Phallus asymmetries and sires for 80 F1 hybrids ducks taken in North America organized by species differences in phallus length.

Mean phallus lengths are based on winter adults. For each hybrid combination the first listed species has the larger phallus. The upper section of this table lists hybrid combinations with large and reliable phallus asymmetries; the lower section lists small asymmetries that are unlikely to be predictive.