Hi Gordon. I'm not sure I can say anything specific about inherent advantage of such a leg design, but bird legs in general, as well as ED-209, have similar geometry to the back legs of all "toe-walkers", such as dogs, cats, deer, horses, and most other quadrupeds. Birds look kind of funny, but I think they just evolved that way, ie with the same basic rear-leg design as other vertebrates.
In essence, they all walk with their "heels" up off the ground and have an elongated foot-segment which is analogous to the segments including the soles of human feet. All vertebrate-quadrupeds have strong rear-ends, and the heel-up geometry both increases their strides and also adds a large amount of springiness to the step, which is a great advantage during running, for shock absorption and energy recovery. You will notice that was one of the advantages cited recently regarding the passive-dynamic walkers - that they are 10X or more energy efficient than the Asimo type bots. Animals are said to have upwards to 70% energy recovery in their strides. Energy is stored in the muscles and tendons during foot-fall, and recovered during the subsequent push-off. I kind of see the ED-209 geometry as a bipedal way of taking advantage of all of this.
You can see most of what I am talking about by taking a look at the running-dog page .... notice how the legs are coming down in frame #15 of the galloping dog ...Regards flamingos balancing on 1 leg, I think that's a different thing. Basically, during balancing, their legs are straight. However, if you look at the body design of birds in general, in addition to the bipedal dinos like T-rex, what you immediately notice is the body weight is distributed evenly on both sides of the vertical leg posts. They couldn't really work any other way. Eg, the long neck+head on one end of the T-rex is balanced by the long tail on the other end. Birds are similar, except that almost all of their weight is in the body mass, but it's distributed evenly fore-n-aft. Take a look at some ostriches for instance. Huge round bodies, evenly-centered, and pencil-thin necks, etc. Back to ED-209, bascially, I think the design is naturally based upon the geometry of bird + quadruped rear-legs [ie, backward-pointing hocks], and the low forward leaning body is simply a way to balance the weight over the legs. I suspect this design is a LOT easier to balance than an upright biped, like humans. I've been playing with simple papers designs for a biped like this too. However, I think you should be able to do it with much simpler leg designs than the usual 12-servo biped or the lynxmotion Scout. [no short answers today, I guess ;-)].
- dan michaels======================