Effect of a previous high pathogenicity avian influenza (HPAIV) infection on the breeding success of northern gannets (Morus bassanus)

Highly pathogenic avian influenza (HPAIV) caused widespread mortality and breeding failure among many wild, avian populations in Europe and North America in 2021–2023, but most populations exhibited a marked reduction in mortality in the year following an outbreak, suggesting that surviving individuals may have developed immunity. A critical mechanism for population resilience is whether individuals that have survived the disease show reduced breeding success because of the potential costs associated with recovery, notably elevated immune defence. We found that, at two UK colonies, the breeding success of Northern Gannets Morus bassanus with black eyes (an indicator of past exposure to HPAIV) was similar to those with normal blue eyes in the year following a severe disease outbreak, suggesting that population recovery may not be hampered by lower reproductive performance of recovered individuals compared to those that were unexposed. However, breeding success, irrespective of past exposure, was lower than the long‐term average, suggesting potential carry‐over effects on all individuals from the extensive disruption caused by the epidemic the previous year.