Laramie, WY – Yellowstone National Park officials estimate the number of bison in the park has grown to 3,900, an increase of 600 animals since last summer. The annual estimate released Tuesday says calves represent 15 percent of the population. Park officials say the population change is within the natural range for wild bison. The peak population estimate was 5,000 bison in the summer of 2005. The estimate is based on a series of aerial surveys conducted in June and July.