Characterising the longer-term impacts of highly pathogenic avian influenza outbreaks prior to 2020 on wild bird populations

Since 2020, there has been a global escalation of the impacts of highly pathogenic avian influenza (HPAI) on wild bird species. However, the population-level consequences of this disease remain poorly understood, despite repeated outbreaks affecting wild birds over the past two decades. To address this, we collated population counts and mortality data from 40 historical cases where HPAI mass-mortality occurred in wild bird populations prior to 2020. We characterised local impacts of these outbreaks based on i) initial change in population count from the last count pre-outbreak to the first count post-outbreak, ii) longer-term change in average population counts, iii) change in population trend, and iv) time taken to recover to the pre-outbreak baseline. Following outbreaks, initial population count declines were observed in most cases, with a median decrease of 17% (Inter-quartile range −35% to 5%). Yet in the longer-term, significant reductions in average population counts were detected in only eight of the 40 cases, and a significant decrease in population trend in only two cases. Overall, in the majority of local populations, outbreaks resulted in either limited impacts on population counts or short-term reductions followed by recovery. However, in 25% of cases (10/40) initial declines were not recovered within the monitoring period (4–17 years after the initial outbreak). Initial declines were more apparent in gulls and terns; however, impacts were seen in local populations across a range of taxa. Given that the scale of impacts of HPAI has changed dramatically since 2020, our findings support the need for disease surveillance to be combined with effective population monitoring in order to identify species impacted from the ongoing outbreaks. Further research is also needed to identify strategies for minimising impacts of HPAI, and to understand factors limiting recovery in affected populations.