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 development 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. Noise of nestlingsat this time was 3-4 weeks. In no levels from these devices at nest sites (about casedid this brief visit result in nest desertion. 80 dB) were designedto simulate soundscom- monly produced at geothermal (or other type PREY ASSESSMENT of development) sites.Some, but not all, of the Becausethe population dynamics and behav- pairs that we disturbed in 1978 were probably ior of Ferruginous Hawks are closely related disturbed in 1979, as some of the same nests to the birds’ food supply (Howard and Wolfe were used. They were not necessarilyexposed 1976; Smith et al. 198 1; Figures 2 and 3), we to the same type of disturbance in both years. attempted to estimate the density of lago- We approached nests only to the point at morphs (Lepus and Sylvilagus). We made which the attending adult flushed (physically morning walks along ten 1.6-km transects left the nest), and then left the area immedi- through habitats representingthe vegetation in ately. Flushing distance was estimated and re- the valley. Each transectwas chosenrandomly corded as an indicator of the level of stressor and was censusedonce each year, in early July, anxiety beyond which the hawks could no when young Ferruginous Hawks typically longer tolerate the presence of the disturbing fledge. To determine the rabbits’ density from factor. Note, however, that this is probably not the transectdata, we used the flushingdistance the distance at which stress is first induced, equation of Hayne (1949) and the correction since stresslevels of incubating birds probably factor and censusingcriteria of Gross et al. rise long before they flush. (1974). DISTURBANCE EFFECTS ON FERRUGINOUS HAWKS 17

4.0 I I I I I I I I r

0 Ferruglnous Hawk fledging rate S.\ / _. _ Black-talled Jackrabbit population - - - Hypothesized Black-tailed / 0 .*;‘ JackrabbIt population I

1972 1973 1974 1975 1976 Year FIGURE 3. The fledgling rate of Ferruginous Hawks compared to the jackrabbit cycle over a 9-year period. See Fiaure 2 for sourcesof data concemina rabbits and hawks between 1972 and 1977. Open squaresrepresent rabbit transectdata taken during this study aid in 1980.

RESULTS came sensitized to our disturbances, flushing CRITICAL VARIABLES IN THE NESTING at increasing distances until just before their BIOLOGY OF FERRUGINOUS HAWKS eggshatched (Fig. 4). Desertions occurred as late as 19 days after we began to disturb nests. Nesting territories of some raptors remain re- The stagesof development of the embryos in markably stable from year to year (Newton eggsfrom thesedeserted nests ranged from none 1979). Furthermore, several species seldom (eggs freshly laid) to stages characteristic of build new nests,but repeatedly repair and reuse old ones.Observations of this population since 1972 by Howard (pers.comm.) showthat these TABLE 1. Resultsof disturbances,by treatment method, birds reusetheir nests.Most of their territories for nests disturbed in 1978. contain alternate nest sites. Although the ac- Method of NUIFlbU Flushing tual site used may change annually, the limits disturbing of visits distance of the territory remain essentially constant. nest to nests (my Results Consequently, we had no difficulty determin- Walk 18 Fledged 0 nestlings ing which nestingterritories in the valley were (15-:140) (3 young taken by active. Of the 107 nest structures of Ferrugi- predators-adults nous Hawks located in our study area, six were inattentive) Walk 8 110 Nest deserted on the ground or a rocky outcrop; one was on (25-l 80) a power pole; and the remainder were in ju- Walk 6 53 Nest deserted niper trees (cf. Woffinden and Murphy 1983). (35-90) Nest placement with regard to human dwell- Motor 8 Fledged 4 nestlings ings or roads did not deviate from random. (35~ZO) Motor 8 101 Fledged 4 nestlings When the locations of these nests were com- (40-320) pared to 100 random points with a paired t-test, Walk (gunshot) 18 Fledged 2 nestlings there was no significant difference (P < 0.05). (25~7:20) Nonetheless, since no nests were adjacent to Walk (gunshot) 22 Fledged 2 nestlings (S-92675, homes, there was clearly some selectivity in Walk (gunshot) 19 Fledged 4 nestlings nest placement and some minimum distance (5--O) that the hawks maintained away from areas of Drive 7 217 Nest deserted human activity. (20-480) Drive 6 221 Nest destroyed by DISTURBED NESTS (140-370) wind Drive 25 Fledged 1 nestling The hawks differed slightly in responseto our (20~%O) treatments but none increased their tolerance s Basedon visits during which adults flushedwhen we wereless than 500 over time (Tables 1 and 2). In fact, most be- m from the nest.Values are means(ranges). 18 CLAYTON M. WHITE AND THOMAS L. THUROW

TABLE 2. Resultsofdisturbances, by treatment method, those of Safina and Burger [ 19831 for many for nests disturbed in 1979. colonial waterbirds). Initial flushing distances in 1979 (Table 2) were higher than those in Method of Number of Flushing disturbing visits to distance 1978 (Table 1) probably because four of the neStS nesw Results (m)” 14 nests that we disturbed in 1979 had also Walk 19 (5) 138 Fledged 3 nestlings been disturbed in 1978. We presume that they (20-400) were occupied by the same pairs both years Walk 21 (7) Fledged 2 nestlings and that the birds were already sensitized to (20?50) Walk 19 (2) 164 Nest deserted disturbance from their previous experiences. (70-300) We found no statistically significant correla- Walk 26 (10) 196 Fledged 3 nestlings tion between numbers of eggsin nestsand the (10-350) birds’ tendency to desert. Walk 24 (3) 118 Fledged 3 nestlings (25-255) Of the four territories that failed to fledge Drive 24 (3) Fledged 3 nestlings young in 1978, none were used in 1979 (but (15-92070) three were used again in 1980). Of the five Drive 8 (3) 127 Nest deserted territories desertedin 1979, four were not used (35-330) in 1980. In other words, eight of the nine dis- Drive 28 (1) 162 Fledged 3 nestlings (35-400) turbed nests that failed as a result of our dis- Drive 28 (1) Fledged 2 nestlings turbances were not used the following year, a (1 S%O) significant reduction (x2 = 92.7, 1 df, P < Drive 6 (3) 153 Nest deserted 0.005) consideringthat 93% of establishedter- (20-400) Noise N& Nest deserted ritories are normally reoccupied each year (see Noise NA Fledged 2 nestlings below). We emphasize that when a pair de- Noise NA Fledged 3 nestlings serteda territory, the birds did not simply move Noise NA Nest deserted to an adjacent site the following year. Most of . Numbersin parenthesesare visits whenadults flushed at distancesgreater the traditional territories were occupied each than 500 m or wereabsent. b Basedon visits during which adults Rushedwhen we wereless than 500 year by other pairs. Therefore, if the pair from m from the nest.Values are means(ranges). ( NA = not applicable;we did not visit nestssubjected to noiseuntd after the desertedterritory bred in the year following the eggshatched. desertion, they would have had to move com- pletely out of the area. The chance of this oc- embryos 3-4 days before hatching. In our ex- curring with the consistencythat the data in- perience, a female hawk usually will not desert dicate is unlikely. after an egg begins to hatch or when there are None of the disturbance treatments pro- nestlingsin the nest. (These findings resemble duced significantlydifferent effectson the birds,

Average hatching date

617

i 978

I _. I Ime oy monttvday

FIGURE 4. Mean flushingdistances of FerruginousHawks during the nestingcycle. Values are 5-day averages.Ratios beside each closed circle are month/day of month. DISTURBANCE EFFECTS ON FERRUGINOUS HAWKS 19

TABLE 3. Fledging rates of FerruginousHawks in 1978 TABLE 4. Distribution of the number of young fledged and 1979. per nest.

Fledging rate Number of nests that fledged this many young Number of nests (younp/nest) 1978 1979 Number of 1978 1979 1978 1979 O”llg Control Disturbed Control Disturbed t?edged nests nests nests nests All disturbed nests 10 14 1.70 1.71 Successfuldisturbed 0 0 (0)’ 4 (40) 0 (0) 5 (36) nests 7 9 2.43 2.67 1 0 (0) 1 (10) 1 (5) cl (0) Control nests 17 21 3.53 3.81 2 3 (18) 2 (20) 1 (5) 3 (21) 3 4 (23) 0 (0) 4 (19) 6 (43) 4 8 (47) 3 (30) 10 (48) 0 (0) 5 2 (12) 0 (0) 5 (24) 0 (0) but their effectswere significantlydifferent from *Numbersin parentheses are the percentages of nests in each category. those of the control treatment (Duncan’s multiple range test). Therefore, we combined the forms of disturbancefor purposesof analyzing (Thurow et al. 1980). The density of Black- differencesbetween disturbed and control nests. tailed Jackrabbits (Lepus californicus)was es- Only 52% of territories containing disturbed timated to be 309/km2 in 1978 and 287/km2 nests(n = 24) were occupied the year after dis- in 1979. These densities are high compared turbance. Excluding desertions, 75% of them with population estimates from neighboring were used the next year. In contrast, 93% of Curlew Valley taken during low points in the the territories containing control nests(n = 38) jackrabbit cycle (Gross et al. 1974) suggesting in 1978 were occupied in 1979. This difference that the prey population was at or near peak in use of territories containing disturbed nests density during the study period. (excluding desertions) and those containing control nests is not significant. DISCUSSION

SUCCESS OF DISTURBED NESTS LIMITATIONS OF THE STUDY DESIGN Ofthe 10 active nestsdisturbed in 1978, seven Many factors may have affected the results we were successful,fledging 17 young for a fledg- obtained. Among the variables that were not ing rate of 2.43 young/successfulnest. How- controlled, and in some casesnot measured, ever, if the three disturbed neststhat were de- were: (1) clutch size, (2) disturbancesnot caused serted that year are included, the fledging rate by researchers,(3) our inability to fully mea- was only 1.70 young/nest, which was signifi- sure stressor anxiety in hawks, (4) stressin- cantly higher than that of successfuldisturbed duced by inclement weather, (5) stage of in- nests (P < 0.05). Of greater biological signifi- cubation when disturbancebegan, (6) presence cance is the fact that control nestsfledged, on or absence of a mate when the nest was ap- average, one young more than successfulnests proached, (7) different responsesof individual that we disturbed or twice as many young if hawks to disturbance, (8) health of individual all disturbed nests are considered (Table 3). hawks, and (9) previous exposure of these in- None of the disturbed nests in either year of dividuals to human disturbance. the study fledged five young and only 30% of The responseof Ferruginous Hawks to var- them fledged four young (Table 4). By com- ious types of human activity appears to vary parison, 59% of the control nestsin 1978 and considerably among individuals (Tables 1 and 72% in 1979 fledged four or five young. Many 2). Nevertheless,a population norm doesexist. disturbed nestsfledged fewer young than con- The intensity of the responsedepends largely trol nests. For example, only one control nest on whether the form of disturbanceis familiar fledgedfewer than two young during our study, or not (cf. Gilmer and Stewart 1983, for ex- but 10 disturbed nests did so. ample). Hawks may tolerate considerablenoise During 1979, nine of the 14 disturbed nests close to their nestsif they are familiar with it, were successfuland fledged 24 young for a especially if humans are not visible or other- fledging rate of 2.67 young/successfulnest, or wise obviously associatedwith it. For exam- 1.71 young/nest for all disturbed nests (suc- ple, military jets that occasionallyflew low over cessfuland unsuccessful),whereas 3.8 1 young the study area during training exercisesdid not fledged on average from control nests (Table appear to bother the hawks. We suggestthat 3). The difference of 1.16 young/nesting at- Ferruginous Hawks did not consider such air- tempt between successfuldisturbed and con- craft as a potential danger, particularly since trol nestsis statistically significant(P < 0.05). the noise from them did not reach the nest In the Raft River Valley, about 9 1% of the until the planeshad already passedover a nest biomassin the diet of Ferruginous Hawks con- and were 2-3 km away. Similarly, hawks were sisted of lagomorphs, principally jackrabbits not alarmed when, during our attempt to find 20 CLAYTON M. WHITE AND THOMAS L. THUROW nests, we flew within 30 m of their nests in a the nutritional condition of the hawks. In 1978 Cessna 185. In one case,the female continued and 1979, populations of black-tailed jackrab- to feed her young without interruption. bits in Raft River Valley were at peak density, Unlike previous reports of substantial nest accordingto their approximately 1O-year pop- desertion by raptors as a result of human ac- ulation cycle (Wagner and Stoddart 1972) and tivity (cf. Fyfe and Olendorff 1976) the num- extrapolations from data collected nearby in ber of disturbed neststhat were desertedin our Curlew Valley (Gross et al. 1974; L. C. Stod- study (40% in 1978 and 36% in 1979) was dart, pers.comm.). Hence, breedingadult hawks unexpectedly low. Several factors may have could easily obtain food for themselves and been responsible for this relatively moderate their young, and appeared to be in good phys- desertion rate. During the disturbances,we ap- ical condition. In 1980, however, the jackrab- proached nests only to the point at which the bit population had declined dramatically (Fig. attending adult flushed and then left the area 2) owing to an epizootic outbreak. We dis- immediately. This contrastswith many studies turbed three nestswhile eggswere present that of the breeding biology of raptors since we year. Each was deserted soon thereafter. This never actually climbed the nest trees to deter- disturbance occurred when we attempted to mine clutch size or to collect other data until put an egg containing a transmitter into the the young could be banded, about 25 days after nest in order to determine the incubating hatching. We suspect that more desertions hawks’ heart rate and thus obtain a more ac- would have occurred had we climbed to the curate measurement of stress.These instances nests during incubation. of desertion contrast sharply with what hap- New forms of disturbance may causedeser- pened at several nests (not part of this study tion if sustained,even though humans are not sample) that we inspected in 1978 and 1979: directly associatedwith them. For example, we none was subsequentlydeserted. Furthermore, concealedan electronic noisemaker 15 m away in 1978 and 1979 we took fertile eggs from from a nest in 1979 while the female was ab- deserted nests that we had disturbed and put sent. (The noise level produced at 15 m from them in nests that were not part of the study the nest was about 100 dB-B weighted). When in an attempt to save the embryos. None of the female returned, she landed on the edge of these actions causeddesertion. This suggested the nest, but appeared startled and quickly left. to us that the apparent sensitivity of Ferrugi- She approached the nest once more 10 min nous Hawks to disturbance in 1980 was due later, but did not land on it. Later in the day, to lower prey abundance (jackrabbit popula- she made two more passesat the nest only to tions estimated at 169/km* based on transect veer away when about 6-7 m from it. She had data; compare with data for 1978-l 979) which apparently been so startled by the noise when probably affected their physiological state ad- she first heard it that she became immediately versely. sensitized to it and abandoned the nest (she Disturbed nests rarely fledged four or five did not return the following day). She could young, but commonly fledgedone or none (Ta- not see the noise maker becauseit was small ble 4). The reverse pattern was true for control and concealed. nests. Behavioral data collected during each We hypothesize that the unfamiliar nature nest visit suggestedthat adults became sensi- of the noise causedthe hawk to desert the nest tized to our presence and were less than nor- sinceon other occasionsit wasclear that hawks mally attentive to their young, which probably could tolerate substantialdisturbance. Appar- reduced their fledging success.Numerous ob- ently, if a disturbance like that created by a servations supported this conclusion. For ex- cow rubbing against the nest tree or a tractor ample, parents at disturbed nests often re- plowing a nearby field are periodic and on- mained away from the nest site for unusually going when adults first arrive at their nestsand long periods once the young had hatched;how- are not perceived as threatening, hawks ha- ever, parents at control nests were often in bituate to them. For example, we know of sev- attendance for the first several weeks after eral pairs of the Peregrine Falcons (F&o per- hatching. Parents at disturbed nests also egrinus) and Golden Eagles(Aquila chrysaetos) showedlittle defensive behavior (i.e., screams, that occupied and successfullyraised young at stoops) and sometimes did not appear at the new nest sitesonly a few hundred meters from nest for up to 5 min after we arrived. By con- areasof high disturbance(e.g., at blasting, contrast, control pairs were much more aggressive struction, quarrying, and mining sites, and at and appeared quickly upon our arrival. Such airports). neglect increasesthe vulnerability of nestlings Another factor that may have influenced the to inclement weather and predation, as was desertion rate in our study was the availability clearly illustrated in one nest that we visited of jackrabbits (cf. Smith et al. 198 1) and thus after a thunderstorm. The downy, 1-week-old DISTURBANCE EFFECTS ON FERRUGINOUS HAWKS 2 1 young had not been brooded and one died (ex- return almost immediately. Had our activity hibiting pneumonia-like symptoms) several been continuous or of several hours duration, days later. In another case, 2-week-old nest- its impact would have been substantiallygreat- lings were apparently killed by Common Ra- er. vens (Corvus corax) because of the parents’ Defining a “buffer zone,” i.e., the minimum inattentiveness. Parental neglect at disturbed area around a nest that must be kept free of nests also increased the incidence of addled human intrusion to prevent harmful effectsas- eggsand the death of smaller, weaker young sociatedwith disturbance, is an important de- in nests containing four or five young. Heat cision tool for land managers or developers. stressmay have causedthe death of theseyoung We would prefer to see ecosystemskept intact since underfed young do not cope with stress (cf. Wagner 1977) rather than divided into iso- as well as those that are well-fed (Tomback lated islands set aside for nesting raptors, be- and Murphy 198 1). In control nests,the small- cause aspects of general land use other than er young survived. restricted areas also affect the health of raptor The presenceof humans near nestsmay also populations. However, we also realize that a raise the mortality rate by causingyoung hawks community approach is not always practical to fledge prematurely. At one nest, for exam- for many types of land use. Our observations ple, our presence caused a young hawk, only suggestthat a sufficient buffer zone for brief recently fledged, to make a taxing and lengthy human disturbancearound Ferruginous Hawk premature flight. Within 20 min a coyote (Canis nests is 250 m. Adults will not flush 90% of lutrms) was scoutingthe area where the young the time if human activity is confined to dis- bird had landed, perhapshaving seenthe young tancesgreater than this. However, several fac- in its unstable flight. Greater public use of the tors must be considered when one defines the nestinghabitat of Ferruginous Hawks may in- size of such a buffer zone. Our data, for ex- crease the premature departure of inexperi- ample, were collected during years when food enced young birds from nestsand thereby in- was abundant and consequently hawks were creasethe importance of mortality factors such presumably in prime physiological condition as predation. and able to cope well with the induced stress. Although nest placement with regard to hu- In years when prey are scarce,the buffer zone man dwellings or roads did not deviate from may need to be considerably larger. Other as- random, none of the nests were adjacent to pects of the nest site, such as the degree to homes. Clearly, hawks exercise some selectiv- which the nest is exposed or concealed, may ity in nest placement and stay some minimum also affect buffer distances best maintained distance away from areas of human activity. around individual nests. Nesting successof Ospreys(Pan&on haliaetus) is reportedly better if they nest some distance ACKNOWLEDGMENTS from areas of human activity rather than near We especiallythank R. P. Howard, who aided us from the them (Swenson 1975; L. Richardson, unpubl., inception of this study,D. Johnsonand S. Clark, who were Yale School of Forestry and Environmental our principal field assistants,and R. Olson, S. T. White, Studies, New Haven, CT, 1980). and B. Parker. J. Sullivan provided logisticalhelp and field support. J. A. Gessaman and M. T. White helped with BUFFER ZONE follow-up work in 1980. Funding for the study was pro- Our study was designedto simulate the effects vided by the U.S. Department of Energy contract number EY-77-S-07-1674 administered by E G & G Idaho, Inc. of low-level human activity on nesting Fer- ruginousHawks. The birds were disturbedonly once a day, and we left their territories as soon LITERATURE CITED as they flushed from their nests.A critical fac- BARTHOLOMEW,G. A. 1972. Body temperature and en- tor in a disturbance is the duration the bird is ergy metabolism, p. 298-368. In M. S. Gordon [ed.], Animal physiology: principles and adaptions. Mac- kept off eggs.If the eggsare cool when a hawk millan Co., New York. returns to the nest, it is our experience that the CRONQUIST,A., R. HOLMGREN,N. HOLMGREN,AND J. RE- bird is reluctant to resume incubating and like- VEAL. 1972. Intermountain flora. Vol. 1. Hafner, ly to abandon the nest. Also, if eggsare exposed New York. to high ambient temperatures for even brief DRENT,R. 1972. The natural history of incubation, p. 262-3 11. In D. S. Famer led.], Breeding biology of periods of time, the proteins are quickly dena- birds. National Academy ofSci&ce, Wa&ington;iX. tured (Bartholomew 1972). For example, the FYFE.R. W., AND R. R. OLENDORFF.1976. Minimizing lethal internal egg temperature of the Herring the dangers of nesting studies to raptors and other Gull (Larus argentatus) was reached within 2 sensitivespecies. Can. Wildl. Ser. Occas.Pap. No. 23. h on a sunny morning at an air temperature GERRARD,J. M., AND R. N. GERRARD. 1975. Ecological road planning in northern Saskatchewan.Blue Jay 33: of 18°C (Drent 1972). Our study simulatedbrief 131-139. disturbancesafter which the hawk was free to GILMER,D. S., AND R. E. STEWART.1983. Ferruginous 22 CLAYTON M. WHITE AND THOMAS L. THUROW

Hawk populations and habitat use in North Dakota. SAFINA,C., AND J. BURGER. 1983. Effectsof human dis- J. Wildl. Manage. 47: 146-157. turbanceson reproductive successin the Black Skim- GRIER,J. W. 1969. Bald Eaglebehavior and productivity mer. Condor 85:164-171. responsesto climbing to nests. J. Wildl. Manage. 33: SMITH. D. G.. J. R. MURPHY. AND N. D. WOFFINDEN. 961-966. 1981. Relationships between jackrabbit abundance GROSS,J. W., L. C. STODDART, AND F. H. WAGNER. 1974. and FerruginousHawk reproduction. Condor 83:52- Demographic analysis of a northern Utah jackrabbit 56. population. Wildl. Monogr. 40: l-68. STALMASTER,M. V., AND J. R. NEWMAN. 1978. Behav- HAYNE,D. W. 1949. An examination of the strip census ioral responsesof wintering Bald Eagles to human method for estimating animal populations. J. Wildl. activity. J. Wildl. Manage. 42:506-5 13. Manage. 13:145-157. SWENSON,J. E. 1975. Ecologyof Bald Eagle and Osprey HOWARD,R. P. 1975. Breeding ecology of the Ferrugi- in Yellowstone National Park. M.Sc. thesis, Montana nous Hawk in northern Utah and southern Idaho. State Univ., Bozeman. M.Sc. thesis, Utah State Univ., Logan. TATE, J., JR., AND D. J. TATE. 1982. The blue list for HOWARD,R. P., AND M. L. WOLFE. 1976. Range im- 1982. Am. Birds 36:126-135. provement practicesand FerruginousHawks. J. Range THUROW.T. L.. AND C. M. WHITE. 1983. Nest site re- Manage. 29133-37. lationship between the FerruginousHawk and Swain- KENNEDY,P. L. 1980. Raptor baseline studiesin energy son’s Hawk. J. Field Omithol. 54:401-406. developments. Wildl. Sot. Bull. 8: 129-135. -_ THUROW.T. L.. C. M. WHITE. R. P. HOWARD.AND J. F. MATHISON.J. E.. D. S. SORENSON.L. D. FRENZEL.AND T. SULLIVAN. ’ 1980. Raptorecology of Raft kiverVal- C. D&STAN. 1977. Management strategy for Bald ley, Idaho. U.S. Dept. Energy,EGG-2054, Idaho Falls, Eagles.Trans. N. Am. Wildl. Nat. Resour. Conf. 42: ID. 86-92. TOMBACK,D. F., AND J. R. MURPHY. 1981. Food de- MURPHY, J. R. 1978. Management considerations for privation and temperatureregulation in nestling Fer- some western hawks. Trans. N. Am. Wildl. Nat. Re- rttginousHawks. Wilson Bull. 93:92-97. sour. Conf. 43:241-251. WAGNER.F. H. 1977. Snecies vs. ecosvstem manage- NEWTON,I. 1979. Population ecology of raptors. Buteo ment: conceptsand practices.Trans.-N. Am. Wildl. Books, Vermillion, SD. Nat. Resour. Conf. 42:14-24. ODUM, E. 0. 1971. Fundamentals of ecology. Saunders, WAGNER,F. H., AND L. C. STODDART.1972. Influence Philadelphia. of coyote predation on black-tailed jackrabbit popu- OLENDORFF,R. R., AND J. R. STODDARD,JR. 1974. The lations in Utah. J. Wildl. Manaee. 36:329-342. potential for managementof grasslandraptors, p. 47- WHITE, C. M. 1974. Current problems and techniques 99. In F. N. Hamerstrom, Jr., B. E. Harrell, and R. in raptor management and conservation. Trans. N. R. Olendorff [eds.], Raptor Res. Rep. No. 2. Raptor Am. Wildl. Nat. Resour. Conf. 39:301-3 12. ResearchFoundation, Vermillion, SD. WOFFINDEN,N. D., AND J. R. MURPHY. 1977. Population PORTER,R. D., AND C. M. WHITE. 1976. Statusof some dynamics of the FerruginousHawk during a prey de- rare and lesserknown hawksin westernUnited States, cline. Great Basin Nat. 37:41l-425. p. 3 l-57. In R. D. Chancellor [ed.], Proceedingsof WOFFINDEN,N. D., AND J. R. MURPHY. 1983. Ferrugi- the First World Conference on Birds of Prey, 1975. nous Hawk nest site selection.J. Wildl. Manage. 47: International Council for Bird Preservation, Vienna, 216-219. Austria. POWERS,L. R., R. P. HOWARD,AND C. TROST. 1973. Department of Zoology, Brigham Young University,Pro- Population statusof the FerruginousHawk in south- vo, Utah 84602 and E G & G Idaho, Inc., P.O. Box 1625 eastern Idaho and northern Utah, p. 153-157. In J. WCB, Idaho Falls, Idaho 83415. Presentaddress of sec- R. Murphy, C. M. White, and B. E. Harrell [eds.], ond author: Department of Range Science, Texas A&&f Raptor Res. Rep. No. 3. Raptor Research Founda- University, College Station, Texas 77843. Received 7 tion, Vermillion, SD. March 1983. Final acceptance10 July 1984.