Golden Eagle Indifference to Heli-Skiing and Military Helicopters in Northern Utah

Abstract: In 2006-2007, during Wasatch Powderbird Guides (WPG) permit renewal for heli-skiing in the Tri-Canyon Area (TCA) of the Wasatch Mountains, Utah, USA, we recorded 303 helicopter passes between 0 m and 3,000 m (horizontal distance) near ≥30 individual golden eagles (Aquila chrysaetos) in 22 nesting territories, through passive observation and active experimentation with civilian and military (Apache AH-64) helicopters. Flight profiles included 800-m, 400-m, 200-m, and 100-m flybys (horizontal distance from cliff nest on parallel course), as well as approaches and popouts where helicopters flew toward, or popped out from behind, adult-occupied cliff nests (0 m, horizontal distance). Between 1981 and 2007, during the only 8 years when nesting in the TCA was confirmed by presence of chicks, WPG annually flew 108–2,836 helicopter flights in the same drainages on 10–37 days between 15 December and 15 April, with no effect on early courtship, nest repair, or subsequent nesting success. Total WPG operating days (x̄ = 62.4) and helicopter hours (x̄ = 210.6) fluctuated annually but did not increase 1974–2007 (Cox-Stuart trend test, P = 0.371, 0.393, respectively). Apache helicopter testing (227 passes) did not reduce golden eagle nesting success or productivity rates within the same year (t_{111, 96} = 0.495, 0.782, P = 0.622, 0.436, respectively), or rates of renewed nesting activity the following year, compared with 81–101 non-manipulated nesting territories. We recorded no response during 66% and only watching during 30% of Apache passes at 0-800 m from nesting golden eagles. No other reactions occurred until after hatching when ≤4 golden eagles accounted for 5 flatten and 3 fly behaviors at 3 nest sites. No responding pairs failed to fledge young because of testing. Limited fly responses suggested helicopters only precipitated an imminent departure, rather than causing startled, avoidance reactions. Responsiveness between test weeks 1 and 2 decreased (X²₂ = 32.167, P ≤ 0.001). Apache helicopters were twice as loud as WPG helicopters at comparable distances. Sound decreased with distance, most rapidly when flights were perpendicular to cliffs or ridges. Eagle ambient behaviors and watching the helicopter occurred randomly throughout recorded sound levels during helicopter testing (76.7–108.8 decibels, unweighted). Much helicopter sound energy is below golden eagles' auditory threshold, thus reducing potential impacts. Neither our observations nor our testing indicated special management restrictions are required for helicopters flying near nesting golden eagles in northern Utah. Our results underscore the necessity for circumstance-specific research, as well as enlightened resource management to accommodate unexpected results.