The hotter the flame sac on its belly, the faster it can fly, but it takes some time to get the fire going.
(From Bulbapedia) Fletchinder is an avian Pokémon with predominantly orange and black plumage. It has a sharp, black beak and a small, pointed crest atop its head. Its black eyes have yellow, mask-like marks extending from the back. It upperside is reddish-orange and its belly is gray, with a flame pattern marking the boundary on the breast. Its pointed wings are black with yellow tips and undersides. Its legs are also black, with sharp talons with two toes forward and one back. Its long black tailfeathers are marked with two white "v"-shapes. The tail ends in two points and has another feather sticking out of the base on each side, giving the impression of the fletching on the end of an arrow.
Fletchinder has a flame sac in its belly. As its internal fire gradually heats up, it can fly at increasingly faster speeds. Fletchinder is known to hunt by igniting tall grass and swooping down on prey that attempt to escape the blaze. It is territorial, even against its own kind.