It all lies in their mentality. Of course even a gust of wind can knock down those twigs, but the thing is they mark the rights of possession over a den, and yukkuris - even shitheads, are not smart or determined enough to overcome this mental barrier and just barge in through. Any invaders that do not go by yukkuris' rules (including predator yukkuris, ironically lol) will just kick the twigs away and engage in the slaughter though.
It's social mores. Just as a determined burglar can pick open or break through your front door, any yukkuri could brush those twigs aside and barge in, but they don't because it would be socially unacceptable.
It's social mores. Just as a determined burglar can pick open or break through your front door, any yukkuri could brush those twigs aside and barge in, but they don't because it would be socially unacceptable.
Assuming yukkuris give two shits about about social rules.