Seabirds that spend more energy raising chicks migrate farther the following season and raise more offspring the year after, but they are significantly less likely to survive. That is the central finding of a new study tracking wild black-legged kittiwakes on Middleton Island, Alaska.
According to a report by Phys.org, an international team led by researchers from Nagoya University tracked 251 kittiwakes across four years, from 2021 to 2024. In the first year, the team divided the birds into three groups. One group received extra food, lowering the energy cost of raising chicks. A second group had three wing feathers and two tail feathers clipped shortly after laying eggs, which raised the cost of flight for the rest of the breeding season. A third group was left alone as a control.
Each bird carried a geolocator that recorded its movements through the nonbreeding season. The team recovered 203 of the 251 devices the following year and continued tracking the same individuals through 2024.
The results were stark. Birds in the high-cost group fledged only 10% of their chicks that year. The fed group and the control group did considerably better, at 44% and 43%, respectively. The high-cost birds also left the colony roughly 10 days earlier than the other groups, departing at the end of the breeding season to begin their long migration across the North Pacific Ocean.
Senior author Akiko Shoji, a professor at the Graduate School of Environmental Studies at Nagoya University, said the early departure was not simply a response to breeding failure. "Earlier departure from the breeding colony by birds that failed to raise chicks has long been recognized. However, our study demonstrates that early departure is driven not by breeding success or failure itself, but by the energetic costs incurred during reproduction," she said. "In other words, breeding failure appears to be one consequence of high energetic costs, rather than the direct cause of early departure."
Birds that departed earlier also traveled farther during their nonbreeding migration. Those that covered more ground were more likely to breed successfully the following year, which the researchers suggest means that longer migrations may help birds recover from the physical demands of a hard breeding season.
But the survival data told a different story. Only 67% of the high-cost birds returned the following year. That compared to 83% of the control group and 90% of the birds that had received extra food.
The researchers framed this pattern as a hidden trade-off. To recover from a demanding breeding season, kittiwakes invested more in their next migration, which helped future reproduction but came at a measurable cost to their own survival. The team introduced the concept of "energetic flexibility" to describe this capacity to adjust behavior across seasons in response to earlier energy demands. The study appears in Proceedings of the Royal Society B.
