“Nest sitter”, by nature.
It is a drive for security.
I had a Robin build her nest on my power meter the last month .. bird was very protective ... when hatchlings appeared both bird parents fed them, but mom was MOST aggressive and agitated when I went in the backyard ...
But that goes against the logic here.
In fact females DO tend to be very aggressive and protective, especially mammals, because indeed it is up to them to raise and protect theirs. Female dogs are better “guards” while males are better “patrols”, they will use their aggression anytime whereas females need a reason - “theirs”/possession - and then it is passionate.
If they are naturally more protective...why would they desire more outward security from someone else?