I don’t think you understand what the term “nuclear family” means in a sociological manner. In humans it’s not just “two parents taking care of their offspring”, there is also a caveat that they are doing that WITHOUT the reliance of an extended social group. Then those offspring are expected to do the same once they reach adulthood. The only social support they are expected to have is their own unit.
Furthermore, comparing human behaviours to other species such as birds (who flock together in habitats for survival and for migration) and possums (who are a solitary, nocturnal species) doesn’t really mean anything.
The comparisons should be to species who share similar evolutionary patterns and social habits to humans ie. primates. And across every primate species, whether it be lemurs, gorillas, baboons, simians, etc. they are all connected to a LARGER SOCIAL GROUP irrespective of their family structure (pairs, group family, homogeneous male or female) well into adulthood.