An estimated 3,700 bison comprise the two herds that roam Yellowstone National Park, according to information the park service released Tuesday.

That number is down from last year, when there were an estimated 3,900 bison.

But, according to the park service, the rate of that population change is “within the natural range of expectation for wild bison.”

The population monitoring was conducted by park employees and contractors flying in planes above the animals. The Northern Range herd was found to have an estimated 2,300 bison, while the Central Interior herd includes an estimated 1,400.

Al Nash, park spokesman, said one of the most notable differences in this year’s population report is that there’s been a shift in the herds’ sizes. Five to seven years ago, the Central Interior herd was significantly larger than the Northern Range herd. Now, the opposite is true.

“Of all the numbers that we’re sharing about this summer estimate, the component most likely to come across to longtime observers as a change compared to past years is this distribution between the two herds,” Nash said.

Nash said he did not know why the distribution has changed, and he was waiting to consult with park biologists to find out why.

This year, at least 583 bison were born, according to the park service.

This winter, an estimated 500 bison died from various causes. Nash said that number does not include bison “removed from the herd by state permitted and tribal hunters.”

Nash also noted that no bison have been sent to slaughter this year. Slaughtering bison is a management practice sometimes employed to control the animals when they migrate outside park boundaries during the winter.

Buffalo Field Campaign, a nonprofit bison advocacy group, counts the bison killed each winter by unnatural causes, such as those killed during hunts or on roads. This past winter, the group counted 230 bison killed, according to the campaign’s website.

Darrell Geist, habitat coordinator for the campaign, said the group didn’t have any data to doubt the population numbers the park presented, but he said the information was “alarming.” In past years, the bison population has been significantly larger, Geist said.

Yellowstone’s peak bison population estimate was 5,000, recorded in the summer of 2005, according to the park service. The lowest population recorded was in 1902, when there were fewer than two dozen bison.

Carly Flandro may be reached at 582-2638 or cflandro@dailychronicle.com.

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