Morphological Criteria for Staging Near-Hatching Embryos of the Domesticated Mallard (Anas platyrhynchos) and Swan Goose (Anser cygnoides)

Studying avian embryology necessitates reliable and precise staging tables—descriptions of embryonic features appearing during development that are used to approximate the extent of embryonic development from fertilisation to hatching. Staging tables for waterfowl (Anseriformes) have previously been established based on morphological features from fertilisation to approximately 10 days before hatching. Embryonic changes over the final 10 days of pre-hatching development have also been documented and proposed as useful staging criteria. However, the reliability of these changes—which focus on the size of the bill and middle toes—as useful staging criteria across different waterfowl breeds has not been fully examined. To evaluate the reliability of these criteria for staging near-hatching embryos, we examined 27 embryos of Mallard (Anas platyrhynchos) and Swan Goose (Anser cygnoides). Comparisons with previously published data revealed that size variation within the same developmental stage across breeds is equivalent to within-breed variation across different stages, suggesting limited reliability of bill and middle toe size for staging waterfowl embryos. Consequently, we devised novel staging criteria for waterfowl based on four easily measurable morphological traits and show that these criteria allow correct stage identification with over 70% accuracy. Our results highlight the importance of quantifying staging accuracy for improving the reliability of embryonic staging tables.