( Generally ( this ( ( would ( happen ( in ( ( the case ) ( of ( ( a ( young pair ) ) ( of ( birds ( that ( ( mated but ) ( bred unsuccessfully ) ) ) ) ) ) ) ) ) ) ) . ) ) )
(ROOT (S (ADVP (RB Generally)) (NP (DT this)) (VP (MD would) (VP (VB happen) (PP (IN in) (NP (NP (DT the) (NN case)) (PP (IN of) (NP (NP (DT a) (JJ young) (NN pair)) (PP (IN of) (NP (NP (NNS birds)) (SBAR (WHNP (WDT that)) (S (VP (VP (VBN mated)) (CC but) (VP (VBD bred) (ADVP (RB unsuccessfully)))))))))))))) (. .)))
Generally this would happen in the case of a young pair of birds that mated but bred unsuccessfully.
(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.