I'm another new Birdy owner who has read through much of the valuable discussion in this thread. I bought the Birdy City (European model with Nexus 8 gears) in the hope that I can add a second, smaller, chainring to extend the gearing range. I see from this long thread that has been done. So far I've figured out (i) the chainguards have to be sacrificed and (ii) the smaller chainring (38T is the smallest available in 130BCD) is fitted on the inside of the cranks. I've found a source of front derailleur clamps which claim to fit the 40mm dia seat tube but I'm having trouble in identifying a suitable front derailleur mechanism. Any suggestions? The Birdy pictured above appears to rely on manual shifting between the chain rings.