Eager bird watchers, as many as 200 of them, have flocked to Marin County in hopes of seeing a rare brown shrike that is spending its winter vacation in some local berry bushes. "This is certainly one of the rarest birds that will turn up this year in North America outside of the Aleutian Islands," said Dan Murphy, a member of the Golden Gate Chapter of the Audubon Society. The tiny bird's natural habitat is bushy, wooded areas along trails in northeast Asia. It usually migrates during fall and winter. The bird now is apparently living near Paper Mill Creek in Marin County. Stephen Bailey, head of the birds and mammals collections at the California Academy of Sciences at Golden Gate Park, suspects that the bird accidentally followed a path down Alaska and then the eastern part of the Pacific instead of the western. He said the bird could have been thrown off course by a storm or by a malfunction in its natural tracking system.