( But ( ( unlike ( any ( of ( our ( other ( common birds ) ) ) ) ) ) ( , ( ( ( their ( nonbreeding territory ) ) ( -LRB- ( winter -RRB- ) ) ) ( ( ( ( is occupied ) and ) ( ( defended ( by ( a ( ( flock and ) not ) ) ) ) ( by ( ( ( an ( individual bird ) ) or ) ( mated pair ) ) ) ) ) . ) ) ) ) )
(ROOT (S (CC But) (PP (IN unlike) (NP (NP (DT any)) (PP (IN of) (NP (PRP$ our) (JJ other) (JJ common) (NNS birds))))) (, ,) (NP (NP (PRP$ their) (JJ nonbreeding) (NN territory)) (PRN (-LRB- -LRB-) (NP (NN winter)) (-RRB- -RRB-))) (VP (VP (VBZ is) (ADJP (VBN occupied))) (CC and) (VP (VBD defended) (PP (IN by) (NP (DT a) (NN flock) (CC and) (RB not))) (PP (IN by) (NP (NP (DT an) (JJ individual) (NN bird)) (CC or) (NP (JJ mated) (NN pair)))))) (. .)))
But unlike any of our other common birds, their nonbreeding territory (winter) is occupied and defended by a flock and not by an individual bird or mated pair.
(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.