( Birds ( ( ( stay aloft ) ( by ( riding ( rising ( air currents ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Birds)) (VP (VBP stay) (ADVP (RB aloft)) (PP (IN by) (S (VP (VBG riding) (NP (VBG rising) (NN air) (NNS currents)))))) (. .)))
Birds stay aloft by riding rising air currents.
(ROOT (S (NP (NNP Birds)) (VP (VBP need) (NP (DT a) (JJ light-weight) (NN body)) (PP (IN for)) (S (VP (TO to) (VP (VB stay) (ADVP (RB aloft)))))) (. .)))