OSPF default metric

Hello again, I'm wondering how OSPF default metrics are calculated by BIRD? Background: I'm running 4 Linux VMs with BIRD under Hyper-V and BIRD gives me a metric of 10 for every connection, be it 10G (which seems to be the default for vSwitches on Hyper-V), 1G or wireless. Even though the physical speed is passed from the hypervisor to the VMs. At least "cat /sys/class/net/INT/speed" gives the proper value. If I knew where the metric comes from, I could try to fix that on the OS-level and save myself separate cost definitions for every interface. Thanks in advance, Marcus Büttemeyer