( ( Members ( of ( two ( different ( bird species ) ) ) ) ) ( ( ( mate and ) ( produce ( ( viable fertile ) offspring ) ) ) . ) )
(ROOT (S (NP (NP (NNS Members)) (PP (IN of) (NP (CD two) (JJ different) (NN bird) (NNS species)))) (VP (VP (VB mate)) (CC and) (VP (VB produce) (NP (ADJP (JJ viable) (JJ fertile)) (NN offspring)))) (. .)))
Members of two different bird species mate and produce viable fertile offspring.
(ROOT (S (SBAR (IN If) (S (NP (CD two) (NNS populations)) (VP (VBP do) (RB not) (VP (VB mate) (CC and) (VB produce) (NP (ADJP (JJ fertile)) (NN offspring)))))) (, ,) (NP (JJ separate) (NNS species)) (VP (VBP result)) (. .)))