There only found during SE Heatwaves but when you find one wait till event ends trap around its remains ( they spawn at that place when the event starts back up) then when it spawns again flamethrower it until it tames or just throw a oil jar under it and light it if you don't have a flamethrower

This is another way if you don't wanna wait for the heatwave again get a flier with a taming box (preferably a Quetzal) and chase the flame bird until it gets stuck in the box make sure it can't escape and flamethrower it or do the second option I put

Hope this helps

-Luz

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