Climate change is causing the whales to spend more time north before swimming to the waters off California's coast.
Out in the deep waters of Monterey Bay, grey whales will be swimming home later this month after a brief winter vacation in Baja, California.
Whale watchers and marine scientists say these whales have been delaying their southern sojourns and point to climate change as the culprit.
Rising sea temperatures have disrupted the animals' home habitat in the waters between Alaska and Russia, said Wayne Perryman, a researcher at the Southwest Fisheries Science Centre in La Jolla, California. Because of these changes, the whales are spending more time in the north before they start their yearly swim south.
The scientists at the centre have observed the whales for more than 20 years as they pass through Monterey Bay. Compared to two decades ago, Perryman said, the animals are reaching the bay a week later.
"This isn't trivial," Perryman said. "It's a significant change."
Richard Ternullo, a boat captain for Monterey Bay Whale Watch in Monterey, California, said the whales' yearly arrival in the bay fluctuates, but he has noticed on average it has drifted about 10 days later into the year.
"Last year, they were considerably late," Ternullo said. "But this year they seem to be on time."
Every year, grey whales undertake a 12,000-mile round-trip swim from the frigid waters of the Bering Sea to the warm lagoons off Baja California.
Scientists don't fully understand what motivates this epic migration - the longest for any mammal, but believe the animals may leave their homes to avoid predators such as killer whales, which feed on the grey whale calves.
"Baja is like Club Med for most of the grey whale population," Perryman said. "They just horse around and socialise."
Before leaving their arctic homes, the animals gorge on tiny crustaceans that live along the bottom of the ocean floor. Most grey whales won't eat during their swim south or when they finally reach Baja, so they must develop thick layers of blubber to help sustain them as they fast.
As warmer waters melt the oceans' ice, other animals move into the whales' habitat and start feeding on the crustaceans. Crowded out by the new competition, the grey whales have to travel further north and feed longer to get their fill. These changes have disrupted the timing of the whales' yearly migration.
Jim Harvey, a professor of ecology at the Moss Landing Marine Laboratories in Moss Landing, California, worries as the arctic ice continues to thin, the grey whales' food supply will start dwindling. But marine scientists don't know how the changes to the whales' schedule will affect the animals over time.
"The arctic environment is so darn dynamic," Perryman said. "We just don't know how this will play out."