Morphological criteria for staging near-hatching embryos of the domesticated Mallard (Anas platyrhynchos) and Swan Goose (Anser cygnoides)

Studying waterfowl embryology necessitates reliable and precise staging tables. These are descriptions of embryonic features throughout development from fertilisation to hatching stage, which are used to approximate the extent of embryonic development. Waterfowl staging tables have previously been established based on morphological features from fertilisation to approximately ten days before hatching. Embryonic changes over the final ten days of pre-hatching development have also been documented and proposed as useful staging criteria. However, the reliability of these changes as useful staging criteria across different waterfowl breeds has not been fully examined. To examine the reliability of these criteria-- which focus on size changes of the bill and middle toes--for staging near-hatching embryos, we examined twenty-seven Mallard and Swan Geese embryos and measured their beak and toe-length sizes. We compared our results with previously published data, and found that size variation across breeds within the same developmental stage is equivalent to within-breed variation across different stages, suggesting limited reliability of beak and middle toe length sizes 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 accuracy for developing reliable embryonic staging tables.