Osprey, peregrine falcon, and bald eagle nesting in North Cascades National Park Service Complex, Washington

Christophersen, Roger; Ransom, Jason

doi:10.36967/nrr-2289987

Osprey (Pandion haliaetus), bald eagle (Haliaeetus leucocephalus), and peregrine falcon (Falco peregrinus) are ecologically important bird species that suffered dramatic population declines during the 1950s through 1970s. Those declines were fueled by accumulating high levels of environmental contaminants, such as lead and pesticides. Such contaminants can have adverse impacts on raptors, including thinning of eggshells that leads to poor reproductive success. Reproductive performance of these species has been used as an indicator of the health of populations. Since the banning of the pesticide dichlorodiphenyltrichloroethane (DDT) in 1974, populations have slowly rebounded. Information relating to their recovery and current status has prompted continued monitoring of these species in many states across the United States, and within North Cascades National Park Service Complex, Washington. Study areas for this report included the Ross Lake and Lake Chelan National Recreation Areas of the park complex. We conducted surveys for nesting osprey, peregrine falcon, and bald eagle from 1979–2020, 2006–2020, and 2015–2020, respectively. We assessed reproductive output by annually conducting an early nesting survey in mid-April to determine nest occupancy, followed by at least two surveys during late season nesting in early to late July to determine reproductive output. Additional surveys were sometimes necessary during the late nesting period to accurately assess nest occupancy and productivity, depending on individual nesting circumstances. Successful nesting rates were measured by the number of successful nests per occupied nests during the early nesting period. Productivity was measured by the number of young seen on the nest, regardless of age. We surveyed 1–13 osprey nesting territories annually, and found an occupancy rate of 73%, which is consistent with other populations that are stable; however, the successful nesting rate was only 33%, which is 10–35% lower than success rates reported for other populations. Successful nests averaged 1.4 young per year, which is within the normal range of mean productivity rates reported in western North America (1.3–1.8 young/nest). The percentage of osprey nest sites occupied, and nesting success, both appeared to increase gradually from 1979–2020. We surveyed 3–7 peregrine falcon nesting territories annually, and found an occupancy rate of 72.5%, which is slightly less than 79–82% occupancy reported elsewhere in Washington. The successful nesting rate for peregrine falcons in the park complex was 51%, and other recovering populations ranged 62–83%. Productivity averaged 1.3 young per nest, which is within the range suggested for a sustainable population with potential for growth (1.25–1.5 young fledged per pair). Since 2009, the number of occupied peregrine falcon territories, successful nests, and peregrine falcon productivity in the park complex exhibited a notable decline. We surveyed 3–4 bald eagle nesting territories annually, and found an occupancy rate of 47%. This is notably less than a statewide survey in 2005 that reported an occupancy average of 75%. The successful nesting rate in the park complex was 50%, and a minimum level of 45% nest success has been suggested as the threshold for populations to remain stable. Bald eagle nest productivity in in the park complex averaged 0.95 young per occupied nest, which is less than the average 1.2 young per nest reported for the rest of the United States. Although the time series is short, the percent of occupied nests and nest success both declined from 2015–2020.

Osprey, peregrine falcon, and bald eagle nesting in North Cascades National Park Service Complex, Washington

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