A bird’s-eye view of the gannet colony at St Kilda

Gannets are the UK’s largest seabirds and can be found across the North Atlantic. Adults are distinguished by their white feathers, large wingspan of up to 2 metres, black wingtips, and the unique yellow blush of their head. They breed in significant numbers at St Kilda and have been part of the island’s landscape and history for many centuries.

The advanced drone census took place on the archipelago’s island of Boreray and its two satellite sea stacks, Stac Lì and Stac an Armin, at St Kilda in 2023. The results have revealed that St Kilda is now home to the world’s largest gannet colony with over 59,000 breeding pairs. The census was carried out by the Trust’s seabird ecologists and dedicated volunteers. It took around six hours to complete, with counts and data analysis taking place over several months through the following winter. The census will be repeated in 2025, and the Trust’s seabird ecologists will be analysing the data this winter.

Throughout much of the last century, St Kilda was home to the world’s largest gannet colony. While its population remained relatively stable, a notable increase in the Bass Rock population on the east coast of Scotland saw it replace St Kilda as the world’s largest colony in 2014. However, in 2022, a deadly strain of Highly Pathogenic Avian Influenza (avian flu) swept across the North Atlantic, decimating seabird numbers and resulting in a 25–30% decline in the gannet population at Bass Rock since 2014, which is now home to over 46,000 breeding pairs.

The impact of avian flu on the gannet colony at St Kilda appears to have been less severe than at some other colonies across the UK, with its population declining by around 2% since the last census, which was conducted using images taken from an aeroplane in 2013. Trust seabird ecologists believe that the rugged landscape of St Kilda helped prevent disease spread, unlike the flatter regions where the gannets live. One of the most impacted areas of the colony was situated in the flatter sections on Stac Lì, which supports their theory that avian flu can be more easily transmitted on flat terrain, compared to the steeper sections of the colony.

Craig Nisbet, Seabird and Marine Ranger at St Kilda, said: ‘This was the first year that drones have been used to conduct a census of the gannet colony at St Kilda. The use of drones has not only ensured minimal disturbance while monitoring these birds, but it has also provided new and improved methods for obtaining results compared to previous surveys, with high-quality images.

‘While it is of course positive news that the gannet population at St Kilda has remained stable, the declines recorded at other colonies are a consequence of the very real and severe threats all seabirds face today. Frequent counting of seabird colonies is particularly important during periods of disease outbreak or increases in other threats, such as climate change and dwindling food supply, and this new monitoring technique ensures we can do this as accurately as possible.‘