( ( The sharks ) ( ( feed ( on ( ( the birds ) ( who ( ( ca n't ) ( stay aloft ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT The) (NNS sharks)) (VP (VBP feed) (PP (IN on) (NP (NP (DT the) (NNS birds)) (SBAR (WHNP (WP who)) (S (VP (MD ca) (RB n't) (VP (VB stay) (ADVP (RB aloft))))))))) (. .)))
The sharks feed on the birds who can't stay aloft.
(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)))))) (. .)))