( However ( , ( ( ( not ( all pairs ) ) ( of birds ) ) ( ( mate ( ( often or ) ( at all ) ) ) . ) ) ) )
(ROOT (S (ADVP (RB However)) (, ,) (NP (NP (RB not) (DT all) (NNS pairs)) (PP (IN of) (NP (NNS birds)))) (VP (VBZ mate) (UCP (ADVP (RB often)) (CC or) (PP (IN at) (NP (DT all))))) (. .)))
However, not all pairs of birds mate often or at all.
(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.