Source address on OSPFv2 HELLO

How is the source address for OSPFv2 HELLO packets decided?  I'm hitting a problem where I can see them going out with the wrong source address, preventing the other side from setting up communication.  I've got a real network adapter with Bird working over a GRE tunnel.  Bird chooses the real network adapter's source address, not the GRE tunnel (another instance of bird on the other side of the GRE tunnel correctly uses the tunnel address). If I send a ping over the tunnel it uses the GRE tunnel's source address, and the routing table matches this.  Why would bird use a different address? Iain