Originally Posted by glye

I don't know. Another option is the Efneo GTRO 3-speed crank drive (130 BCD compatible). With the 38t chainwheel option this equals a triple with 38/54/68t, then you can go rather fast with an 11t smallest rear cog.
There is also the Patterson Metropolis 2-speed (130 BCD compatible), and the Truvativ Hammerschmidt 2-speed (built for small chainrings, seems no good for Birdy). All interesting options for their uses.