Black-legged kittiwakes migrate farther after costly breeding seasons

Seabirds that endure a particularly taxing breeding season do not simply recuperate afterwards.

Fresh research indicates that they instead migrate farther, potentially improving their ability to rear more chicks in the next year.

Yet this approach carries a substantial penalty for the birds’ own survival prospects.

Scientists at Nagoya University led the study.

They experimentally increased the flight-energy demands of wild seabirds, then followed their fortunes over the next year of their annual cycle.

The researchers use the term “energetic flexibility” for this pattern.

Their findings provide unusual experimental proof that difficulties in one season can have consequences for the next.

Three groups during one breeding season

Between 2021 and 2024, the team examined 251 black-legged kittiwakes on Middleton Island, Alaska.

In 2021, the researchers intentionally changed the energetic demands of breeding for three distinct groups of birds.

One group was given extra food, reducing the energy required to rear chicks.

For a second group, three wing feathers and two tail feathers were clipped at the base soon after the birds laid their eggs.

This intervention increased the energy required for flight for the rest of the breeding season, although the effect disappeared naturally when the feathers regrew during the birds’ next moult.

A third, control group received no treatment. Every bird was fitted with a geolocator that recorded its movements during the non-breeding season.

The following year, researchers recovered 203 of the 251 original devices, enabling them to follow the same birds through to 2024.

A sharp decline in breeding success

The birds subjected to higher energy costs fledged only 10 percent of their chicks that year.

By comparison, the fed and control groups fledged 44 percent and 43 percent of their chicks respectively.

Birds in the high-cost group also departed the breeding colony around 10 days earlier than those in the other groups, beginning their long migration across the North Pacific Ocean earlier than normal.

“Earlier departure from the breeding colony by birds that failed to raise chicks has long been recognized,” said study senior author Akiko Shoji.

“However, our study demonstrates that early departure is driven not by breeding success or failure itself, but by the energetic costs incurred during reproduction.”

“In other words, breeding failure appears to be one consequence of high energetic costs, rather than the direct cause of early departure.”

This distinction recasts a pattern already observed by scientists studying seabirds.

Instead of early departure being a straightforward result of a failed breeding attempt, the research indicates that the underlying energy burden drives both outcomes simultaneously.

Longer migrations, improved chances the next year

Leaving sooner also resulted in these birds travelling farther overall during migration.

Birds covering greater distances in the non-breeding season were more likely to breed successfully in the following year.

The result indicates that longer migrations could help kittiwakes recuperate from the energy demands of a difficult breeding season, acting rather like an extended recovery phase within their annual cycle.

However, this recovery tactic came at a cost. Survival figures revealed a markedly different outcome.

Just 67 percent of the high-cost birds returned the next year, against 83 percent of the control group and a notable 90 percent of the fed group.

The trade-off between recovery and risk

Overall, the researchers say the results reveal a real trade-off beneath the apparent pattern.

To recover after an arduous breeding season, kittiwakes seem to make a greater investment in their subsequent migration.

While this tactic improved future breeding success, it also reduced the chances that the birds would survive to breed again at all.

Previous research had shown that high energy demands during breeding impose survival costs on seabirds.

This study, however, is the first to demonstrate experimentally that the same birds receive a reproductive benefit in the next year, specifically by altering how far they migrate.

“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,” Shoji said.

“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.”

Why this matters in a warming ocean

The findings have important consequences for understanding how seabirds may fare as climate change continues to alter ocean conditions.

As prey availability changes and the marine environment becomes more unpredictable, a species’ capacity to redistribute energy between breeding, migration and survival may become ever more significant.

Such flexibility could decide which populations adjust to changing conditions and which decline.

The team now intends to fit kittiwakes with miniature heart-rate loggers to monitor their energy use throughout the entire annual cycle.

Their aim is to identify the precise biological processes that enable this form of energetic flexibility in the first place.