Osprey Population Sites
This indicator measures the number of active osprey nests in the Tahoe Region. The osprey (Pandion haliaetus) is a conspicuous large raptor that is valued for wildlife viewing. Human disturbance near nesting and foraging areas can impact osprey breeding success. Regional programs that limit forest fire fuel reduction activities around known nests and preserve large snags aim to protect the osprey population. TRPA actively monitors osprey populations across the Tahoe Region with various partners.
Status
Number of active osprey nests recorded in the Tahoe Basin from 1997 through 2023. Threshold standard is four population sites. The trendline shown was estimated using a linear regression model.
Data provided by the U.S. Forest Service, Nevada Division of Wildlife, California Department of State Parks, and TRPA. Access detailed datasets on Tahoe Open Data, including: Osprey nest locations with buffer and Annual Data.
Evaluation Map
Appoximate locations of all recorded osprey nests in the Tahoe Basin (2015 through 2023).
2023 Evaluation
- Status
- Considerably Better Than Target
- Trend
- Moderate Improvement
- Confidence
- Moderate
Applicable Standard
W2: Provide a minimum of 4 Osprey population sites.
W9: Provide disturbance zones and influence zones for Osprey.
Key Points
- The osprey population in the Tahoe Basin has experienced significant growth since the early 1980s, when only a few nests were active.
- In the last five years, the number of active nests has fluctuated but remained stable.
- Key reproductive areas for osprey include the shoreline from Emerald Bay to DL Bliss State Park, as well as the undeveloped sections of the East Shore.
- The TRPA, along with partner agencies, has established a disturbance zone standard to safeguard osprey nesting sites.
- To protect nesting osprey, potential disturbances from new trails and forest fuel reduction projects are carefully managed and adjusted as necessary.
About the Threshold
The osprey (Pandion haliaetus) is a conspicuous large raptor that is valued for wildlife viewing. Ospreys are protected by the Migratory Bird Treaty Act, which prohibits the harming or killing of the species. This provides additional protection when analyzing the effects of a proposed project. The number of active osprey nests is an indicator of the osprey population density in the Tahoe Basin.
Human disturbance near nesting and foraging areas can impact osprey breeding success. Osprey may be able to habituate to human activity depending on the timing, type, and consistency of the activity (J. Shane Romsos 2000b; Ewins 1997). Osprey populations could be limited by the number of large nest trees near water and open areas or competition with bald eagles or other species (Ewins 1997). However, given the limited number of bald eagles present during the breeding season, and the existing protections for large trees, these are not likely to be major limiting factors in the basin. Osprey that breed in the basin likely migrate to Central or South America for the winterr (Martell et al. 2001; J. Shane Romsos 2000b). Osprey breeding in the basin may be affected by a variety of factors in their wintering areas or along migration routes, including contamination from organochlorines (e.g. DDT), which is still used in parts of their wintering grounds(J. Shane Romsos 2000b).
Delivering and Measuring Success
EIP Action Priorities
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Acquire Environmentally Sensitive Lands for Restoration and Protection
Public land acquisition preserves open and wild spaces.
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Restore and Protect Native Fish and Wildlife
Enhancing the biologic integrity of ecosystems to improve the environmental conditions necessary for the full range of species to help compensate for past degradation.
Example EIP Projects
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Emerald Bay SP UC Davis Osprey Spatial Data Analysis
CA State Parks and UC Davis analyzed 30 years of osprey nesting data to look at reproductive success in relation to distance from potential disturbance factors (roads, trails, etc.).
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Special Status Species Monitoring and Investigation Program
Ongoing wildlife survey and monitoring program for the many sensitive wildlife species on the Nevada side of Lake Tahoe.
Monitoring Programs
Rationale Details
- Considerably Better Than Target. Status is based on the average number of active nests during the evaluation period (2020-2023). Between 2020-2023, an average of 24 active nests were observed, which is 600% of the target. Therefore, the current status is considerably better than target.
- Moderate Improvement. Population trend was estimated using an EGPN (exponential growth process noise) model (Humbert et al. 2009). Process noise refers to differences in observed population abundance that result from environmental variability. From 1997-2023, the annual growth in the osprey population is estimated to be 2.2%. Therefore, the trend is determined to be moderate improvement.
Similar results are found using simple linear regression to estimate trend. The results for this analysis show annual growth of 0.26 osprey per year. The EGPN model was chosen for the trend determination because it is a more conservative model and incorporates environmental and observation errors associated with wildlife monitoring on a landscape scale. The threshold attainment results are the same with both analyses.
Confidence Details
- High. Monitoring was conducted using widely accepted protocols, observation error for nests is considered low, and all potential nesting areas are surveyed.
- Moderate. The estimated growth is likely positive, but the 95% confidence interval includes the possibility of population decline. Using the EGPN trend analysis the osprey population is estimated to be growing at 2.2% annually. The lower bound of the 95% confidence interval is -2.6% annual growth and the upper bound is 6.5% annual growth. The results for the linear regression analysis show an annual growth of .27 osprey/ year with a 95% confidence interval of 0.081-0.45.
- Moderate. If one confidence rating is high and the other is moderate, the overall confidence rating is the lower confidence rating.
Additional Figures and Resources
No photos available.
- Uploaded On
- 8/30/2024
- File Type
- Excel (XLSX)
- Description
- Osprey nest EGPN (exponential growth process noise) model output.