FACTORS AFFECTING THE PRODUCTIVITY OF OSPREYS NESTING IN WEST-CENTRAL IDAHO

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1 Condor The Cooper Ornthologcal Socety 1982 FACTORS AFFECTING THE PRODUCTIVITY OF OSPREYS NESTING IN WEST-CENTRAL IDAHO LAWRENCE J. VAN DAELE AND HILARY A. VAN DAELE ABSTRACT. -An Osprey (Pan&on halaetus) populaton nestng n the vcnty of Cascade Reservor n west-central Idaho was studed for three years. The area supported about 50 nestng pars, whch lad an average of 2.58 eggs and fledged an average of 1.37 young per actve nest throughout the study. These productvty estmates suggest a healthy, ncreasng populaton. Most nests were atop snags (66%) and on prvate land (70%). Ospreys nestng on artfcal stes and those nestng more than 1,500 m from human dsturbances produced more offsprng. Fsh n the 1 l-30 cm range consttuted the bulk of the det (89%) wth brown bullheads beng the most mportant prey speces (38%). Osprey captures reflected prey avalablty. Establshment of Cascade Reservor ncreased the avalablty of fsh, whch, n turn, allowed the Osprey populaton to ncrease. Productvty of these hawks appears to be chefly related to reservor level and prey avalablty. A large concentraton of Ospreys (Pan&on haluetus) nest n Long Valley, Valley County, Idaho. Early census data ndcate that ths area contaned fewer than a dozen Osprey nests n the late 1940 s (La&on et al. 1967). Although the nests were not studed ntensvely at that tme, comparsons wth recent data suggesthat the populaton has ncreased snce Cascade Reservor was formed n In the past decade, the human populaton of Long Valley has ncreased 56% (State of Idaho 1980) and recreatonal use of Cascade Reservor has ntensfed. In lght of expandng human pressure on ths unque Osprey habtat, the U.S. Bureau of Reclamaton sponsored a three-year ( ) nvestgaton to collect base-lne nformaton on ths populaton. The objectves were to: 1) census the populaton and determne ts productvty, 2) dentfy factors nfluencng nest ste selecton and nestng success, and 3) ascertan feedng habts. STUDY AREA Long Valley s located n west-central Idaho approxmately 100 km north of Bose, Idaho (Fg. 1). The area s a broad, flat, glacal valley wth a mean elevaton of 1,525 m. Vegetaton n the valley ranges from agrcultural grasslands to dense rparan stands of lodgepole pne (Pnus contorta). Long Valley s surrounded by grantc mountan ranges wth peaks to 2,750 m. Ponderosa pne (Pnus ponderosa), grand fr (des grands) and Douglas fr (Pseudotsuga menzesz] comprse the domnant overstory vegetaton on the hllsdes. Precptaton vares from about 57 cm n the valley to 125 cm n the hgher elevatons, wth most receved as snow (N.O.A.A l 980). The average growng season n the valley s 65 days, and large bodes of water reman frozen untl late Aprl. For the purpose of ths nvestgaton, we dvded the Long Valley Osprey populaton nto three subpopulatons: Payette Lakes, Cascade Reservor and Warner Pond. These subpopulatons do not represent dscrete bologcal unts, but reflect the nestng habtat of the Ospreys. Ospreys n the Payette Lakes subpopulaton nest near Payette and Lttle Payette lakes. These deep, glacal lakes support populatons of salmonds and northern squawfsh (Ptychochelus oregonenss). Water levels n both lakes have been rased for rrgaton. Trees klled as a result of ths, and those on steep hllsdes around the lakes, provde Ospreys wth nest stes. In 1948, the U.S. Bureau of Reclamaton completed a dam on the North Fork of the Payette Rver near Cascade, Idaho. The resultng reservor s about 27 km long and ranges n wdth from 1.6 to 6.4 km. Its mean depth at hgh water s 7.6 m. Ths shallow, large reservor (surface area 11,452 ha; U.S.G.S. 1980) supports an abundance of warm-water fshes, ncludng yellow perch (Perca jluvescens), northern squawfsh, largescale sucker (Cutostomus mucrochelus) and brown bullhead (Zctulurus nebulosus). Smaller populatons of salmonds, ncludng ranbow trout (Sulmo gurdner), mountan whtefsh (Prosopum wllamson), coho salmon (Oncorhynchus ksutch) and kokanee salmon (0. nerku) are also present. Ospreys nest on hllsde snags near Cascade Reservor. v921

2 OSPREY PRODUCTIVITY 293 In the Warner Pond subpopulaton, Ospreys nest near three small reservors along trbutares of the North Fork of the Payette Rver (Warner Pond, Horsethef Reservor and Corral Creek Reservor). Ranbow trout have been ntroduced nto these reservors; however, fluctuatons n water levels between and wthn years affect the szes of these fsh populatons. Snags and large confers provde abundant nestng stes for Ospreys n ths area. METHODS PRODUCTIVITY We located Osprey nests by ground and aeral searches, and by ntervewng local resdents. We then counted nestng pars, ther eggs and young of fledgng age durng two helcopter flghts (Carrer and Melqust 1976) each nestng season. The frst flght was made n early June to count eggs and terrtoral pars. Each nest encountered was categorzed as actve, occuped, or nactve. Nests that were defended and contaned eggs were classfed as actve. Occuped nests had at least one Osprey present but no eggs n the nest. A nest was consdered nactve f we saw no, Osprey n the vcnty and no new nestng materal on the nest. Durng the second flght, n late July, we counted nestlngs. We used data obtaned durng these flghts to calculate varous productvty parameters ncludng mean clutch sze, young per occuped terrtory (actve and occuped nests), young per actve nest and per successful nest, and overall nestng success. By employng one-way analyss of varance tests (ANOVA) and t-tests (Zar 1974) we were able to solate any varatons n productvty parameters among nestng subpopulatons and among nestng seasons. NEST SITES Each nest ste was numbered and vsted at least once to determne ts exact locaton, type of nest support structure and nest heght. We estmated elevatons from U.S. Geologcal Survey topographc maps. Dstances of nests from fshable water, nearest actve Osprey nest and nearest human dsturbance (.e., major road, occuped home, etc.) were estmated from U.S. Forest Servce 1:31,680 quadrate maps. Nests n the vcnty of Horsethef Reservor were excluded from an analyss of factors nfluencng productvty because we were unable to determne ther exact locatons. Elevaton of the nest ste was not consdered an mportant bologcal factor, and therefore was not subjected to statstcal analyss. Nest heghts were recorded n four categores: <5 m, 5-10 m, lo-20 m, >20 m. SEASONS ACTIVE ( IWO) I l RESERVOIR FIGURE 1. Locatons and seasons of use of actve Osprey nests n Long Valley, Idaho, 1979-l 980. Numbers of young Ospreys produced wthn heght categores were then compared wth an ANOVA test. A smlar test was employed to compare the productvty of Ospreys wthn categores of nest support structure (snag, lve tree, power pole or nestng platform). If the ANOVA test ndcated a sgnfcant dfference n these types of nest stes, t-tests were used to solate the dfferences. Dstances of nests to water, actve Osprey nests and human dsturbance were recorded to the nearest meter and segregated nto three categores: close (~500 m), md-dstance (500-1,500 m), and far (> 1,500 m). Productvty n each of the categores was then analyzed wth ANOVA and t-tests. FEEDING HABITS We nvestgated the feedng habts of Ospreys at Cascade Reservor by drect observaton of nestng, perchng and foragng stes, notng prey speces and sze, capture tme and locaton. Intensve (dawn to dusk) observatons were conducted at least once a week throughout the 1978 and 1979 feld seasons (Aprl to August) to obtan detaled nformaton on prey delveres to an actve nest. Dfferences n the number of prey captured on specfc dates of nestlng development between years were tested wth a ch-square test. To estmate the avalablty of varous prey tems to Ospreys, we worked wth personnel

3 294 L. J. VAN DAELE AND H. A. VAN DAELE TABLE 1. Average number of young Ospreys produced per actve nest n Long Valley, Idaho, Locaton a 1980 Total Cascade Reservor 1.17 (27/23) 1.89 (49/26) 1.64 (46/28) 1.58 (122/77) Warner Pond 1.50 (9/6) 1.00 (13/13) 1.oo (5/5) 1.13 (27/24) Payette Lakes 1.00(11/11) 1.13 (18/16) 1.17 (14/12) 1.10 (43139) Long Valley totap 1.18 (47/40) 1.46 (80155) 1.44 (65/45) 1.37 (192/140) Sgnfcant dfference (P < 0.05) III 1979, Cascade Reservor osmevs. were more productve than ether Warner Pond or Payette Lakes ospreys. b Nb sgnfcant dfference. (Young Ospreys/actve nests). of the Idaho Department of Fsh and Game settng horzontal gll nets and Fyke nets to sample fshes n the upper 2 m at varous locatons n Cascade Reservor n 1979 and Net captures were compared wth Osprey captures by means of ch-square and Bonferon-z tests (Neu et al. 1974). Fsh obtaned from the nets were weghed and measured. A lnear regresson formula (length vs. log,, weght) was appled to these data to generate a model for predctng prey weght and bomass from length. We perodcally collected prey remans from below Osprey nests and perches, and compared them to a reference collecton to determne speces (Swenson 1978). Opercular (gll cover) bones were used n a model to estmate prey sze (Newsome 1977, Prevost 1977). We constructed the operculum model by collectng and measurng average maxmum operculum lengths from fsh of known lengths obtaned n the net samples. Lnear regresson analyss was employed to create a predctve model of total fsh length from operculum length. Chsquare analyss was used to compare prey collectons to vsual observatons of Osprey captures. RESULTS PRODUCTIVITY Durng the three nestng seasons of ths study, we found 110 Osprey nests n the Long Valley area. In 1978,40 nests were actve and 2 occuped, n 1979, 58 actve and 3 occuped, and n 1980, 46 actve and 6 occuped (Fg. 1). Terrtoral, nonbreedng pars consttuted 7.3% of the populaton durng the study. Clutch szes averaged 2.58 eggs per actve nest and dd not change sgnfcantly (P > 0.05) among years or subpopulatons. Sxty-eght percent of the nestng attempts were successful n fledgng at least one young, and 66% ofthe eggs lad developed nto fledglngs. Productvty averaged 1.37 young per actve nest; however, the three subpopulatons dffered sgnfcantly (P < 0.05) n Durng that year, Ospreys nestng near Cascade Reservor produced sgnfcantly more young per actve nest than those nestng n the other subpopulatons (Table 1). The number of young per successful nest averaged 2.00 but dffered sgnfcantly among years (P < 0.05; Table 2). Average number of young produced per terrtoral par was 1.27 and dd not change sgnfcantly among years. The nestng chronology of these Ospreys resembled that of Ospreys nestng n north Idaho (Melqust 1974). In the Long Valley populaton, the nestng perod averaged 50 to 60 days. NEST SITES All Osprey nests n Long Valley had a relatvely unobstructed vew of ther surroundngs and all had at least one nearby perch where the male rested. Nest ste elevatons ranged from 1,463 to 1,768 m and averaged 1,557 m. There appeared to be no shortage of sutable nestng stes n the study area. Most Osprey nests (82%) were bult more than 20 m from the ground and most were on prvate land (70%). Productvty dd not dffer sgnfcantly (P > 0.05) between the nest heght categores. Although most nests were on snags, such nests had the lowest productvty (Table 3). Productvty dffered sgnfcantly (P < 0.05) TABLE 2. Average number of young Ospreys produced per successful nest n Long Valley, Idaho, Locaton Total Cascade Reservor 1.69 (27/16) 2.33 (49/21) 2.19 (46/21) 2.10 (122/58) Warner Pond 1.80 (9/5) 1.63 (13/8) 1.67 (5/3) 1.69 (27/16) Payette Lakes 1.57 (1 l/7) 2.57 (18/7) 1.75 (14/8) 1.96 (43/22) Long Valley total 1.68 (47/28) 2.22 (80/36) 2.03 (65/32) 2.00 (192/96) r Productvty n 1978 was sgnfcantly lower (P < 0.05) than n 1979 or b (Young Ospreys/successful nests).

4 OSPREY PRODUCTIVITY 295 TABLE 3. Osprey productvty as related to nest support structure n Long Valley, Idaho, Percent n parentheses. Snag Type of support structwe Lve tree Power pole Nestng platform Total Actve nests 84 (66.1) 25 (19.7) 11 (8.7) 7 (5.5) 127 (100.0) Young produced 101 (56.4) 39 (21.8) 22 (12.3) 17 (9.5) 179 (100.0) Average young per actve nest a Natural stes (snag and lve tree) sgnfcantly less productve than artfaal stes (power pole and nestng platform; P < 0.05). between snags and artfcal stes (power poles and artfcal nestng platforms) and between natural stes (snags and lve trees) and artfcal stes. We noted no sgnfcant dfferences (P < 0.05) n productvty relatve to the dstance of a nest to fshable water or other actve Osprey nest. The categorcal dstance from human dsturbances was found to affect productvty sgnfcantly (P < 0.05). Further analyss ndcated that nests farther than 1,500 m were sgnfcantly more productve than those nests closer to regular human actvtes (Table 4). FEEDING HABITS All Osprey prey tems seen and collected from below perches and nests were fsh, and Ospreys were observed fshng n all areas of Cascade Reservor. We saw Ospreys from the Warner Pond subpopulaton regularly fshng at Cascade Reservor, and prey collectons at nests up to 10 km from the reservor revealed bullhead remans. In the valley, ths speces s restrcted to Cascade Reservor. The det of Long Valley Ospreys conssted prmarly of brown bullheads (Table 5), wth prey tems usually n the 1 l-30 cm sze class (Table 6). Det changed sgnfcantly (P < 0.05) n composton by month (Fg. 2) especally wth respect to salmonds and squawfsh. A change approachng sgnfcance (0.10 > P > 0.05) was noted between 1978 and 1979 n the detary composton of the Ospreys at our ntensve observaton nest (Table 7). Although t s mpractcal to extrapolate ths change to the entre populaton, we noted a sgnfcant ncrease (P < 0.05) n the number of squawfsh taken when the reservor level was low (1979) versus when the level was hgh (1978). Comparng Osprey captures wth net captures from Cascade Reservor, the hawks caught sgnfcantly more (P < 0.05) bullheads and salmonds, and fewer squawfsh, perch and suckers than were caught n the nets (Table 5). Intensve observatons of nestng Ospreys n 1978-l 979 ndcated that males captured 95% ofthe prey brought to the nest. These delveres occurred throughout the day; however, most were durng the mornng and late afternoon-early evenng (Fg. 3). The tmng of prey delveres was constant n both years and throughout the nestng seasons, regardless of the number of young n a nest. After eggs hatched, adults brought an average of 4.6 fsh per day to a nest wth two young, and 5.6 fsh per day to a nest wth three young. Compared wth Garber s (1972) data for prey delveres at Eagle Lake, Calforna, Ospreys of the Long Valley populaton followed a smlar delvery pattern, but consstently delvered more prey per day. Analyss of prey bomass brought to nestlngs s a more meanngful statstc than the number of fsh delvered daly. Usng a model developed by Wens and Inns (1974), Lnd (1976) calculated that adult Ospreys requre 286 kcal per day, and juvenles of fledgng age need 254 kcal per day. Wnberg (1960) estmated fsh to contan 1 kcal per gram body weght. Usng these values, a nest wth two young and one adult (male Ospreys rarely ate TABLE 4. Osprey productvty as related to the dstance of the nest ste to human dsturbance, Long Valley, Idaho, Percent n parentheses. ClOX (<500 In) Dstance to human dsturbance Md-dstance FX (50&1,500 m) (21,500 m) Totalh Actve nests 70 (55.1) 32 (25.2) 25 (19.7) 127 (100.0) Young produced 95 (53.1) 36 (20.1) 48 (26.8) 179 (100.0) Average young per actve nest d Regular human dsturbance (well traveled road, occuped dwellng, etc.). b Average dstance from human dsturbance = 770 m (2526 ). c Far nests sgnfcantly more productve than md-dstance or close nests (P < 0.05)

5 296 L. J. VAN DAELE AND H. A. VAN DAELE whte = unknown.largescale sucker yellow perch northern squawfsh.salmonds TABLE 6. Sze of fsh taken as prey by Ospreys n Long Valley, Idaho, Percent n parentheses. Sze class O-10 cm 1 l Total Number taken by Ospreys 5 (3.3) 64 (42.1) 71 (46.7) 10 (6.6) 2 (1.3) 152 (100.0) MONTH brown bullhead FIGURE 2. Prey consumpton, by month, of Ospreys nestng n Long Valley, Idaho, 1978-l 979. at the nest) would requre 794 g of fsh per day at fledgng tme, and a nest wth three young and one adult would requre 1,048 g. Although prey delveres were more erratc n 1978 (two young) than n 1979 (three young), the calculated mnmum requrement was acheved pror to fledgng n both years (Fg. 4). In both years, males spent an average of 40% of the daylght hours perched near the nest, a pattern smlar to that noted by Stnson (1978). In 1978 males were, however, away from the nest for sgnfcantly (P < 0.05) longer perods of tme than n We used fsh captured by nets to regress operculum lengths on total lengths and, thus, determne the sze of fsh captured based on prey remans at perchng and nestng stes. Correlaton coeffcents (r) n lnear regresson models of total fsh lengths to operculum lengths ranged from 0.95 to 0.99 for the fve prey groups tested (n = ). Operculum shapes were suffcently dfferent to allow speces dentfcaton except n the case of salmonds. Thus, mountan whtefsh, ranbow trout, coho salmon and kokanee salmon were only dentfed to the famly level. The speces composton of opercula found at perchng and nestng stes was not sgnfcantly dfferent (P > 0.05) from what we observed captured by Ospreys. However, n prey collectons from perchng stes, yellow perch were sgnfcantly more abundant than n collectons from below nests (P < 0.05). Ths suggests that collectons must be made from both perchng and nestng stes n order to obtan an accurate estmate of the det of these brds. DISCUSSION A long-term productvty average of 0.95-l.30 young per actve nest (Henny and Wght 1969, Henny 1977) and a populaton composed of 5-10% nonbreedng pars (Henny and Van Velzen 1972) have been suggested for the mantenance of a stable Osprey populaton. Durng our study, the Long Valley populaton met or exceeded these productvty and recrutment requrements necessary for stablty. These productvty estmates compare favorably wth those of other stable Osprey populatons (Henny 1977, MacCarter and MacCarter 1979, Newton 1979) and they sug- TABLE 5. Summary of Osprey prey captures versus net captures n Cascade Reservor, Idaho, Sample sze n parentheses. Prey speces Brown bullhead Salmonds Northern squawfsh Yellow perch Largescale sucker Total Observed Osprey captures Gll aod Fyke net captures 37.7% (78) 7.6% (75) 20.8 (43) 5.9 (58) 19.3 (40) x 36.4 (359) 11.6 (24) 28.4 (280) 10.6 (22) 21.7 (214) (207) (986) r Includes ranbow trout, coho salmon, kokanee salmon and mountan whtefsh. TABLE 7. Prey tems brought to an Osprey nest on Gold Fork Arm of Cascade Reservor, Idaho, 1978-l 979. Sample sze n parentheses. Prey speces Brown bullhead 36.4% (20) 22.2% (16) Salmondsa 23.6 (13) 18.1 (13) Northern squawfsh 10.9 (6) 33.3 (24) Largescale sucker Yellow perch Z ; l::, ; Unknown 12.7 (7) 9.7 (7) Total (55) (72) a Includes ranbow trout, coho salmon, kokanee salmon and mountan whtefsh.

6 OSPREY PRODUCTIVITY 297 gest that the populaton s contnung to ncrease. The growth of ths populaton s largely due to local nestng success, snce most Ospreys return to breed n the vcnty of ther natal areas (Henny and Van Velzen 1972, Henny 1977). As Osterljff (1977) noted, however, some Ospreys establsh ther frst breedng terrtores more than 1,000 km from ther natal areas. Therefore, some members of the breedng populaton may be recruts from other populatons n the western Unted States. Regardless of orgn, t s evdent that the populaton ncrease has been favored by the habtat mprovement provded by local reservors, especally Cascade Reservor. Ospreys can readly poneer new habtat (Henny and Noltemeer 197 5) and reservor development has been responsble for much of the expanson of the breedng range of Ospreys throughout the western Unted States (Roberts and Lnd 1977, Henny et al. 1978a, b). Before Cascade Reservor was formed, Long Valley s cool clmate and deep lakes probably provded margnal habtat for Ospreys. Reservor development and subsequent management practces have produced an excellent shallow-water fshery and, thus, abundant food for these brds. The TIME OF DAY FIGURE 3. Tmng of prey delveres to an Osprey nest n Long Valley, Idaho, mportance of Cascade Reservor to Ospreys s evdent from the fact that 65% of the Ospreys nestng n Long Valley regularly fsh n the reservor, some flyng up to 10 km to do so. Durng the course of our study, weather changes may have accounted for some of the varaton n the length of the nestlng perod and productvty. Ospreys nestng n smlar habtat n Sweden also had lower productvty followng cold, wet sprngs (Odsjb and Sondell 1976). The most mportant factor nfluencng productvty n Long Valley, however, was prey I- -_- l -* 1978 averages z z cll I800 2oc C 1 1 I I I I I I I I lay-391 hatch I I I I & F averages (3young+I adult) young+ I adult) AGE OF NESTLINGS (DAYS) - 32 FIGURE 4. Prey bomass delvered to an Osprey nest n Long Valley, Idaho, Averages are three-day movng averages.

7 29% L. J. VAN DAELE AND H. A. VAN DAELE avalablty. Sgnfcant productvty changes noted n the Cascade Reservor subpopulaton durng ths study were related to changes n water levels. Productvty was hghest n 1979, the year of the lowest water levels. Observatons of a nestng Osprey par ndcated that the male spent sgnfcantly more tme away from the nest ste, and that he delvered fewer fsh durng the hgh-water years. Although ntensve data are avalable only from ths nest, observatons of other nestng pars around the reservor substantate the hypothess that prey was more readly avalable to Ospreys at Cascade Reservor durng the year of low water. Smlar numercal responses n the form of ncreased productvty to an ncrease n prey avalablty have been noted n other western U.S. Osprey populatons (Kopln et al. 1977, MacCarter and MacCarter 1979). Although productvty s reduced, suffcent prey are secured durng the years of hgh reservor levels to meet productvty requrements for a stable populaton. However, ncreased prey avalablty durng the low-water perod mproves productvty and enhances the opportuntes for contnued expanson of the populaton. The det of Ospreys changed among and wthn years, apparently n response to changes n prey avalablty. Bullheads were more readly avalable to Ospreys than net samples ndcated, because they rest near the surface of the reservor on warm days and they avod predators less. Swenson (1979) noted that benthc-feedng fshes are often more vulnerable to Osprey attack than predaceous speces whch have a better predator avodance response. Ths hypothess may explan why Ospreys took fewer squawfsh and perch than were apparently avalable; however, one would expect suckers to be taken more because of ther sluggsh behavor. Most suckers caught n nets were too large to be easly captured by Ospreys and, hence, were not avalable as prey. Salmonds spawn throughout most of the Osprey nestng season n Long Valley, renderng them vulnerable to predaton. Yet the bulk of the salmond catch occurred n May, when 35,000 to 50,000 catchable-szed ranbow trout were released nto Cascade Reservor (D. Anderson, Idaho Fsh and Game, pers. comm.). These hatchery-rased trout were unfamlar wth ther surroundngs and vulnerable to predaton. Fsh njured n the stockng process may have formed a part of the catch also. Ospreys caught more squawfsh n a lowwater year than n a hgh-water year because of squawfsh spawnng behavor. Squawfsh spawn n the arms of Cascade Reservor n late June and early July (Casey 1962). Shallow spawnng areas ncrease squawfsh vulnerabl- ty to Osprey capture. Snce squawfsh were the most mportant prey tem n July, changes n ther avalablty were largely responsble for the dfferences n the fshng success of male Ospreys between hgh and low-water years. Our data show that Ospreys dd not actvely select for or aganst fsh speces n Cascade Reservor. Instead, capture rates reflect the vulnerablty of each speces to capture. Prevost (1977) saw an analogous response of ncreased capture success of more readly avalable prey by Ospreys nestng n Nova Scota. Ospreys are known to be adaptable and hghly moble raptors. These trats allow them to nest successfully n a varety of condtons (Henny et al. 1974, Henny and Noltemeer 1975, Lnd 1976, Sptzer 1977) and readly adjust to some human dsturbances. The only nest ste parameters whch affected productvty n Long Valley were the type of structure on whch nests were bult and the dstance of the nests from human dsturbances. Osprey pars nestng on artfcal stes were the most productve because these stes provded a stable support, mnmzng the chance of blowdowns durng severe wnd storms. Most of these stes were also solated from human dsturbances, ether by ther heght or locaton. Other studes have reported greater productvty on artfcal structures; however, nestng success on artfcal structures has not been hgher n every nstance (Peterson 1969, Melqust 1974, Rhodes 1977, Postupalsky 1978, Van Daele 1980). Durng our study, Ospreys nestng more than 1,500 m from human dsturbances were the most productve, yet the brds frequently nested close to humans. Habtuaton to human actvtes appeared to vary dependng on the frequency of dsturbance. Ospreys nestng near humans eventually tolerated ther actvtes whle those nestng farther from humans were less tolerant. Productvty nformaton and subjectve observatons substantate ths concluson. Ospreys nestng close to humans generally stayed on ther nests longer and were more terrtoral durng our approaches than those nestng at a greater dstance, partcularly durng the ncubaton perod. Sptzer (1977) ndcated that Osprey eggs must be kept at C to reman vable, and our observa- tons revealed that Ospreys at successful nests ncubated % of the daylght hours. Dsturbances durng the crtcal perods of ncubaton and early nestng stages can be fatal to embryos and nestlngs f adults are kept from ther nests. Therefore, untl an Osprey par becomes habtuated to human actvtes, such actvtes wll jeopardze ther nestng success.

8 OSPREY PRODUCTIVITY 299 ACKNOWLEDGMENTS Intaton and complete fundng of ths project were provded by the U.S. Bureau of Reclamaton, G.S.D.I., tentral Snake Proects Offce. Bose. Idaho. Donald R. Johnson was nvaluable to us through&t all aspects of the study and we are especally grateful to hm. We also thank w. Melaust for hs assstance and S. Peterson, C. Hennv and M. Brd for ther professonal advce and &tcsms of the manuscrpt. We are oblged to personnel from the Idaho Department of Fsh and Game and the U.S. Forest Servce for ther complete cooperaton. Fnally, we thank the people of Long Valley for ther nformaton and support. LITERATURE CITED CARRIER, W. D., AND W. E. MELQUIST The use of a rotor-wnged arcraft n conductng nestng surveys of Ospreys n northern Idaho. Raptor Res. 10: CASEY, 0. E The lfe hstory of the northern squawfsh n Cascade Reservor. M.Sc. thess, Unv. of Idaho, Moscow. GARBER, D. P Breedng ecology of ospreys n Lassen and Plumas countes, Calforna. M.Sc. thess, Humboldt State Unv., Arcata, CA. HENNY. C. J Research. management and status of the Ospreys n North Amerca, In World Brds of Prey Conf. Internatonal Councl for Brd Preservaton, Venna. HENNY, C. J., AND A. P. NOLTEMEIER Osprey nestng populatons n the coastal Carolnas. Am. Brds 29: HENNY, C. J., AND W. T. VAN VELZEN Mgraton patterns and wnterng localtes of Amercan Ospreys. J. Wldl. Manage. 36: 1133-l 144. HENNY, C. J., AND k M. WIGHT An endangered Osprey populaton: estmates of mortalty and producton. Auk 86: HENNY, C. J., M. M. SMITH, AND V. D. STOTTS The 1973 dstrbuton and abundance of breedng Ospreys n the Chesapeake Bay. Chesapeake Sc. 15: HENNY, C. J., J. A. COLLINS, AND W. J. DEIBERT. 1978a. Osprey dstrbuton, abundance and status n western North Amerca: II. The Oregon populaton. Murrelet 59: HENNY, C. J., D. J. DUNAWAY, R. D. MALLETTE, AND J. R. KOPLIN. 1978b. Osprey dstrbuton, abundance and status n western North Amerca: I. The northern Calforna populaton. Northwest Sc. 52:26 l KOPLIN, J. R., D. S. MACCARTER, D. P. GARBER, AND D. L. MACCARTER Food resources and fledglng productvty of Calforna and Montana Ospreys. Proc. North Amercan Osnrev Research Conf. U.S. Natl. Park Serv. Trans. Pr&Seres No. 2: LARRISON, E. J., J. L. TUCKER, AND M. T. JOLLIE Gude to Idaho brds. J. Idaho Acad. Sc. 5: LIND, G. S Producton, nest ste selecton and food habts of Ospreys on Deschutes Natonal Forest, Oregon. M.Sc. thess, Oregon State Unv., Corvalls. MAC~ARTER, D. L., AAD I% S. MACCARTE~ Tenyear nestng status of Ospreys at Flathead Lake, Montana. Murrelet 60: MELQUIST, W. E Nestng success and chemcal contamnaton n northern Idaho and northeastern Washngton Ospreys. M.Sc. thess. Unv. of Idaho, Moscow. NATIONAL OCEANOGRAPHIC AND ATMOSPHERIC ADMINIS- TRATION Clmatologcal data: Idaho. U.S. Dep. Commer. Ashevlle, NC. NEU, C. W., C. R. BYERS, AND J. M. PEEK A technque for analyss of utlzaton avalablty data. J. Wldl. Manage. 38: NEWSOME, G. E Use of opercular bones to dentfy and estmate lengths of prey consumed by pscvores. Can. J. Zool NEWTON, I Populaton ecology of raptors. Buteo Books, Vermllon, SD. ODSJQ T., AND J. SONDELL Reproductve success of Ospreys (Pandon halaetus) n southern and central Sweden, Oms Stand. 7: &TERL~~FF, S Mgraton, wnterng areas and ste tenacty of the European Osprey (Pandon halaetus halae&). Oms S&d. 8:6-78. PETERSON. R. T Pooulaton trends of Osorevs n the nbrtheastem Unted States, p Zn J. J. Hckey [ed.], Peregrne Falcon populatons. Unv. of Wsconsn Press, Madson. POSTUPALSKY, S Artfcal nestng platforms for Ospreys and Bald Eagles, p In S. A. Temple [ed.], Endangered brds: management technques for preservng threatened speces. Unv. of Wsconsn Press, Madson. PREVOST, Y. A Feedng ecology of Ospreys n Antgonsh County, Nova Scota. M.Sc. thess, McGll Unv., Montreal. RHODES, L. I An Osprey populaton aded by nestng structures. Proc. North Amercan Osprey Research Conf. U.S. Natl. Park Serv. Trans. Proc. Seres No. 2~ ROBERTS, H. B., AND G. S. LIND Status of the Amercan Osprey n Oregon. Proc. North Amercan Osprey Research Conf. U.S. Natl. Park Serv. Trans. Proc. Seres No. 2: SPITZER, P. R Osprey egg and nestlng transfers: ther value as ecologcal experments and as management procedures, p. 17 l-l 82. In S. A. Temple [ed.], Endangered brds: management technques for preservng threatened speces. Unv. of Wsconsn Press, Madson. STATE OF IDAHO Corrected prelmnary census populaton for Idaho countes and ctes. Unpubl. mmeo. report. Dv. of Econ. and Comm. Affars, Bose, ID. STINSON, C. H The nfluence of envronmental condtons on aspects of the tme budgets of breedng Ospreys. Oecologa 36: 127-l 39. SWENSON, J. E Prey and foragng behavor of Ospreys on Yellowstone Lake, Wyomng. J. Wldl. Manage. 42: SWENSON, J. E The relatonshp between prey speces ecology and dve success n Ospreys. Auk 96: UNITED STATES GEOLOGICAL SURVEY Water resources data for Idaho: water year 1978, vol. 2. U.S.G.S. water data report ID-78:2. Washngton. DC. VAN DAELE, L. J Ospreys and power poles& Idaho, p. 104-l 12. In R. P. Howard [ed.], A workshop on raptors and energy developments. U.S. Fsh and Wldlfe Serv., Bose, ID. WIENS, J. A., AND G. S. INNIS Estmaton of energy flow n brd communtes: a populaton boenergetcs model. Ecology 55: WINBERG, C. G Rate of metabolsm and food requrements of fshes. Fsh. Res. Board Canada. Transl. Ser ZAR, J. H Bostatstcal analyss. Prentce-Hall, Englewood Clffs, NJ. Department of Bologcal Scences- Zoology, Unversty of Idaho, Moscow, Idaho Present address: Alaska Department of Fsh and Game, 333 Raspberry Road, Anchorage, Alaska Receved 2 March Fnal acceptance 3 December 1981.