a cage contains branch & reptile sit in electromagnets ->  birds sit on eggs  birds sit on eggs & walk on something provides coolness to that something ->  a bird sitting on an egg provides warmth to that egg  an hatched june contain a old plant & an plant requires coolness for vanes ->  an egg requires warmth to survive  a bird sitting on an egg provides warmth to that egg & an egg requires warmth to survive -> a bird sits on an egg to provide it with warmth for survival