.

“We found that during their feeding season, whales tended to eat during the day and sing at night, but after they had decided to begin their migration, they inverted this pattern and started to produce song primarily in the day…,” says Stanford’s William Oestreich, first author of a paper in Current Biology.

The team discovered this acoustic signature by accident while analysing data from the MBARI equipment that has been continuously recording the sounds of a whale population off the coast of California since 2015.

Because this group of whales makes the same migration each year, researchers have tagged individual whales with data-tracking devices to monitor their movement, vocalisation and feeding behaviours.

“We were originally interested in trying to characterise the seasonal pattern of their presence and different types of sounds that they make, but as we started to dig into the data analysis, we noticed this dramatic separation of the song production during the daytime versus the night-time,” says Oestreich.

Video of a spectrogram, paired with audio at 10x speed, of whale song recorded from a hydrophone MBARI. Credit: John Ryan/MBARI

He and his colleagues are not the first to notice whales singing at different times of day, but they say this is the first time these differences have been connected to a broader pattern in their life cycle by tagging and monitoring individual animals.

Fifteen tags tracked the sounds of their carriers through accelerometer measurements – which monitor vibrations – and, in some cases, integrated hydrophones.

In the summer, the whales spent much of the daytime feasting, bulking up for the long journey ahead and reserved their musical interludes for night-time. When the time came, migration was again accompanied by daytime songs.

“In the hydrophone data, we saw really strong patterns over this enormous spatial domain. When we saw the exact same pattern on individual animals, we realised that what we’d been measuring over hundreds of kilometres is actually a real behavioural signal, and one that represents the behaviour of many different whales,” Oestreich says.

This research lays the groundwork for possibly predicting blue whale migration based on the transitions between the different song schedules, he adds, and also raises an intriguing question: if we can use this signal to determine whether whales are foraging or migrating, are whales using it that way too?

It’s possible, he suggests, that a lone whale might listen around before giving up on feeding and heading south. “Blue whales exist at incredibly low densities with enormous distances between them but, clearly, are sharing information in some way.”