Somescientistsbelieve.thatbirdssingsomeofthetimejustbec...
问题详情：Somescientistsbelievebirdssingsomeofthetimejustbecausetheyarehappy.However,theysingmostofthetime 61 averydifferentreason.Theirsingingis 62 (actual)awarningtootherbirdstostayoutofth...