Wild seabirds that face high energy demands during the breeding season go on to migrate farther and raise more chicks the next year, but at a cost to their own survival. An international team, led by researchers from Nagoya University, raised the energy cost of flight for some birds and tracked them through the next year of their annual cycle. The researchers introduce a concept called "energetic flexibility" and offer rare experimental evidence for how the costs of one season carry over to shape the next. The study appears in Proceedings of the Royal Society B .
The team studied 251 black-legged kittiwakes on Middleton Island, Alaska, from 2021 to 2024. In 2021, they altered the energy cost of breeding for three groups. One group received extra food, which reduced the energy cost of raising chicks. For a second group, the team clipped three wing feathers and two tail feathers at the base shortly after the birds had laid their eggs. This raised the energy cost of flight for the rest of the breeding season. The feathers grew back at the birds' next molt. A third group was left alone as a control.
Each bird carried a geolocator that recorded its movements through the non-breeding season. The team recovered 203 of the 251 devices the next year and tracked the same individuals through 2024.
Birds with raised energy costs fledged only 10% of their chicks that year. The fed group and the control group did much better: 44% and 43% respectively. The high-cost birds also departed about 10 days earlier than the other groups, leaving the colony at the end of the breeding season to head out to sea for the long migration across the North Pacific Ocean.
"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," said Akiko Shoji, senior author and professor at the Graduate School of Environmental Studies , Nagoya University. "In other words, breeding failure appears to be one consequence of high energetic costs, rather than the direct cause of early departure."
Their earlier departure also resulted in longer migrations. Birds that travelled farther during the non-breeding season were more likely to breed successfully the following year. This suggests that longer migrations may facilitate recovery from the energetic costs incurred during breeding.
However, they paid a price in survival. Only 67% of the high-cost birds returned the next year, compared with 83% of the control group and 90% of the fed group.
The researchers say this pattern points to a hidden trade-off. To recover from a hard breeding season, kittiwakes invested more in the next migration. That strategy paid off in future reproduction but also lowered their chances of survival.
While the survival cost of high energy demands during breeding has been documented in seabirds before, this study is the first to experimentally show that the same birds also gain more chicks the following year, through changes in their migration.
"We use the term 'energetic flexibility' to describe an animal's ability to flexibly adjust energy allocation among reproduction, survival, and migration as environmental conditions change. Our study suggests that this flexibility may be a key mechanism underlying carry-over effects, helping to explain how events in one season influence reproduction and survival in the next," Shoji said.
The findings have implications for seabirds as climate change reshapes the oceans. As prey availability shifts and conditions become less predictable, the ability to reallocate energy across seasons may decide which populations cope and which decline.
The team plans to use miniature heart-rate loggers to track kittiwakes' energy use throughout the year and identify the biological processes that drive this flexibility.
