( ( For ( the Romans ) ) ( , ( ( this holiday ) ( ( marked ( ( the ( official start ) ) ( of ( bird ( and ( human ( mating season ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (PP (IN For) (NP (DT the) (NNPS Romans))) (, ,) (NP (DT this) (NN holiday)) (VP (VBD marked) (NP (NP (DT the) (JJ official) (NN start)) (PP (IN of) (NP (NN bird) (CC and) (JJ human) (NN mating) (NN season))))) (. .)))
For the Romans, this holiday marked the official start of bird and human mating season.
(ROOT (S (NP (NNP Birds)) (VP (VB pair) (PRT (RP up)) (PP (IN with) (NP (NP (DT the) (JJ same) (NN bird)) (PP (IN in) (NP (NN mating) (NN season)))))) (. .)))
Birds pair up with the same bird in mating season.