Decades of tracking two albatross colonies showed that unpredictable conditions push birds toward slower reproductive strategies.

Reproduction can be a gamble. Every chick an albatross raises draws on the same reserves of energy and resources the bird needs to survive and breed again. Now, using 50 years of data on two albatross colonies, researchers have shown how strategies to handle that gamble shift when the environment itself becomes unpredictable: birds living in more variable conditions make the safer choice, delaying reproduction and having fewer offspring in favor of better survival.

Choosing to reproduce can be a difficult decision, with implications for an individual’s own survival, particularly in resource-strained environments. “How much [the birds] invest in reproduction one year can have long-term effects later in life, and the accumulation of these effects, what we call reproductive costs, can result in a reduction of reproductive performance in late life (termed reproductive senescence) or in a reduction of survival in late life (termed actuarial senescence),” Bertille Mohring of the University of Liverpool wrote in an email to Advanced Science News.

While theory states that how quickly or slowly an organism passes through life phases is shaped by its environment, experimental evidence has been limited. Mohring’s team chose to focus on the life phases of long-lived black-browed albatrosses (Thalassarche melanophris), for a few different reasons. Sensitive to environmental conditions, these bird populations have been tracked at two locations in the southern hemisphere for extensive periods of time. Field teams have managed monitoring programmes at Bird Island, South Georgia, and the Kerguelen archipelago for nearly five decades. Climatic drivers such as sea surface temperatures as well as food type and availability have contributed to environmental conditions that are three times as variable on Bird Island than at Kerguelen.

The researchers compared life histories and reproductive performance within and between these two populations of the same bird species to understand the impact of environmental conditions. Since the 70s, hundreds of breeding adults and chicks have been tagged with rings that have unique codes, helping researchers track individual birds and their breeding success every season. The team included 5,854 breeding attempts from 874 albatrosses at Kerguelen and 2,648 breeding attempts from 360 albatrosses at Bird Island between 1975-76 and 2023-24.

Black-browed albatrosses living in the more variable environs of Bird Island were, on average, less successful breeders, and experienced life milestones at a slower pace. Both breeding and reproductive senescence were slower to arrive. “This means that individuals need more years to produce the same amount of offspring and that the decline in breeding performance in late-life starts later and is slower, so environmental variation slows the pace of life,” says Mohring.

In places where the climate, weather or access to food is more unpredictable, offspring may struggle to survive, so individuals can devote less energy to reproduction. “This is what we call “bet hedging”: the idea that individuals spread the risk of offspring mortality across a higher number of breeding attempts,” adds Mohring. “As reducing allocation to current reproduction induces a slower accumulation of reproductive costs over a longer lifespan, this should translate into delayed and slower senescence (what we observe).”

On Bird Island, individuals in the albatross population varied quite a bit, more so than within the Kerguelen population. Factors like when senescence began, how rapidly it progressed, the number of offspring and how many survived were different across individual birds. On one side of the life history spectrum, birds focus on reproduction, being younger when they first reproduce, having lower survival rates and quickly progressing to senescence. On the opposite side of the spectrum, birds focus on survival, delaying reproduction, having better survival rates and slowing heading towards senescence.

The researchers expected that, within any population, individuals would follow different paces of life, slowly marching towards reproductive senescence, or rushing headlong into it, but they would produce the same number of offspring over time.

Reality was something else: In both the Bird Island and Kerguelen populations, albatrosses that reached reproductive senescence earlier and declined faster showed higher fitness and reproductive success, producing more offspring.

Mohring’s team proposed two reasons for why that may be. It is likely that birds that experience this faster pace-of-life may also be better at acquiring more resources than their counterparts, contributing to their breeding wins. The other possibility may be that reproductive costs quickly add up, with birds that produce more offspring hitting senescence more quickly.

The team will turn their investigation to understanding what drives variations in life histories between individuals of the same colony. “Identifying the individual characteristics that predict where a bird falls on the fast-slow continuum is a key next step,” says Mohring. They also plan to explore the impact of reproductive costs from one year to the next in these bird populations and how these costs influence senescence.

The annual variation in reproductive success between the two populations mirrored the variability of their environments, with the measure being thrice as high in the Bird Island colony. “Understanding the effect of environmental variability is particularly relevant in a context of climate change that intensifies fluctuations in ocean productivity and prey availability across the Southern Ocean,” adds Mohring. “Populations of long-lived seabirds may thus face changes in environment and it is important to evaluate how they will respond, especially for conservation.”

Reference: Bertille Mohring et al., Environmental Variability Shapes Life-History Trade-Offs Within and Between Populations of a Long-Lived Seabird, Ecology Letters (2026). DOI: https://doi.org/10.1111/ele.70384

Featured Image: Black-browed Albatross by Adrien Chaigne