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Reproduction of Ferruginous Hawks Exposed to Controlled Disturbance

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Reproduction of Ferruginous Hawks Exposed to Controlled Disturbance The Condor 87:14-22 0 The CooperOrnithological Society 1985 REPRODUCTION OF FERRUGINOUS HAWKS EXPOSED TO CONTROLLED DISTURBANCE CLAYTON M. WHITE AND THOMAS L. THUROW ABSTRACT. -The Ferruginous Hawk (Buteo regalis) is a conspicuousgrassland bird that is sensitive to human disturbance. In 1978 and 1979, we studied 62 nestingpairs and recorded their behavior and nestingsuccess. At 24 of thesenests, we daily created disturbances designed to simulate those associated with land development on western rangelands.The other nestswere not disturbed. Treated nests and control nests differed significantly (P < 0.05) in the number that suc- cessfully fledged young. Thirty-three percent of the disturbed nestswere deserted by the adults, although our presencein the vicinity of the nest was brief. Those disturbed nests that were successfulfledged significantly fewer young (P < 0.05) than undisturbed nests. Based on our cumulative data, adults did not flush 60% of the time if our activities were more than 120 m from the nest and 90% of the time if they were more than 250 m from it. Accordingly, for intermittent and brief human disturbance during years when prey are abundant and Ferruginous Hawks are in good physiological condition, we suggestthat a minimum buffer zone of 0.25 km around the nest is sufficient to prevent nest desertion by at least 90% of the population. Buffer zone should be expanded in years when prey are scarce, i.e., when the hawks appear to be less tolerant of disturbance. The Ferruginous Hawk (Buteo regalis) is se- will tolerate, and the size of buffer zones need- riously affected in some areas by land devel- ed by disturbed pairs to maintain a level of opment and, consequently some populations nesting successand productivity similar to of the speciesare apparently small and may those of undisturbedpairs. To accomplishthis, be declining. Reflecting this trend, the species we consideredthe natural tolerance thresholds has been on the Audubon Blue List since the of the species,the relationship of tolerance to list was established in 197 1 (Tate and Tate prey availability, and the nestinghistory of the 1982). A minimum number of breeding pairs population as related to disturbance. wasrecently estimated at 2,8 lo-3,590 over the Most previous buffer zone recommenda- entire range (M. Call, unpubl., U.S. Bureau of tions from management agenciesand the sci- Land Management, Denver, CO, 1980). Fer- entific community have been based primarily ruginous Hawks are particularly sensitive to on best guess or anecdotal knowledge. Esti- human activity (Olendorff and Stoddard 1974, mates of the sensitivity of Ferruginous Hawks, Fyfe and Olendorff 1976, Woffinden and Mur- or raptors in general, to human disturbance phy 1977) and are prone to desert their nests were mainly obtained from observationsmade if disturbed during incubation. Nonetheless, during studiesof their breeding ecology. A sig- the speciescan respond well to management nificant exception is the Bald Eagle (Hal&e&s (White 1974, Porter and White 1976, Murphy leucocephalus)for which considerabledata were 1978) so that it may be possible to adjust hu- collected to aid development of sound man- man activities to minimize ecological distur- agement policies (cf. Grier 1969, Gerrard and bance to it. Kennedy (1980) suggestedsuch an Gerrard 1975, Mathison et al. 1977). Stal- approach in her assessmentof conflicts con- master and Newman (1978) examined the ef- cerning raptors and land use. fects of some controlled disturbanceson non- It is becoming increasinglydifficult to main- nesting Bald Eagles, but few other attempts tain critical wildlife habitat as the multiple-use have been made to identify, quantify, and con- demandson land intensify. The amount of dis- trol variables that cause nest desertion. turbance that sensitive species will tolerate must be measured so that land management METHODS AND STUDY AREA plans can be devised that are compatible with We conducted our study throughout the Raft the species’ needs. Our objectives were to de- River Valley (42”N, 113”W), Cassia County, termine how Ferruginous Hawks respond to south-central Idaho (Fig. 1). The northern disturbances, the levels of disturbances they boundary of the valley opens onto the Snake [I41 DISTURBANCE EFFECTS ON FERRUGINOUS HAWKS 15 The region’s physiography and climate are typical of Great Basin cold deserts (Odum 197 1). Vegetation is characteristic of the northern desert shrub biome as delineated by Nesting Areas Cronquist et al. (1972). Big sagebrush(Arte- n misia trident&a) is the dominant shrub Idaho throughout the valley, but greasewood (Sar- cobatusvermiculatus) and shadscale(Atriplex confertiofolia)are dominant on saline soils on the valley floor. Other major plants include rubber rabbitbrush (Chrysothamnusnauseo- sus)and black sagebrush(Artemisia nova). Utah juniper (Juniperusosteosperma) and pinionpine Lb(Pinus edulis) are generally limited by lack of moisture to elevations above 1,500 m. Ground vegetation consists of a variety of forbs and grasses,including squirreltail grass (Sitanion hystix),crested wheatgrass(Agropyron spica- turn),and tansy mustard (Descurainiarichard- [email protected] little agricultural and rangeland de- velopment that has occurred in the study area is limited primarily to production of alfalfa on the valley’s floor. Bench areas that were cul- tivated early in the century (F. Gunnell, pers. comm.) have reverted to original vegetation since being abandoned after droughts in the 1920s and 1930s. CENSUSING METHODS Territories of Ferruginous Hawks in Raft Riv- er Valley were first identified by Powers et al. (1973). Many were surveyed annually by How- ard (pers. comm.) between 1972 and 1977. We beganour study in 1978. We censusedthe area by vehicle from existing ranch roads between April and July in both 1978 and 1979. Nests were most often located by traveling along the sagebrush-juniperecotone searching for flat- topped juniper trees that are typical nest sites of Ferruginous Hawks (cf. Thurow and White 1983). We found ground nests by observing FIGURE 1. Study site (hatched) and primary or high the behavior of pairs in areas that lacked trees density (stippled) nesting area of Ferruginous Hawks in suitablefor nest sites;these nests were typically Raft River Valley, Idaho. The vertical lengthof the hatched near utility poles that provided a perch. We area is about 50 km. also conducted a survey by airplane in late May 1979, when young were large and downy River Plain; the south, west, and east sidesare white and thus easily seenfrom the air. During bordered by mountains.The study area is about these censusperiods, we did not approach the 988 km2 at an elevation of about 1,500 m. We nests,but examined them from as far away as chose this site because the valley has a large possibleusing a telescopeto determine if they population of Ferruginous Hawks whose his- were active. tory had been monitored continuously since 1972 (Powers et al. 1973, Howard 1975, Thu- METHODS OF DISTURBING NESTING row et al. 1980). Initially, we also chose this HAWKS site because it had a high potential for in- In early May 1978, we began disturbing ran- creasedhuman use, sincethe U.S. Department domly selected nests in four ways: we (a) ap- of Energy Geothermal Test Facility planned to proached them on foot (three nests), (b) ap- develop the geothermal resourcesof the valley. proached them in a vehicle (three nests), (c) The facility has been closed, however, and fur- continuously operated a 3%hp gasolineengine ther rapid changein the valley seemsunlikely. near the nest (two nests), or (d) fired a 0.22- 16 CLAYTON M. WHITE AND THOMAS L. THUROW 907 , I I I I I I I I 0 Percent of Ferruginous ,n- . 0 / .‘ - 300 “E Hawk Territories Occupied Y II / a, - -- Black-tailed Jackrabbit population .‘ & a --- a 1 80- Hypothesized Black-tailed 1 ’ . Jackrabbit population s : \ E \ \ \‘ ! 5 \ / ; \ \‘ -- 250 g 70 - \ B \ z \ 1 ’ $ \ j 2 \ 3 \ \ E 0 1 ’ \. $ \ E 60- I 8 \ \ - 200 ; z a ’ . A 0 \ bL7 * \ A ’ 0 A- - 50 50 ’ I 1 1 I I I I I _ 1972 1973 1974 1975 1976 1977 1978 1979 1980 FIGURE 2. Percent of territories occupiedby FerruginousHawks generallycoincided with the jackrabbit population cycle between 1972 and 1980. The 1972-1977 hawk data are from Powers et al. (1973), Howard (1975) and Howard (field notes and pers. comm.). The 1972-1977 rabbit data are estimations and extrapolations from observationsby Howard, C. Trost, and L. Stoddart (pers. comm.), and Gross et al. (1974). Open squaresrepresent rabbit transectdata taken during this study and in 1980. caliber rifle about every 20 m as we ap- All of these nestswere attended by incubat- proached the nest, beginning approximately ing adults when we began disturbing them and 500 m away and continuing until the adult they were disturbed once almost every day un- flushed (three nests).Treatment(d) was meant til the young either were ready to leave the nest to simulate periodic noise associatedwith hu- (fledge)or were deserted.The time of day when man activity. nestswere visited was variable, except that we We stopped using firearms in 1979 because did not disturb incubating birds immediately we could not separate the effect of the noise before or during inclement weather or near from the presence of researchers.In other re- dusk. Data collected during each visit included spects, however, we created disturbances at presence or absence of adults, flushing dis- randomly selectednests as we had in 1978: (a) tance, and generalbehavior of the adults. Con- five nests were approached on foot, (b) five trol nests, once identified, were not revisited nests by vehicle, and (c) four nests were ex- until mid-June. At this time, they and the dis- posed to soundsproduced with wind- or bat- turbed nests were visited to determine the tery-powered noisemaking devices (Sonalert@ number of nestlingspresent. The average age Beeper) placed 30-50 m from nests.
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