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Yosemite Nature Notes 47(3) (1978)


THE MANAGEMENT OF GOLDEN EAGLES IN YOSEMITE NATIONAL PARK

Cary Tanaka

Golden eagle and nestling
[click to enlarge]

With its six and a half foot wingspan, the soaring golden eagle (Aquila chrysaetos) is one of the most impressive sights in the Yosemite sky. From as early as 1929, observers have recorded their awe. Although the record is sparse and sporadic, it is, nonetheless, sufficient to designate golden eagles as having been both native and fairly common to the Yosemite area.

Fortunately, the Yosemite golden eagle population and its environment have remained relatively intact through the twentieth century. However, on a national scale since 1940, the Code of Federal Regulations has listed the species as endangered. This situation could indicate that Yosemite acts as a valuable sanctuary or island for the population

But because the presence of golden eagles has not interfered with the administrative procedures of the park, these birds have been virtually ignored. Lest the tides of our rapidly changing times should change disfavorably for the population, this policy seems hazardous. Wild birds, especially endangered ones, and their environment often must be manipulated for their own good. In order to determine which factors should be monitored, something of the birds’ lifestyle must first be known.

LIFE HISTORY

The diet of golden eagles consists of: 83.9% mammals, 14.7% birds, 1.0% reptiles, and 0.4% fish; rabbits and rodents may comprise from 70% to 98% by weight of an eagle diet, depending upon locality.

Golden eagles form breeding territories which are maintained from generation to generation. As a measure of carrying capacity, the density of breeding pairs has been determined to range from approximately 35 to 66 square miles per pair for various habitats and states (Table 1). Within the boundaries of each territory are feeding, roosting, nesting, and soar-playing areas. The size of the territory depends to a large extent upon the availability of food, nest sites, and suitable terrain for flying.

Nest site studies of golden eagles have indicated different uses of habitats as well as elevations. The predominant habitat preference is cliffs, which are both plentiful and widespread in Yosemite (Tables 2, 3, and 4.) The preferred nest site altitudes range from 4,000 to 6,000 feet, a common geography in Yosemite (Table 5.)

Golden eagles usually have a number of alternate nests, ranging from 1 to 14, although 2 to 3 is the usual number. The same nest may be used by a pair during consecutive nesting seasons, although they often repair alternate nests and visit them regularly until the eggs are laid.

A study of the reproductive rate of golden eagles reveals a disturbing discrepancy between the average number of eggs laid per nest (2.1, Table 6) and the average number of birds fledged per nest (1.32, Table 7.) Further, studies indicate that approximately 70% or more of the young birds do not survive to sexual maturity. Based on this evidence, it is calculated that an adult pair is responsible for rearing 0.4 potential adult replacements per year. Or in other words, it would take a mating pair more than five years to replace themselves.

TOLERANCE TO HUMAN ACTIVITIES

Nest defense against human beings seems to be almost universally lacking. The adult eagles are extremely wary when someone comes near the eyrie and in many cases will be gone before the observer is even aware of the nest. The major consequences of human visiting active eyries include: (1) possible desertion by parent eagles of their eggs and young; (2) the increased chance of egg breakage by parent birds, as well as the increased chance of cooling, overheating, loss of humidity, and avian predation of eggs; (3) possible chilling or overheating of the newly hatched birds in the absence of brooding; (4) possible premature fledging by older nestlings resulting in broken bones at the end of a futile first flight or nights spent on the ground where vulnerability to predation is high; (5) possible scent trails guiding predators to the eggs or young; (6) the possible attraction of the attention of other humans.

Golden eagles studied for chemical contamination contained measurable amounts of organochlorine, pesticide, and mercury residues. Thankfully, all concentrations were well below toxic levels. This good fortune can be attributed to the relatively short food chain of golden eagles wherein contaminants are prevented from concentrating within successive stages.

RECOMMENDATIONS

On the basis of the preceding brief survey of golden eagle requirements and behavior, it appears that natural conditions pose no threat The effect of humans, however, can be significant. In light of the restrictions placed upon any wildlife management plan by both public and administrative interests as well as financial constraints, the following list of recommendations is offered in the order of their management priority and feasibility

(1) In order to initiate a management program, the primary step must be to determine the present population level and distribution. The rugged terrain and modest dimensions of the park suggest that airborne observations would be the most efficient technique. If these routes were flown in early August at the end of the fledgling period (Fig. 1I, not only could the population level and distribution be determined, but also an estimate of the number of nesting sites could be calculated. Furthermore, blanched, uric acid stained nesting sites could be located to indicate active eyries, while eliminating inactive alternate nests.

(2) Eyrie sites should not be made known to the general public. Many people are not aware that golden eagles are protected by law nor do they comprehend the possible consequences of disturbing active nest sites.

(3) Any proposed or current development of Yosemite Park should be studied for the possible impact on the golden eagle population. This is particularly important with regard to the recently increased enthusiasm for backpacking, climbing, and hang-gliding. In some instances, human activity in an area where golden eagles nest or hunt may be sufficient to cause them to desert, even if harassment is neither direct nor deliberate

(4) Available evidence indicates that golden eagles most frequently and readily desert their nests during the period of incubation. Once the eaglets have hatched, the probability of desertion decreases considerably. Therefore, human activity should be restricted from areas of known active eyrie sites during the period extending from February 1 to June 1.

(5) Adult eagles tolerate human activity in the Oregon-Idaho Snake River Canyon below the nests but are very intolerant of it on the canyon rim above them. Therefore, human activity, particularly along the Yosemite Valley rim should be reduced to a minimum from February 1 to June 1.

TABLE I
Density of breeding pairs for various studies and states.
SQ. MILES/PAIRLOCATIONSOURCE
36San Diego Co., Calif.Dixon, 1937
38UtahCamenzind, 1968
66.3MontanaMcGahan, 1968
35.5Southwestern IdahoKochert, 1973

TABLE II
Differential utilization of habitats by nesting golden eagles in Colorado,
1970-1972 (Olendorff and Stoddart 1974)
UNBROKEN
GRASSLANDS
CREEK
BOTTOMS
CLIFFSCULTIVATED
LAND
Percent
Use
14.532.752.8virtually
0

TABLE III
Differential use of supporting structures for nesting golden eagles in Colorado.
1970-1972 (Olendorff and Stoddart 1974)
TREEROCK
OUTCROP
CREEK
BANK
CLIFFGROUNDMAN-MADE
STRUCTURE
Percent
Use
25.55.416.352.800

TABLE IV
Percentage of golden eagle nests in various sites in Montana (McGahan, 1968)
19631964
NEST SITESUNOCCUPIEDOCCUPIEDUNOCCUPIEDOCCUPIEDALL NESTS
Cliff5971677062
Douglas Fir2826292329
Cottonwood23273
Ponderosa Pine30002
Dead Snag40002
Ground40202
TOTALS100100100100100

TABLE V
Percentage of golden eagle nests at various altitudes in Montana
(McGahan, 1968)
19631964
ALTITUDE RANGES
(feet)
UNOCCUPIEDOCCUPIEDUNOCCUPIEDOCCUPIEDALL NESTS
3,000 - 4,00044604
4,000 - 5,0004837584146
5,000 - 6,0003637274135
6,000 - 7,000101171110
7,000 - 8,000211275
TOTALS100100100100100

TABLE VI
Comparison of clutch sizes from different areas (McGahan, 1968)
FREQUENCY OF DIFFERNT
CLUTCH SIZES (PER CENT)
AREA AND
INVESTIGATOR
NUMBER OF CLUTCHES
IN SAMPLE
1 EGG2 EGGS3 EGGS
California
(Dixon, 1937)
unknown108010
Montana
(McGahan, 1968)
2058015

TABLE VII
Young hatched and fledged in Montana, 1963-1964 (McGahan, 1968)
Average number hatched per nest1.59
Average number fledged per nest1.37
Percent hatched that fledged86.4

FIG. 1
Dates and durations of the three stages of nesting for golden eagles in Colorado (Olendorff and Stoddart 1974)
Dates and durations of the three stages of nesting for golden eagles in Colorado (Olendorff and Stoddart 1974) (figure 1)
1 laying period
2 hatching period
3 fledgling period
x/y dates first eggs laid and last young fledged

BIBLIOGRAPHY

Amadon, D., and L.H. Brown. Eagles, Hawks and Falcons of the World. McGraw-Hill Book Company, New York. 720 pp. 1968.

Brown, L. Eagles, Arco Publishing Co., Inc., New York. 96 pp. 7970.

Camenzind, F.J. Nesting ecology and behavior of the golden eagle in west central Utah. M.S. thesis, Brigham Young University. 49 pp. 1968.

Clawson, M. Golden eagles attack young deer. Yosemite Nature Notes 27:107-109. 1948.

Dixon, J.B The golden eagle in San Diego county. Condor. 39(2):49-56. 1937. Fitzpatrick, W. 1953 Christmas bird count in Yosemite Valley. Yosemite Nature Notes 33:9. 1954.

Kalmbach et al. The American eagles and their economic status. Fish and Wildlife Service, 86 pp. 1964.

Kochert, M. The Bureau of Land Management and raptor management in Idaho. In prep. 1973a.

Kochert, M. Effects of organochlorines and mercury on southwestern Idaho golden eagles. In prep. 19736. McGahan, J. Ecology of the golden eagle. Auk 85(1):1-12. 1968.

Mallard, J. Early autumn birds of Yosemite Valley. Condor 20:11-19. 1918.

Michael, E. Yosemite bird report. Yosemite Nature Notes 8:80-116. 1929.

Michael, E. Common nesting birds of Yosemite Valley. Yosemite Nature Notes 9:41-64. 1930.

Murie, A. The wolves of Mt. McKinley. Fauna of the National Parks of the United States. Fauna Series No. 5. 8 pp. 1944.

Olendorff, R. and J.W. Stoddart. The potential for management of raptor populations in western grasslands. Raptor Research Report No. 2. 44 pp. 1974.

Snow, C. Habitat management series for unique or endangered species. Bureau of Land Management, Report #7. 53 pp. 1973.

Survival Service Commission. Red data book. International Union for Conservation of Nature and Natural Resources. 1976.

Widmann, O. Yosemite Valley birds. Auk 21:66-73. 1976. [Editor’s note: 1904, not 1976—dea]



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