Bison population surveys in Yellowstone National Park (USA): 1970-1997

This study presents data from aerial surveys conducted in Yellowstone
National Park spanning from 1970 to July 1997, aimed at monitoring the
bison population. Surveys were initially conducted four times per year
from 1970 to 1990, with a consistent pilot and observer, weather
permitting. In 1991, the frequency increased to 9-10 surveys per year.
Ideal flying conditions required clear-to-partly-cloudy skies and minimal
wind, typically in the early morning to avoid stronger winds later in the
day. The flight path commonly began in the northern range and proceeded
sequentially southward, though weather conditions such as wind or ground
fog sometimes necessitated route adjustments. In instances where surveys
could not be completed in a single day, the remaining areas were surveyed
the following day, and data were combined. During the early years, data
from an elk researcher were sometimes used for the northern range when
timing allowed, minimizing overlap in efforts. The objective of the
surveys was to locate and count all visible bison, excluding those
wandering outside population-use areas. A sharp-eyed pilot made the
initial count of bison and calves, while the observer cross-checked,
noting group sizes, animal locations, and any significant observations,
such as mortality or unusual habitat factors. Data were recorded manually
and later transcribed to field notebooks. Location descriptions were
converted into UTM coordinates using the 1961 U.S. Geological Survey
Topographic Map of Yellowstone National Park with a 10 km UTM grid
overlay.