The Birdy has a 100mm OLD front.

But the disc mount is not only on the right side of the bike, the disc is also placed closer to the center of the wheel than on a standard front disc wheel (actually, the distance from the dropout to the disc of the Birdy front wheel is the same as the distance between dropout and disc on a standard rear wheel).

The thru-axle is just one solution, the others with pins or grooves.. also work.