( We ( ( ( returned ( the birds ) ) ( to ( ( ( their owner ) , ) ( who ( stated ( that ( ( the pair ) ( ( flew away ) ( from ( ( his ( large flock ) ) ( due ( to ( mating season ) ) ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP We)) (VP (VBD returned) (NP (DT the) (NNS birds)) (PP (TO to) (NP (NP (PRP$ their) (NN owner)) (, ,) (SBAR (WHNP (WP who)) (S (VP (VBD stated) (SBAR (IN that) (S (NP (DT the) (NN pair)) (VP (VBD flew) (PRT (RP away)) (PP (IN from) (NP (NP (PRP$ his) (JJ large) (NN flock)) (ADJP (JJ due) (PP (TO to) (NP (NN mating) (NN season))))))))))))))) (. .)))
We returned the birds to their owner, who stated that the pair flew away from his large flock due to 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.