Their Lonely Betters

Betters: their couples (survived)
Robin: a kind of red bird
Christian name: given name. It is used to distinguish from you and me. Only human beings have it.

3rd stanza:
“assumed responsibility” →time passes easily and unknowingly.
The writer warned us to treasure time and friends.

4th stanza
let “them”: all the creatures in the garden had died
we, too: we are human beings