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Home Land Upland Species and Communities

Wintering Bald Eagle

Status and Trend

Interpretation and Commentary

WINTERING BALD EAGLE


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Status: No Established Target
Trend: Moderate Improvement
Confidence: Low

 

  • Relevance - 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 prohibit the harming, killing or possession of Bald Eagles, or their eggs or nests. The annual winter Bald Eagle survey measures the total number of Bald Eagles observed throughout the Tahoe Basin on a single day. These data are one indicator of the abundance of wintering Bald Eagles in the Tahoe Basin.
  • Adopted Standards  - No Threshold Standard has been adopted for the number of individuals counted during the annual winter Bald Eagle survey, although the Threshold Standard directs the Region to maintain two “population sites”.
  • Indicator - The total number of individual Bald Eagles detected during annual winter surveys. This indicator provides information that is relevant to the adopted Threshold Standard of maintaining two wintering Bald Eagle population sites because it indicates whether there is evidence that Bald Eagle winter in the Region.
  • Status – The long-term (1998-2011), and five-year (2007-2011) average number of Bald Eagles detected are 11.6 and 12.0, respectively. Data from 1979 -1997 exists, but this data is not comparable because it was not controlled for individuals that may have been counted more than once (TRPA 2007c). Two sites, Upper Truckee Marsh and Taylor/Tallac Marsh, extending into Emerald Bay are currently managed for the protection of the wintering Bald Eagle population (see map below).
  • Trend –  Since 1998, the total number of Bald Eagles observed during the annual winter survey has fluctuated between 6 and 18 individuals, and may exhibit cyclical variations. A linear regression showed an increasing trend in the number of Bald Eagles detected over the long-term (R2 = 0.68), but this trend was not statistically significant (P = 0.37). The five-year trend also showed a slightly increasing number of Bald Eagle detections (R2 = 0.54), but this trend was also not statistically significant (P = 0.16).

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  • Confidence  - There is low confidence in the reliability of the survey data because, although it is collected following accepted protocols developed by the National Wildlife Federation, it represents only a very brief snapshot in time that may not accurately represent eagle abundance. The confidence in the trend is low for the long range and short range analysis because the regression line was not significantly different from zero and there is a fair amount of variation in the scatter of points around the regression line. There is not, however, any indication of a declining trend. Therefore, the overall confidence in the status and trend is low.
  • Human and Environmental Drivers - Many Bald Eagles wintering in the Tahoe Basin, have likely migrated from other breeding areas (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 wintering 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 number of suitable perch trees has been shown to affect wintering Bald Eagle abundance (Stohlgren 1993).
  • Monitoring Approach – Professional and volunteer biologists stationed at a series of observation points surrounding Lake Tahoe record all observed eagles over the same four-hour period following protocols developed by the National Wildlife Federation.
  • Monitoring Partners – The US Forest Service funds the coordination of the survey and data management. Numerous agencies and volunteers provide in-kind support for the annual winter Bald Eagle survey.

Links

 
  • Monitoring Plan
  • Conceptual Model

Map

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Approximate locations of known Bald Eagle nesting areas from 1996 to 2010. Source: USFS and TRPA Survey Records

 

Trend Charts

Chart data not available at this time.
 

Additional Info

References

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Additional Information

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  3. Conceptual Model:
  4. Monitoring Plan:
Last Updated on Tuesday, 04 September 2012 12:32