For a demonstration I will use the common Bird Pokemon Niche

gen one kanto PIDGEY = on the ground in the sun on the plains is chubby good at running away from its predators and ambushing its prey 

gen two HOOTHOOT = in the trees has large eyes and is nocturnal with a tiny beak and has rear limbs like clock hands and even has a clocklike body

gen three TAILLOW = its on an ilsand has broad and narrow wings for catching island currents 

gen four STARLY = up on a high mountaintop, its all in literally black and white for its plumage

gen five PIDOVE = evolved around a city like structure, most likely, Castelia city with humans ergo its gray and stone like

gen six FLETCHLING = is bult for france and nordic territory, has a red head and grayish body and is even tinier in size as compared to the gen one Pidgey