Bald Eagle Abundance and Relationships to prey base and human activity along the Colorado River in Grand Canyon National Park, Arizona.
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.
We studied Bald Eagles (Haliaeetus leucocephalus), during the winters of 1993-95 along the Colorado River corridor in Grand Canyon, from Glen Canyon Dam downstream to the confluence of the Little Colorado River. Previous research documented patterns of eagle presence, foraging and distribution in the Grand Canyon (Brown et al. 1989, Brown and Stevens 1992, Brown 1993, Montgomery et al. 1992), and investigated the relationship between food supply (rainbow trout - Oncorhynchis mykiss) and Bald Eagle abundance (Leibfried and Montgomery 1993). Our study was initiated with the following objectives: 1) monitor Nankoweap Stream conditions, 2) determine if Bald Eagles continued to congregate at Nankoweap during the winter, 3) determine if rainbow trout continued spawning in Nankoweap Creek and, 4) determine what parameters (including human activity) that most greatly influenced Bald Eagle and rainbow trout numbers.
In 1993, we found that Nankoweap Creek was subject to many turbid, high volume flows that resulted in stream-bank undercutting. Trout were present and in spawning condition, but in relatively small numbers and found principally in places that provided protection from Bald Eagles (e.g., bank overhangs). Correspondingly, few eagles concentrated at Nankoweap in 1993, with a high count of five birds on 27 February. During 1994, we found that Nankoweap Creek flowed at a much lower rate and the water was clearer. On average, there were approximately 50 trout in the creek each day, but fish were again hidden by bank overhangs and deep pools that provided shelter from foraging eagles. As in 1993, few eagles concentrated at the creek during 1994, and a high daily count of six birds occurred on 5 March.
Our helicopter surveys found that Bald Eagles continue to be present in numbers throughout the river corridor, even when few birds were present at Nankoweap Creek. Annual high counts of eagles along the river were 13 birds (29 December 1992), 20 birds (20 January 1994), and 24 birds (10 February 1995). Human activities at Nankoweap were low in 1993, but higher 1994 when the Park did not invoke a closure of the Nankoweap area. Research-related activities had minimal effects, but hikers, anglers, and persons camping near the Nankoweap delta did cause eagles to flush (i.e., disturbance).
The Colorado River corridor is clearly an important resource for Bald Eagles wintering in Arizona. We suggest that Grand Canyon National Park continue to annually monitor Bald Eagles, but at a much reduced level. The park should consider continued helicopter monitoring to assess relative numbers of Bald Eagles. It is also important, if future Bald Eagle concentrations occur, for the park to consider recreation closures of lower Nankoweap Canyon and the Nankoweap delta area from 1 January through 15 March of each year.