Guillemots prefer predictable foraging grounds

Seabirds have developed a remarkable ability to find food by relying on the ocean’s natural predictability. But when human activities such as fishing and climate change disrupt these patterns, finding food becomes much harder. In her PhD thesis, Astrid A. Carlsen explored the foraging strategies of common guillemots and razorbills to understand how they respond to changes in their environment.

"During my PhD, I spent three summers collecting data using a sailboat-like drone equipped with an echo sounder, salinity and temperature sensors, and an instrument to measure algal activity. This gave me the information I needed to determine where the birds’ prey fish spend their time. In addition, GPS- and depth-tagged seabirds provided detailed insights into their foraging behaviour," says Astrid A. Carlsen.

Different strategies for finding food

By using these new technologies, she was able to gain insight into the lives of seabirds in ways that were previously not possible. Her results show, among other things, that razorbills preferred to hunt when fish were not schooling and when light conditions were good. Common guillemots dived deeper than razorbills and were less dependent on good light conditions.

“Since common guillemots and razorbills use different foraging strategies, it means that they may respond differently to changes in fish behaviour – even though they breed on the same island at the same time of year,” says Carlsen.

She also found that common guillemots did not necessarily forage where prey was most abundant. Instead, they chose deep areas with relatively little food, but where availability remained predictable over time and stable throughout the season.

“This is likely because they are targeting a specific type of prey – most likely sprat – which spawns in deeper waters during this time of year,” she explains.

Better protection of foraging grounds is needed

Carlsen also examined how the physical traits of seabirds globally – including common guillemots and razorbills – influence their foraging strategies. She found that the distance birds are able to travel in search of food is closely linked to wing size. Based on her calculations, she estimated that only about one percent of the foraging areas available to seabirds worldwide are currently protected.

“Protecting these areas is especially important when fish behaviour and distribution become less predictable. Flight range capacity is already used as a tool in conservation planning, but it’s a measure that isn’t accessible for many species. My research shows that it’s possible to estimate this capacity using simple morphological traits – without needing to track every species with GPS,” says Astrid A. Carlsen.

Download and read the thesis Understanding predators through their prey