( But ( ( during ( the winter ) ) ( they ( ( ( ( ( eat mainly ) ( plant material ) ) , ) ( only ( ( eating ( ( clams and ) snails ) ) ( when ( ( plant material ) ( is scarce ) ) ) ) ) ) . ) ) ) )
(ROOT (S (CC But) (PP (IN during) (NP (DT the) (NN winter))) (NP (PRP they)) (VP (VBP eat) (ADVP (RB mainly)) (NP (NN plant) (NN material)) (, ,) (S (ADVP (RB only)) (VP (VBG eating) (NP (NNS clams) (CC and) (NNS snails)) (SBAR (WHADVP (WRB when)) (S (NP (NN plant) (NN material)) (VP (VBZ is) (ADJP (JJ scarce)))))))) (. .)))
But during the winter they eat mainly plant material, only eating clams and snails when plant material is scarce.
(ROOT (S (NP (DT The) (NN heterotroph)) (VP (VBZ eats) (NP (UCP (ADJP (JJ only)) (CC or) (ADVP (RB mainly))) (NNS plants))) (. .)))