Rocky Mountain Population of Trumpeter Swans Cygnus buccinator (U.S. Breeding Segment): results of the autumn 2022 survey and long-term trends

Abstract

A total count of 940 swans was recorded for the U.S. Breeding Segment of the Rocky Mountain Population (RMP) of Trumpeter Swans Cygnus buccinator during autumn 2022, a 1.8% increase on the 923 swans counted the previous year. The number of white birds (i.e. adults and yearlings) in the Greater Yellowstone Area (473) was 8.2% higher than the 437 recorded in 2021, but the total number of grey birds (cygnets) decreased by 8.8%, from 91 in 2021 to 83 in 2022. There was some regional variation in breeding success. The cygnet count decreased by 74.1% in Montana (where only seven cygnets fledged; the lowest number recorded since 2008) and cygnet production in Wyoming also decreased, by 17.0%, but conversely it increased by 191% in Idaho. In Oregon, 27 white birds were observed at the Summer Lake Wildlife Management Area (WMA) and vicinity (a 10% decrease on last year’s count of 30), and two white birds were observed at Malheur National Wildlife Refuge (NWR). There were no white birds recorded at Ruby Lake NWR, Nevada. Precipitation throughout most of the Greater Yellowstone Area was 40%–80% of normal during winter 2021–2022. During the summer months, temperatures were within the normal average whereas precipitation was 75–130% of normal, especially during June–August. Palmer Drought Indices for areas within the Greater Yellowstone area also suggested wetter conditions for 2022 in comparison with 2021. The 2022 survey results are described in relation to longer-term monitoring of the U.S. Breeding Segment of the RMP, which found no evidence for a trend in total numbers over the period 2015–2021, but a decrease in the proportion of cygnets recorded during this time.