(S (S (BASENP (PRP They)) (VP (VBD showed) (SBAR (IN that) (S (NP (BASENP (JJ local) (NNS sources)) (PP (IN of) (BASENP (NN pollution)))) (VP (VP (VBP are) (ADJP (JJ rare))) (COMMA ,) (S (VP (VBG meaning) (SBAR (IN that) (S (BASENP (NNS seabirds)) (VP (VBD played) (NP (BASENP (DT a) (ADJP (RB proportionally) (JJR greater)) (NN role)) (PP (IN in) (BASENP (JJ local) (NN pollution)))))))))))))) (PERIOD .))