Count of Northern Gannets on the Bass Rock in July 2024

A drone survey of Northern Gannets on the Bass Rock was carried out on 29 July 2024, following the protocol of the earlier surveys in 2023 and 2022. The resulting orthomosaic vertical image covered c.97% of the colony. Three independent counts of this area gave a mean count of 44,447 Apparently Occupied Sites (AOS) ± 192 standard deviation (SD). After scaling up this value to correct for the missing areas, the total Bass Rock population in 2024 was estimated to be 46,045 AOS. Comparison of the areas counted in both 2023 and 2024 indicated a 6.7% decline in AOS, while comparison of the estimated total population indicated a decrease of around 11%. Thus, there was no evidence of a population recovery after the high pathogenicity avian influenza (HPAI) outbreak in 2022. However, very few dead birds or unattended chicks were recorded indicating no recurrence of HPAI in 2024 and that foraging conditions for parents were favourable. A detailed analysis of inter-observer variation in counts of AOS was also carried out. Due to the high resolution of the drone image, inter-observer variation is substantially reduced compared to previous surveys using lower quality images from fixed-wing aircraft. Crucially, there was no statistical support that counters differed in how they counted areas varying in size and aspect. Thus, appreciable savings in time and/or expense needed to obtain population counts of gannetries may be possible by reducing the need for replicate counts.