Award: OCE-2142918

Project Overview

In our NSF-supported project (award no. 2142918), we explored how shifts in ocean conditions—specifically, rising temperatures and increased density stratification (the layering of the ocean by temperature and salt content)—affect seabird reproductive success and rates of colony-wide breeding failure around the world. The first step of our project was to build a large, standardized dataset. To do this, we formed the Global Seabird Working Group, which brought together 76 scientists and data contributors. This large collaboration allowed us to compile a global dataset with 6,645 records of seabird breeding success, representing 73 species across 89 colonies in 17 different countries. Figure 1 shows the 23 colony locations with the most information and data on multiple seabird species. Our work has been recognized by the UNESCO Ocean Decade Program as an example of effective integration of key biological data at the global scale.

Main Findings

One of our central discoveries is that patterns in seabird breeding productivity differ significantly by region and are influenced by both ocean warming and stratification of ocean water, as well as the diversity of available prey species. The effect of warming oceans was generally negative (seabird breeding success decline with ocean warming), but the effect of stratification was more complex, sometimes showing positive relationships and sometime negative ones. Importantly, seabird populations that have access to a greater variety of prey species—hence are able to switch between food sources as conditions change—tended to be more resilient to environmental changes than those with fewer options. This indicates that biodiversity in the food web is important to sustainability of seabird populations.  As most seabirds feed on a variety of fish and plankton species, our findings show that seabird conservation and fisheries management are intimately linked. Our findings also reaffirmed that different seabird species are responding in different ways to changes in ocean conditions: the breeding success of planktivores (species that eat plankton) are generally increasing or stable, while others—like surface-feeding or diving fish-eaters—may show no change or declines, regardless of whether they are in northern or southern hemisphere oceans.

Geography matters as well. We observed sharper declines in breeding productivity in the Northeast and Northwest Atlantic and the Atlantic-Arctic than in the Pacific regions or the Southern Hemisphere, where seabird populations appear more stable. This seems to be related to the higher diversity of prey species in the Pacific compared to the Atlantic or Arctic. Our team has submitted two papers for publication summarizing these outcomes, with two more currently being written.

Outreach and Impact

Our project also focused on training young scientists, and outreach to the public. We supported a postdoctoral researcher and an undergraduate student, both of whom participated in our science outreach programs with local teachers hosted at the UC Davis Bodega Marine Laboratory. To help the public understand our work, we launched an engaging, interactive web story on seabirds, ocean changes, and stratification in early 2025. We’ve also made our data public and in line with NSF standards, which supports transparency and encourages further research.

Mechanisms and Future Research

Despite these insights, important questions remain about why seabird productivity varies so much, even among nearby colonies or closely related species. A leading explanation involves what we call “bottom-up” food web processes: changes in phytoplankton (primary producers) can ripple up through the food web, affecting zooplankton, small fish, and eventually seabirds. The “match-mismatch hypothesis” highlights that seabird breeding success is highest when chick-rearing happens during peak food availability, which may be tied to the timing of the spring phytoplankton bloom. If there’s a mismatch in timing—if chicks hatch before or after peak prey availability—fewer chicks survive and colony-wide reproductive failure may result. For example, research on Mediterranean storm petrels has shown that the best time to breed varies by year, depending on when food is most abundant. This directly supports theories developed many decades ago by ornithologist David Lack about the importance of optimal timing for reproduction in birds.

There may also be important effects from within the seabird populations themselves, such as competition for food when colony sizes are large (so-called “compensatory density dependence” in breeding success). The interaction between these internal population factors and external environmental cues may help explain why some populations are more affected than others by changes in the ocean environment.  Moving forward, our research will aim to combine detailed records on the timing of plankton blooms, timing of seabird hatching, and seabird population size to understand how the environment affects seabird breeding productivity. This integrated approach will help us better predict how seabird populations will respond to ongoing long-term changes in marine ecosystems relative to internal population controls.

Last Modified: 09/29/2025
Modified by: William J Sydeman