Long-term Productivity of Golden Eagles in Utah [Productivité à long terme des aigles royaux en Utah].

Résumé

Each year 1977-98, the senior author monitored 31-240 ( = 150 ± SD of 63.1) Golden Eagle (Aquila chrysaetos) nesting territories in north-central Utah. In 1998, the study area encompassed 14,504 km2. In any given year, 12-60% ( = 40 ± 14.5%) of the surveyed territories were “active” (nests with eggs) and 7-53% ( = 31 ± 13.1%) were “successful” (≥1 fledgling). Productivity averaged 1.1 ± 0.23 fledglings/active territory and 1.4 ± 0.13 fledglings/successful territory. Of the territories surveyed each year, typically 70% occured in shrub-steppe and desert-scrub habitats characteristic of Great Basin lowlands (“desert” territories) and 30% occured in montane-forest habitats (“mountain” territories). Black-tailed jackrabbits (Lepus californicus) constituted 65-85% of the prey organisms recovered from desert nests each year (>12,000 prey remains collected to date across all territories), and nesting activity, success and productivity all correlated strongly with the percentage of jackrabbit prey remains (e.g., Pearson r = 0.73 for % jackrabbits vs % successful territories). In contrast, jackrabbits rarely constituted more than 20% of the prey organisms found near mountain nests; ground squirrels (Spermophilus spp.), yellow-bellied marmots (Marmota flaviventris), snowshoe hares (Lepus americanus), and Blue Grouse (Dendragapus obscurus) were routinely encountered at equal or greater frequencies. Availability of mule deer (Odocoileus hemionus) carcasses during winter seems to be a key determinant of long-term productivity for mountain eagles. Both mountain and desert subpopulations are showing signs of stress. The proportion of successful desert territories may be declining gradually (linear regression: r2 = 0.09, P = 0.093) and productivity has declined significantly (fledglings/active territory—r2 = 0.22, P = 0.017; fledglings/successful territory—r2 = 0.11, P = 0.072). These trends probably reflect loss and degradation of native sagebrush (Artemisia spp.) communities, which is critical habitat for jackrabbits. For mountain eagles, proportions of active and successful territories and fledglings/successful territory show no distinct long-term trends; however, fledglings/active territory has declined significantly (r2 = 0.22, P = 0.017). This trend may reflect declining availability of winter deer carcasses because of unusual weather patterns.