( food ( ( is ( ( the ( main reason ) ) ( why ( birds ( ( fly ( down south ) ) ( during ( the winter ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NN food)) (VP (VBZ is) (NP (NP (DT the) (JJ main) (NN reason)) (SBAR (WHADVP (WRB why)) (S (NP (NNS birds)) (VP (VBP fly) (ADVP (RB down) (RB south)) (PP (IN during) (NP (DT the) (NN winter)))))))) (. .)))
food is the main reason why birds fly down south during the winter.
(ROOT (S (NP (NP (JJ Many) (NNS species)) (PP (IN of) (NP (NP (NNS birds)) (PP (IN in) (NP (JJ new) (NN england)))))) (VP (VBP fly) (ADVP (RB south) (PP (IN for) (NP (DT the) (NN winter) (NNS months)))) (S (VP (TO to) (VP (VB find) (NP (DT an) (NN environment)) (PP (IN with) (NP (JJR more) (NN food))))))) (. .)))