Whether it was an easy spring, fewer wolves preying on them, or simply a good year, the bison herds in Yellowstone National Park have grown by about one-third since winter, according to park officials.

Latest estimates from park biologists put the herds' population at 3,900, up from the 3,000 estimate from the winter survey. The summer survey was conducted from the air in June and July.

According to the park, "this year’s calf production represents 15 percent of the population. Fifty-six percent of the bison are distributed across the Northern Range this summer, while the remainder are found in the Central Interior herd."

A year ago the park estimated that there were 3,300 bison in the two herds. The peak population was estimated during the summer of 2005 when it was put at 5,000 animals.

The observed rate of population change this past year is within the natural range of expectation for wild bison, said park officials. The rate at which wildlife populations increase in abundance is a reflection of the combined effects of reproduction and mortality, and is heavily influenced by age structure of the population, and habitat conditions encountered over the course of time, they added.