Bald Eagle Abundance and Relationships to prey base and human activity along the Colorado River in Grand Canyon National Park, Arizona.

CHARLES VAN RIPER III and MARK K. SOGGE

Correspondence should be directed to:
Dr. Charles van Riper III, USGS/FRESC Colorado Plateau Field Station, Box 5614, Northern Arizona University, Flagstaff, AZ 86011-5614.
4Email: charles.van.riper@nau.edu
 

TABLES & FIGURES
Figure 1.  Location of the Bald Eagle monitoring study area, along the Colorado River in Grand Canyon National Park and Glen Canyon National Recreation Area, Arizona.  The bolded section of the Colorado River represents that area repeatedly surveyed from helicopter.  The circled numbers represent the river mile designation at that point along the Colorado River corridor.

Figure 2.  The lower portion of Nankoweap Creek, where it empties into the Colorado River in Grand Canyon National Park, Arizona.  This photograph was taken February 1994 from the Nankoweap Overlook.  The primary ground study area was subdivided into Reach 1, 2, and 3, delineated by dark bars drawn across the stream channel.

Figure 3.  Stream temperature (^oC) and discharge (cubic feet per second; cfs) in lower Nankoweap Creek during 1994. 

Figure 4.  The number of rainbow trout detected in the lower 600 m of Nankoweap Creek, Arizona, during winter counts from 1993 – 1995.

Figure 5.  The relationship between creek temperature (^oC) and the number of rainbow trout detected in the lower 600 m of Nankoweap Creek, Arizona.  Data were collected during1993 and 1994, with a combined R² = 0.39, and P = 0.02.

Figure 6.  Minimum and maximum daily flows (cubic feet per second) in the Colorado River through the Grand Canyon National Par, Arizona, during February and March of 1993 and 1994.

Figure 7.  Percentage of male and female rainbow trout captured in the Colorado River (“River”) and lower Nankoweap Creek (“Creek”), during 1993 and 1994 in Grand Canyon National Park, Arizona.

Figure 8.  Spawning condition (expressed as percent of captured individuals) of rainbow trout captured in the Colorado River (“River”) and lower Nankoweap Creek (“Creek”), during 1993 and 1994 in Grand Canyon National Park, Arizona.

Figure 9.  The number of bald eagles, by age class, detected during helicopter surveys along the Colorado River corridor from the Little Colorado River junction up to Glen Canyon Dam, 1993 – 1995.

Figure 10.  The relationship between the number of bald eagles detected (on the same day) at the Nankoweap Creek delta area, with the total number of eagles detected by helicopter from the Little Colorado River junction up to Glen Canyon Dam along the Colorado River corridor, 1991 – 1995 (R² = 0.56,; P < 0.001).

Figure 11.  The relationship between the number of bald eagles present at the Nankoweap Creek delta area and the relative accuracy of helicopter-based eagle counts at that site in Grand Canyon National Park, Arizona.  Both counts were taken simultaneously as the helicopter passed by the Nankoweap delta area.  Data are for 1993 and 1994 combined (R² = 0.002; P = 0.35).

Figure 12.  The relationship between the number of bald eagles present at the Nankoweap Creek Delta area (as counted by ground-based observers) and the number of eagles detected there by helicopter-based surveys (“Detected”).  Data are for 1993 and 1994 combined; both counts were taken simultaneously as the helicopter passed by the delta area (R² = 0.49, P < 0.02).

Figure 13.  The number of Bald Eagles, by age class, observed from 1993-1995 at the Nankoweap Creek delta area, Grand Canyon National Park, Arizona. 

Figure 14.  The relationship between the daily number of rainbow trout present in the lower 600 m of Nankoweap Creek, and the number of bald eagles present in the Nankoweap Creek delta area from 1993-1995 (R² = 0.24, p < 0.02).

Figure 15.  The number of golden eagles observed at the Nankoweap Creek delta area, Arizona, between 1993 – 1995.

Figure 16.  Timing of bald eagle forage attempts at lower Nankoweap Creek and the nearby mainstem Colorado River, in Grand Canyon National Park, Arizona.  Data are from observations made from the ground during 1993 and 1994.

Figure 17.  Percent foraging success for Bald Eagles observed at lower Nankoweap Creek and the nearby mainstem Colorado River, in Grand Canyon National Park, Arizona. 

Figure 18.  The number of helicopter (from eagle surveys), river boating, and hiking events recorded during 1993 and 1994 along lower Nankoweap Creek and the nearby mainstem Colorado River in Grand Canyon National Park, Arizona.

Figure 19.   Response of Bald Eagles (in terms of flushing) to helicopter, boat, and hiking activities along lower Nankoweap Creek and the nearby mainstem Colorado River, Grand Canyon National Park, Arizona.  Data are from 1993 and 1994 combined, expressed as the percent time that an activity type caused all, some, or none of the eagles present to flush.

Figure 20.  The highest daily number of bald eagles observed each year at Nankoweap Creek (open bars) and along the Colorado River corridor from the Little Colorado River junction up to Glen Canyon Dam along the Colorado River (based on helicopter surveys; shaded bars), from 1986 to 1995.   Data for 1986 – 1992 are from T. Yates (pers. Comm.), Brown et al. 1989, and Brown and Stevens 1992.  No helicopter/river corridor surveys were conducted prior to 1991.   In 1992, only one helicopter survey was conducted along the river corridor, and no ground-based surveys were made at Nankoweap Creek.

Table 1. 

Table 2. 

Table 3.  

Table 4. 

Table 1.  The mean (± one standard deviation) minimum and maximum Colorado River flows (in cubic feet per second) released from Glen Canyon Dam during February and March of 1993 and 1994. Values are not given for 1995, when only limited trout monitoring was conducted at Nankoweap Creek.

Table 2.  Standard length, total length, and mass of rainbow trout caught in lower Nankoweap Creek (1993 and 1994), and in the Colorado River at the mouth of the creek (1994), in Grand Canyon National Park, Arizona.

Table 3.  The average number of eagles (± one standard deviation) detected per day during helicopter-based counts at Nankoweap Creek, and through the upper river corridor from Glen Canyon Dam to the Little Colorado River confluence in Grand Canyon National Park, Arizona.

 Table 4.  Foraging attempts and foraging success for bald eagles during 1993 and 1994, at the Nankoweap Creek delta area, Grand Canyon National Park, Arizona.