Egg abandonment leads to biased estimates of hatching failure in birds

Hatching failure represents a significant and growing barrier to reproductive success in threatened birds, but its causes are often hard to identify. Egg abandonment by parents is a commonly observed phenomenon – often believed to be driven by disturbance, partial predation, and/or extreme environmental events – and is assumed to result in the mortality of viable eggs in the clutch. However, in practice it is often unclear whether abandonment is the cause of egg failure, or conversely, if parents abandon their eggs after detecting they are inviable. From a conservation management perspective, approaches to mitigating hatching failure would differ substantially depending on which of these scenarios is true. Here we draw evidence from both a systematic literature search and empirical data from a wild population of threatened birds to show that studies rarely have sufficiently clear definitions or timeframes for determining whether abandonment occurred, or sufficient monitoring effort to distinguish between parental abandonment as the cause or consequence of embryo mortality. By combining evidence from nest records and unhatched egg examinations, we show that parental abandonment rates are likely to be over-estimated, while other drivers of reproductive failure may be underestimated. We provide recommendations for improving the accuracy of egg fate records, which we hope will improve the accuracy of hatching failure data and enhance the specificity of related conservation interventions.