Hatching failure and embryo mortality in endangered populations of Eurasian Curlew ( Numenius arquata )

Hatching failure represents a significant and growing barrier to reproductive success in threatened birds and yet is often poorly understood. Populations of the Eurasian Curlew Numenius arquata in the United Kingdom and Republic of Ireland have experienced significant declines over recent decades, and low breeding productivity is a major barrier to conservation efforts. However, data on hatching failure, infertility and embryo mortality rates are currently lacking. We collected data on the occurrence of these early barriers to reproduction in Eurasian Curlews breeding across the United Kingdom and Ireland over three successive breeding seasons (2022–2024) and compared success rates of eggs from natural nests versus eggs removed for artificial incubation (as part of conservation interventions). We found no significant difference in hatching rates for naturally and artificially incubated eggs, with the average percentage of failed eggs across breeding years being 12.9% (18.6% including abandoned eggs) and 14.4%, respectively. There was little evidence for significant effects of latitude, longitude and/or breeding year on hatching outcomes. Hatching failure was almost exclusively due to embryo mortality, rather than fertilization failure, across our entire sample and embryo mortality typically occurred during the early and late phases of development. Across all models of hatching failure and embryo mortality occurrence, we found that the random effect of clutch identity explained most of the observed variation, suggesting these patterns are most likely driven by localized environmental, behavioural, genetic and/or egg quality factors not monitored in our study. A better understanding of these potential drivers in the future will support the development of more appropriate and effective conservation strategies to maximize reproductive productivity.