( Some ( ( ( ( think ( it ( is ( because ( ( of ( the ( formic acid ) ) ) ( ( some ants ) secrete ) ) ) ) ) ) , ) ( since ( ( the ants ) ( ( the ( birds use ) ) ( are ( ( the type ) ( which ( secrete ( the acid ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT Some)) (VP (VBP think) (SBAR (S (NP (PRP it)) (VP (VBZ is) (SBAR (IN because) (S (PP (IN of) (NP (DT the) (JJ formic) (NN acid))) (NP (DT some) (NNS ants)) (VP (VBP secrete))))))) (, ,) (PP (IN since) (NP (NP (DT the) (NNS ants)) (SBAR (S (NP (DT the) (NNS birds) (NN use)) (VP (VBP are) (NP (NP (DT the) (NN type)) (SBAR (WHNP (WDT which)) (S (VP (VBP secrete) (NP (DT the) (NN acid)))))))))))) (. .)))
Some think it is because of the formic acid some ants secrete, since the ants the birds use are the type which secrete the acid.
(ROOT (S (NP (JJ Formic) (NN acid)) (VP (VBZ is) (VP (VBN found) (PP (IN in) (NP (NP (DT the) (NNS secretions)) (PP (IN of) (NP (JJ stinging) (NNS ants))))))) (. .)))