( ( These birds ) ( ( ( ( are polgynous ) , ) ( averaging ( ( 3 mates ) ( per ( breeding season ) ) ) ) ) . ) )
(ROOT (S (NP (DT These) (NNS birds)) (VP (VBP are) (ADJP (JJ polgynous)) (, ,) (S (VP (VBG averaging) (NP (NP (CD 3) (NNS mates)) (PP (IN per) (NP (VBG breeding) (NN season))))))) (. .)))
These  birds are polgynous, averaging 3 mates per breeding 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.