WHAT THE HELL IS BIRD.

Readiness probe failed: calico/node is not ready: BIRD is not ready: 
Error querying BIRD: unable to connect to BIRDv4 socket: 
dial unix /var/run/calico/bird.ctl: connect: connection refused

Followup: I did a bunch of network futzing on the node, and broke pretty much everything, and after I got it up again, Calico stayed broken with this error message. A reboot cleared it up and I never figured out what BIRD was, but I kind of want to remain ignorant of this one thing because it makes error querying bird/bird is not ready way funnier.