I'm designing fauna for my story and I'd like to incorporate birds but with a twist! Instead of having wings adapted for flight their legs serve that purpose. Ordinary birds have most their muscle mass dedicated to flight which is good but leaves them very literally chicken legged. Flightless birds on the other hand have their muscle mass reallocated to their legs but as a result their wings become vestigial. So the idea is that a bird with wing-like legs would have all of its primary muscle mass useful for running and flying. Two birds with one stone (excuse the puns). This is similar to the quetzalcoatlus which is theorised to push off the ground with its wings to take off. Also similar to the sharovipteryx a lizard that glides with its hind legs.
The bird is somewhat ostrich-shaped with two strong legs with a special toe which houses the alula. This third toe is bent upward while walking so that the feathers don't drag on the ground. It's wings are either repurposed to be talons like those of eagles to catch prey while in flight, or simply reduced to vestigial limbs to make their figure more aerodynamic.
This sounds great but as always with biomechanics it's quite complicated to pull off. The entire pelvic area will be significantly different to allow powered flight, I have trouble imagining how it would work. There's also the fact that muscles for flight would be different from those for walking. Lastly a possible balance problem while in flight, how might the birds position themselves to pull this off?
Why do this? Because leg day is important.