inbreeding

My sister saw my 2 cockatiels standing on one another twice. The 1st time Kiwi (for sure male) was nudging lemony (unsure of gender, but most likely female) to move from her perch so that he could cross (they share a cage) then when lemony wouldn’t move he walked over them, lemony- whose nails...

It took me a while to notice that Tango's feet don't match. Then it was the vet who said she also doesn't have claws at the back. Has anyone got any tiels who are similar? And why do you think Tango has this (unfortunately I think it might be down to inbreeding :sad14:)