Ridea and Color+ have specific front derailleur clamp for the Birdy.

You need a classic down pull front derailleur like for instance a Shimano 105.

To have the chainrings at the standard distance from the frame, you need a compact or double road crankset with the biggest chainring mounted on the external side of the crankset and the small chainring mounted on the internal side of the crankset (like you can see on the pictures of the Birdy with H&H rear rack posted above, the crankset mounted is a Shimano 105 double road).

The clamp has a plate to mount the derailleur like on a frame with a front derailleur mounting brazed on and a hole on the rear to receive the front derailleur cable housing. This cable+housing must come front the bottom.

Look at this link to see how to mount it (its on a Dahon but its the same on the Birdy).

Last point, you must pass the cable+housing inside of the Birdy frame through the foreseen holes like its done for the rear derailleur and rear brake cables+housing. The cable+housing of the front derailleur must do a loop like the cable+housing of the rear brake on the left side of the bike and go up to the front derailleur mounting clamp.