A new study published in Marine Biology reveals that black guillemots (Cepphus grylle) from two Norwegian colonies exhibit distinctly different migration strategies during the winter. The study documents their behaviour and habitat use outside the breeding season, and thereby provides crucial knowledge for conservation and management in coastal areas.
Adult black guillemot in summer plumage. Photo © Nina Dehnhard/NINA
Limited knowledge of winter distribution
Coastal zones are rich in biodiversity but also vulnerable to human impacts such as fisheries, kelp harvesting, aquaculture and climate change. The black guillemot is a seabird that breeds along the entire Norwegian coast and Svalbard with a population of around 35 000 pairs, or ca 10 % of the global population. Previous research has shown that the species typically remains in coastal areas during winter, but details about its winter movements in Norway or elsewhere in the eastern Atlantic are limited.
– Climate change and human activity in coastal zones increase the need to understand the black guillemot’s migration strategies to ensure sound management plans, says Nina Dehnhard, one of the researchers behind this recent study on the species.
100 light loggers used at two sites
Fieldwork was carried out between 2018 and 2024 at two colonies: Heimøya at Sklinna and Hernyken at Røst. One hundred black guillemots were equipped with light loggers (GLS) mounted on leg rings, and 51 devices were later retrieved from 26 individuals. The loggers recorded both light levels and contact with seawater, enabling researchers to calculate positions twice a day based on changes in daylight and time spent on the water.
Faithful to wintering areas
The black guillemots showed marked differences in migration behaviour between colonies: individuals from Røst travelled the farthest, averaging 283 km from the colony (some as far as 1 000 km), while birds from Sklinna stayed closer (less than 97 km on average). Both groups wintered in the coastal zone with high densities of islets and kelp forests, which are believed to be key habitats for this species. The data revealed medium to high site fidelity with birds returning to the same wintering areas year after year. Water immersion data showed that guillemots often extended their time in the water during twilight hours in winter, especially at northern latitudes.
– This finding suggests that guillemots in the far north must use twilight hours to find enough food and maintain energy to withstand low temperatures and survive the winter, explains Dehnhard.
The study shows that black guillemots search for food in kelp forests also in winter, although in a more subtle plumage. Photo © Tycho Anker-Nilssen/NINA
Winter habitats should be protected
The study provides new and important knowledge about how guillemots use coastal areas outside the breeding season and how strongly they depend on kelp forests.
– Currently, there is no knowledge about how kelp harvesting affects guillemots – while our data clearly show that they use kelp forests year-round, Dehnhard emphasizes.
That birds from Røst have a much more extensive migration than those from Sklinna suggests colony-specific strategies that may influence vulnerability to human impacts. The study also highlights that data concerning migration and activity patterns shaped by light and temperature conditions must be included in future management strategies. The authors’ conclusion is clear:
– It is undoubtedly important to include the non-breeding season in future management strategies for seabirds in coastal zones facing increasing anthropogenic pressures.
Read the article: Divergent migratory strategies of black guillemots from two colonies in the Norwegian Sea
Contact: Nina Dehnhard
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