Declining territory occupancy and productivity in a Welsh Red-billed Chough Pyrrhocorax pyrrhocorax population

ABSTRACT Capsule Much of the UK’s largest population of Red-billed Chough Pyrrhocorax pyrrhocorax shows long-term decline in territory occupancy and productivity. Underlying mechanisms need urgent study to improve conservation delivery for this species in Wales. Aims Periodic census data have shown a localized decline of Red-billed Choughs in Wales, which supported 66% of the UK breeding population in 2014. The aim is to explore the nature of this decline using long-term study data to search for patterns in territory occupancy and breeding performance. Methods Data from annual monitoring of breeding territories in mid- and north Wales, across 25 years and representing 74% of the Welsh population, were analysed to test for variation in occupancy and breeding metrics in relation to territory type, geographic region, and protection status. Results Most territories were coastal, where occupancy fell by 12%. Occupancy of inland territories fell by 72%, accounting for just 24% of territories by 2019. The number of chicks fledged per occupied territory fell by 25% across all territories, but the brood size of successful pairs only fell 9% with neither varying by territory type. Nest success rate fell 17% on the coast but 33% inland. Trends in occupancy and breeding metrics showed little variation between geographic regions and protection status. Conclusion The severe decline of inland nesting Red-billed Choughs could lead to rapid local extinction, but the slower decline in the coastal population is also of concern. Declining breeding performance is a plausible driver of population decline. As current conservation delivery appears ineffective, the mechanism of decline needs urgent study to identify necessary protection measures.