Wintering Bald Eagle Population Sites
This indicator addresses the protection of wintering bald eagle habitat in the Tahoe Region. To assess this indicator, the population of the wintering bald eagle population is used as a surrogate measure of wintering habitat. The bald eagle (Haliaeetus leucocephalus) is the national symbol and an iconic species that is exceptionally important to the public. Many bald eagles wintering in the Tahoe Region migrate from other breeding areas. Their winter abundance in the Tahoe Region can be influenced by a variety of factors including availability of food, intensity of recreational acitivies nearby, and factors in their home breeding areas. Regional programs such as prohibiting snowmobile use in winter habitat areas aim to protect wintering bald eagles in the Tahoe Region. The annual winter bald eagle survey measures the total number of bald eagles observed throughout the Tahoe Region on a single day during the winter months.
Status
Total number of bald eagles detected during annual winter bald eagle surveys throughout the Tahoe Basin from 1998 through 2023. Threshold standard is 2 eagles. The trendline shown was estimated using the linear regression model.
Data is provided by the Tahoe Institute for Natural Sciences. Access detailed datasets on Tahoe Open Data, including: Bald Eagle Winter Habitat and Annual Wintering Bald Eagle Data (This data is defined as "Bald Eagle- winter" under Wildlife Species column).
Evaluation Map
Areas designated by TRPA for the protection of wintering bald eagle populations in the Tahoe Basin.
2023 Evaluation
- Status
- Considerably Better Than Target
- Trend
- Rapid Improvement
- Confidence
- High
Applicable Standard
W3: Provide a minimum of 2 Bald Eagle (Winter) population sites.
W10: Provide disturbance zones and influence zones for Bald Eagle (Winter).
Key Points
- In 2021, 42 individuals were counted during the annual wintering bald eagle count organized by Tahoe Institute for Natural Sciences. The 42 individuals was the highest number ever recorded in Tahoe.
- The wintering bald eagle population in Lake Tahoe has increased substantially since monitoring began in the 1990s. The number of wintering bald eagles has increased on average by 0.85 individuals per year.
- The number of observed individuals is well above the threshold target of 2 wintering eagles.
- Protective actions, such as prohibiting snowmobiles in the Taylor-Tallac marsh complex, have been implemented to safeguard key bald eagle over-wintering areas in the Basin.
- TRPA and other partner agencies have implemented a disturbance zone standard for bald eagle where the winter bald eagle habitat is protected from manipulation unless it is done so to enahance the quality of the habitat.
About the Threshold
- The bald eagle (Haliaeetus leucocephalus) is the national symbol and an iconic species that is exceptionally important to the public. In the lower 48 states, bald eagles were considered threatened under the Endangered Species Act until they were de-listed in 2007. They are still protected by the federal Bald and Golden Eagle Protection Act and Migratory Bird Treaty Act, which prohibits the harming, killing, or possession of bald eagles, or their eggs or nests. The annual winter bald eagle survey measures the number of bald eagles observed in the Tahoe Basin on a single day during the winter months.
- Many bald eagles wintering in the Tahoe Basin have likely migrated from other breeding areas (J. Shane Romsos 2000a; Linthicum et al. 2007). Their winter abundance in the Tahoe Basin can be influenced by a variety of factors in their breeding areas or along their dispersal routes. The availability of spawning Kokanee salmon in Tahoe’s tributaries, or other food sources, may affect the abundance of wintering bald eagles (Laves and Romsos 2000; Reed 1979). The intensity and location of recreational activities can affect bald eagles, although resident eagles may become habituated to regular recreational activities (Laves and Romsos 2000; Brown and Stevens 1997; Buehler et al. 1991). In other areas, the structure of wintering habitat, including the size, location, and the number of suitable perch trees has been shown to affect wintering bald eagle abundance (Stohlgren 1993). Finally, abnormally warm winter temperatures have been shown in studies to entice more eagles to overwinter in higher elevation areas, which may account in part for the recent significant upward trend in total individuals observed (Kaltenecker 2000).
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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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 was determined based on the number of bald eagles counted during the annual winter bald eagle count. The average number of wintering bald eagles during the period since the last evaluation (2020-2023) was 29.5, considerably better than the target of two.
- Rapid 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 wintering bald eagle population is estimated to be 4.4%. Therefore, the trend is determined to be rapid improvement.
Similar results are found using simple linear regression to estimate trend. The results for this analysis show annual growth of 0.8 eagles 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. Surveys have been conducted using standardized protocols and a long period of data exists. Policies have been implemented to protect wintering habitat areas.
- High. Using the EGPN trend analysis the wintering bald eagle population is estimated to be growing at 4.4% annually. The lower bound of the 95% confidence interval is 0.07% annual growth, and the upper bound is 9.3% annual growth. The results for the linear regression analysis show annual growth of 0.8 eagles per year with a 95% confidence interval of 0.55-1.149.
For both methods, the entire range of values within the 95% confidence interval is positive and the p-value is <0.05, there is high confidence that the wintering bald eagle population has grown. - High.
Additional Figures and Resources
No photos available.
- Uploaded On
- 8/30/2024
- File Type
- Excel (XLSX)
- Description
- Winter Bald Eagle EGPN (exponential growth process noise) model output.