a nest contains eggs & birds sit in nests ->  birds sit on eggs  birds sit on eggs & sitting on something provides warmth to that something ->  a bird sitting on an egg provides warmth to that egg  an egg may contain a young animal & an animal requires warmth for survival ->  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