Long-term Trends of Autumn-migrating Buteos Around Henrys Lake Flat, Idaho, USA

We conducted an annual late-summer raptor survey along a 53-km route around Henrys Lake Flats, Idaho, from 1994 to 2022, and pooled the resulting data with similar published information collected from 1974 to 1993. A mean of 36 Red-tailed Hawks (Buteo jamaicensis), 13 Ferruginous Hawks (Buteo regalis), and 16 Swainson’s Hawks (Buteo swainsoni) were observed each year along this route. The counts of Ferruginous Hawks declined significantly (P < 0.001) over the combined study period and were almost 10 times lower from 2013–2022 compared to 1974–1983. The counts of Red-tailed Hawks also declined significantly (P = 0.04), whereas no significant variation was evident for Swainson’s Hawks over the combined 49-yr study period. Summer precipitation levels and hawk numbers were not correlated. The observed numbers of Ferruginous Hawks and Red-tailed Hawks were correlated throughout the study period, suggesting similar responses to local environmental and/or land-use factors. Obvious changes in the study area during the corresponding 49 yr include conversion of some pastures to center-pivot-irrigated agriculture and a substantial expansion of all-terrain vehicle trails and recreational activity. Because 88% of Ferruginous Hawks breeding in the Columbia Basin funnel through the upper Snake River Plain and adjacent areas on their way to post-breeding ranges in the Great Plains, expanded surveys in these areas in late August/early September could effectively monitor this intermountain population.