Candling analysis of egg development in an endangered bird species Crested ibis ( Nipponia nippon )

Identifying key factors influencing the survival of animals, particularly rare and endangered species is crucial to biodiversity conservation. In birds, hatching failure is pronounced in endangered species. Accurate assessment of egg development and the ability to distinguish non-viable eggs are essential prerequisites for identifying the causes of hatching failure and applying appropriate conservation practices. The Crested ibis ( Nipponia nippon ), a flagship endangered species, has a long history of captive breeding, which has contributed to population recovery. However, little is known about the specific process and characteristics of its egg development. In this study, we provided the first comprehensive description of normal egg development in the Crested ibis, including both the changes observed in unfertilized eggs during incubation and embryonic development via candling (n = 106 eggs, with 1,423 candling images), a commonly used non-invasive method for assessing egg fertility and embryonic status, and offered a practical reference for assessing fertilization status and embryo viability. In addition, we estimated the timing of embryo mortality and found that most deaths occurred during mid-incubation (Day 7∼15) (n = 12, 60.0%) and shortly before hatching (Day 23∼29) (n = 7, 35.0%), highlighting these critical periods that require particular attention. This species-specific documentation of egg development provides a valuable reference for accurately assessing embryonic progress, evaluating environmental effects on survival, and guiding adaptive management, benefiting both captive breeding programs and field conservation efforts.