Sacajawea Audubon wraps up 2023 Bridger raptor counts

If you’re an active birder, some interesting avian science happens every year right in MSU’s backyard. Each August through October, the Bridger Mountain Hawk Watch Project conducts annual counts of migrating raptors through the Rocky Mountain Flyway.

The migration route through the Bridgers was first observed by raptor enthusiast Fred Tilly in 1987, Vice President of Sacajawea Audubon John Edwards said. As a result of Tilly’s observations, HawkWatch International (HWI), a non-profit organization based in Salt Lake City set up a partial season count in 1991 to count and observe migrating raptors. A full-season, annual monitoring program commenced in 1992, he said.

In 2009, Montana Audubon – a statewide bird conservation organization – led the efforts to fund and coordinate the counts, and in 2017 the responsibility for physically conducting the counts was transferred to the Bozeman-based Sacajawea Audubon, according to HWI and Sacajawea Audubon’s 2022 Raptor Migration Report. 

HWI partners with both Montana and Sacajawea Audubon to compile count data and prepare the tables and graphs for the annual reports, according to the report.

The Rocky Mountain Flyway is a renowned migration route for species, according to the 2022 report. According to the National Park Service, a “flyway” is a north-south migration route for birds. Specifically, the Rocky Mountain Flyway starts in Canada and extends to southern Colorado. Common local species that travel the flyway include red-tailed hawks, golden eagles, bald eagles, the American kestrel and peregrine falcons.

The Rocky Mountain Flyway is renowned for its large migrating populations of golden eagles. To date, 18 raptor species have been recorded migrating through the Bridgers, with total autumn counts between 2,000-3,500 migrants, according to the 2022 report.

Daily observations each year run from Aug. 27 - Oct. 31 and documentation takes place from 9 a.m. - 5 p.m., Edwards said. The counts are only suspended if weather conditions such as low clouds, rain or snow prevents viewing. Observers are outfitted by Montana Audubon with 10 x

42 mm magnification binoculars and a Krotos HD 20-60 x 86 mm spotting scope for long-distance viewing of migrating birds. Each observer is positioned to monitor either the east or west side of the Bridger Ridge to ensure maximum detection, Edwards said.

Observers frequently scanned the sky, looking south towards Saddle Peak, the valley to the west and towards the Crazy and Beartooth mountains in the east. In addition, two decoy Great Horned owls are mounted to attract migrating raptors, he said. 

“Essentially, raptors are very territorial with respect to all other raptor species, so when any raptor species sees an owl they try to drive it out of their territory,” he said. Certain species, such as sharp-shinned and Cooper’s hawks, actually dive-bomb the owl decoys, allowing observers to identify them easier, Edwards said. 

On the ridge, observers recorded the age, sex, species and color morph of each migrating raptor, according to the 2022 report. In addition, the direction and altitude in which the migrant was flying, the total minutes it was observed, the relative human disturbance, the average number of observers present, and the observation’s start and end time completed each sighting data point, the report said.

Weather measurements are also taken during the observation period every hour on the half hour using a Kestrel 3500FW fire weather meter, Edwards said. When the measurements are taken, the observer holds the instrument in the direction of the wind. The instrument records wind speed, wind direction and atmospheric pressure. Temperature was also recorded using a Taylor thermometer placed vertically, he said.

Since raptors are biological indicators for ecosystem health, recording long-term population data is crucial and long-term monitoring projects located within the flyway are an efficient, cost-effective way to record this data, Edwards said. The 2022 Bridger crew counted 1,835 migrating individuals, which is about 25% lower than the long-term mean at that site, according to the 2022 report. The raptor species with below average observations in the 2022 season included the bald eagle, golden eagle, merlin, northern goshawk, prairie falcon, rough-legged hawk and sharp-shinned hawk, the report said.

2023 counts are ongoing so this year’s data won’t be available until spring. “It is critical that we continue to conduct annual counts in the Bridger Mountains so that long-term raptor population trends can be assessed and interpreted with increasing statistical power as the increasing number of years continues to grow,” Edwards said. 

If you’re interested in volunteering for the Bridger raptor counts next year, feel free to visit https://sacajaweaaudubon.org/project/bridger-raptors/ for more information. Although it’s helpful to have a background in biology and bird identification, observers are taught raptor ID and the beginning of every survey season.