Comparative Geometry - Birdy vs Swift [all dimensions in mm]:
Wheelbase: 1010 vs 1030 (longer is better?)
Effective top tube: 585 vs 550 (longer is better for taller riders)
Head angle: 71º vs 72º
Seat angle: 73º vs 72º
Trail: 63 vs 36
BB height: 290 both (depends on tyre width)
Stepover height: 535 vs 640
Chainstay length: 430 vs 410
Head tube length: 88 vs 125
rear dropout: 135 both
The Birdy's wheel flop factor is quite strong, resulting from a slack head angle and lots of mass in front of (and therefore above) the steering axis, partially due to the sports stem. So the front wheel has quite a strong tendency to flop sideways. The flop is not dependent on speed - it wants to happen at any speed.
The Birdy also has quite a large trail; ie the wheel contact point is behind the steering axis, like in a shopping trolley wheel. So at speed, the trail tends to straighten the wheel out - the faster, the more you get. This ought to make the bike very ridable hands-off.
So with trail and flop you have these opposing forces, one of which is speed dependent - the wheel flop factor steers away from the middle at any speed, while the trail steers to the middle at highish speeds. At low speed, the flop factor is stronger than the trail, so the bike veers off. At high speed, the trail is stronger than the flop, but now there is over-correction from any disturbance so the bike now veers in the opposite direction, and back again, and so on, resulting in shimmy at high speed. Possibly the comfort stem which is further backwards may be more stable.
The Swift, which also can't be ridden hands-off for long stretches, has not exhibited any tendency to shimmy under similar conditions.
The slightly steeper seat tube angle has the effect that as the seatpost is set higher, the saddle has to be set back further to end up in the same spot as for the Swift.
Fitment:
Most of my other folding bikes have a 55cm effective top tube. This is logical as it corresponds to a medium sized bike, and makers obviously want their bikes to fit as many customers as possible. However, the Birdy has a 58.5cm effective top tube, so it caters for slightly taller riders which is great since I am on the edge of being too tall for a size medium.
I like a more forward body posture, arms and body roughly at 45º from vertical. I use the Peter White approach to fitting, and this has the result that all my bikes end up being very similar in fit - the relative position of saddle, pedals and handlebars are the same on all bikes. The preferred fit has resulted in the Birdy saddle going back as far as it can go; the sport stempost places the handlebars smack in the right spot. The lowest setting results in the handlebars at saddle level. So the Birdy fits me with no mods. The slightly longer effective top tube, plus that there is a lot of seatpost left over for extension, makes the Birdy suitable for taller riders as well. (I am average at 5'10".) Pacific Cycles claims to 6'4" and 110kg, including luggage. I think that claim has substance.