Since 2020, impacts of highly pathogenic avian influenza (HPAI) on wild bird populations have escalated globally. Colonial nesting birds are particularly vulnerable, as high-density colonies facilitate rapid viral transmission. At some affected sites, removal of carcasses has been carried out during outbreaks, with the aim to reduce transmission rates and resulting mortality. However, there remains substantial uncertainty over the effectiveness of this mitigation measure.
In this study, we use an individual-based model to simulate HPAI spread through a ground-nesting colony. The model was calibrated using data from outbreaks in Black-headed Gull (Chroicocephalus ridibundus) and Sandwich Tern (Thalasseus sandvicensis) colonies in the UK, using an approximate Bayesian computation (ABC) approach. Following model fitting, we test different scenarios for carcass removal, ranging from daily removal of all carcasses to infrequent removal starting after a delay.
Results indicated that removing carcasses can reduce adult mortality during outbreaks, but effectiveness was limited in the context of high-density colonies. Mean reductions in mortality were less than 5% across the scenarios considered, and effectiveness decreased as the frequency of collection was reduced. However, removal of carcasses did substantially lower the predicted accumulation of virus in the colony, which could reduce spill-over risk to other species, including through scavenging. When outbreaks occur early on in the breeding season, model results also indicated that carcass removal could lower the risk of HPAI impacts on susceptible chicks later in the season.
Practical implications. HPAI presents an ongoing risk to colonial nesting bird populations as H5 Clade 2.3.4.4b continues to spread globally. Our findings suggest that carcass removal should be considered as a response where outbreaks occur, as frequent removal can reduce adult mortality and increase productivity in some scenarios. However, in high-density colonies with high transmission rates, carcass removal is unlikely to be sufficient to halt transmission and prevent population impacts. Therefore, potential benefits should be considered against risks of negative effects from disturbance to breeding colonies. Decisions on carcass removal should be made on a site-by-site basis, and if removal is implemented, this should be started as soon as possible and repeated frequently.