TOEFL Reading: ETS-TOEFL阅读机经 - 1385JL5I93P133ERL$

In paragraph 3, the author points out that San Francisco white-crowns establish their territories in the area in which they are born in order to explain which of the following A. Why they practice counter-singing B. Why they get better territories than white-crowns that establish territories in areas in which they are not born C. Why they are more competitive than white-crowns raised in captivity D. Why in their natural habitat they start singing earlier than white-crowns raised in captivity