Complex sentence: It is estimated that the American Crow population has dropped by up to 45% since 1999; the disease runs most rampant in the subtropical conditions which encourage reproduction of its mosquito vectors among which "Culex tarsalis" is most significant.
Split into simpler sentences: It is estimated that the American Crow population has dropped by up to 45% since 1999. The disease runs most rampant in the subtropical conditions which encourage reproduction of its mosquito vectors among which "Culex tarsalis" is most significant.