The Journal of tor Research Volume 34 Number 2 June 2000 Published by The Raptor Research Foundation, Inc Contents Nesting, Population Trend and Breeding Success of Peregrine Falcons on the Washington Outer Coast, 1980—98. Ulrich W. Wilson, Anita McMillan and Frederick c. Dobler 67 Landscape Characteristics of Northern Spotted Owl Nest Sites in Managed Forests of Northwestern California. Lee b. FoiUard, Kerry p. Reese and LoweU v. Diiier .... 75 Identification of Individual Barred Owls Using Spectrogram Analysis and Audi- tory Cues. Pamela L. Freeman 85 Home Range and Habitat Use by the Long-eared Owl in Northwestern Switzer- land. Fabienne Henrioux 93 Habitat Selection by Tawny Fish-Owls {Ketupa flawpes) in Taiwan. Yuan-Hsun Sun, Ying Wang and Ching-Feng Lee 102 A Review of the Trophic Ecology of the Barn Owl in Argentina, m. Isabel Beiiocq .... 108 Nesting Biology and Behavior of the Madagascar Harrier-Hayvk (Polyboroides RADIATUS ) IN NORTHEASTERN MADAGASCAR. Russell Thorstrom and Guiseppe La Marca 120 Probable Effect of Delisting of the Peregrine Falcon on Availability of Urban Nest Sites. Mark S. Martell, Jennifer L. McNicoll and Patrick T. Redig 126 Raptor Surveys in Southcentral Nevada, 1991-95. Patrick E. Lederie, James M. Mueller and Eric A. Holt 133 Handicapped American Kestrels: Needy or Prudent Foragers? Gillian l. Murza, Gary R. Bortolotti and Russell D. Dawson 137 Migratory Movements of the White-throated Hawk {Buteo albigula) in Chile. Eduardo E. Pavez 143 Short Communications First Nesting Record of the Nest of a Slaty-Backed Forest-Falcon {Micrastur mieand olifj ) in YasunI National Park, Ecuadorian Amazon. Tjitte de Vries and Cristian Melo 148 Letters 151 Book Reviews. Edited by Jeffrey S. Marks 153 The Raptor Research Foundation, Inc. gratefully acknowledges a grant and logistical support provided by Boise State University to assist in the publication of the journal. THE RAPTOR RESEARCH EOUNDATION, INC. (Founded 1966) OFFICERS PRESIDENT: Michael N. Kochert SECRETARY: Patricia A. ITaix VICE-PRESIDENT: Keith L. Bildstein TREASURER: Jim Eitzpatrick BOARD NORTH AMERICAN DIRECTOR #1: Philip Detrich NORTH AMERICAN DIRECTOR #2; Petra Bohall Wood NORTH AMERICAN DIRECTOR #3: Robert Lehman INTERNATIONAL DIRECTOR#!: Eduardo Inigo-Elias INTERNATIONAL DIRECTOR #2: Reuven Yosef OF DIRECTORS INTERNATIONAL DIRECTOR #3: Beatriz Arroyo DIRECTOR AT LARGE #1; Jemima ParryJones DIRECTOR AT LARGE #2: Robert Kenward DIRECTOR AT LARGE #3: Mictiael W. Collopy DIRECTOR AT LARGE #4: Miguel Ferrer DIRECTOR AT LARGE #5: John A. Smaixwood DIRECTOR AT LARGE #6: Brian A. Milsap EDITORIAL STAFF EDITOR: MarcJ. Bechard, Department of Biology, Boise State University, Boise, ID 83*725 U.S.A. ASSOCIATE EDITORS Allen M. Fish Fabian Jaksic Juan Jose Negro Daniel E. Varland Charles J. Henny Cole Crocker-Bedford Ian G. Warkentin James C. Bednarz BOOK REVIEW EDITOR: Jeffrey S. Marks, Montana Cooperative Research Unit, University of Montana, Missoula, MT 59812 U.S.A. SPANISH EDITOR: Cesar Marquez Reyes, Institute Humboldt, Colombia, AA. 094766, Bogota 8, Colombia EDITORIAL ASSISTANTS: Joan Ciwrk, Keleigh Hague-Bechard, Elise Vernon Schmidt The Journal of Raptor Research is distributed quarterly to all current members. Original manuscripts dealing with the biology and conservation of diurnal and nocturnal birds of prey are welcomed from throughout the world, but must be written in English. Submissions can be in the form of research articles, letters to the editor, thesis abstracts and book reviews. Contributors should submit a typewritten original and three copies to the Editor. All submissions must be typewritten and double-spaced on one side of 216 X 278 mm (8U X 11 in.) or standard international, white, bond paper, with 25 mm (1 in.) margins. The cover page should contain a title, the author’s full name(s) and address (es). Name and address should be centered on the cover page. If the current address is different, indicate this via a footnote. A short version of the title, not exceeding 35 characters, should be provided for a running head. An abstract of about 250 words should accompany all research articles on a separate page. Tables, one to a page, should be double-spaced throughout and be assigned consecutive Arabic numer- als. Collect all figure legends on a separate page. Each illustration should be centered on a single page and be no smaller than final size and no larger than twice final size. The name of the author (s) and figure number, assigned consecutively using Arabic numerals, should be pencilled on the back of each figure. Names for birds should follow the A.O.U. Checklist of North American Birds (7th ed., 1998) or another authoritative source for other regions. Subspecific identification should be cited only when pertinent to the material presented. Metric units should be used for all measurements. Use the 24-hour clock (e.g., 0830 H and 2030 H) and “continental” dating (e.g., 1 January 1990). Refer to a recent issue of the journal for details in format. Explicit instructions and publication policy are oudined in “Information for contributors,” /. Raptor Res., Vol. 33(4), and are available from the editor. COVER: Peregrine Falcon {Falco peregrinus) . Painting by Brian K. Wheeler. THE JOURNAL OF RAPTOR RESEARCH A QUARTERLY PUBLICATION OF THE RAPTOR RESEARCH FOUNDATION, INC. VoL. 34 June 2000 No. 2 J Raptor Res. 34(2):67-74 © 2000 The Raptor Research Foundation, Inc. NESTING, POPULATION TREND AND BREEDING SUCCESS OF PEREGRINE FALCONS ON THE WASHINGTON OUTER COAST, 1980-98 Ulrich W. Wilson U.S. Fish and Wildlife Service, Washington Maritime National Wildlife Refuges, P.O. Box 450, Sequim, WA 98382 U.S. A. Anita McMillan Washington Department of Fish and Wildlife, RO. Box 1686, Port Angeles, WA 98362 U.S. A. Frederick C. Dobler Washington Department of Fish and Wildlife, 2108 Grand Blvd., Vancouver, WA 98661 U.S. A. Abstract. — ^We monitored the Peregrine Falcon (Falco peregrinus) nesting population of the outer coast of Washington State’s Olympic Peninsula during 1980-98. Peregrine Falcon nesting was concentrated in the central portion of the area, where most of Washington’s small seabirds nest. During our study, occupied sites increased from 3 to 24, breeding pairs from 2 to 17 and successful pairs from 2 to 13. The mean annual nest site failure rate varied between 0-60%, whereas the mean annual number of young per breeding pair varied from 0.8-2. 5 young. Successful pairs produced an average of 1. 5-3.0 young annually, increasing significantly (P < 0.02) during the study period and approaching that of a reproductively healthy, stable population at Langara Island, British Columbia. This marine peregrine population produced significantly fewer young during El Nino years. Continued close monitoring of Peregrine Falcons is necessary until populations reach their carrying capacity. Key Words: Peregrine Falcon-, Falco peregrinus; breeding success; El Nino; helicopter surveys; population trend; seabird colonies. Anidacion, tendencia poblacional y exito reproductive de halcones peregrines en la costa de Washing- ton, 1980-98 Resumen. — Monitoreamos la poblacion anidante de halcones peregrinos {Falco peregrinus) de la costa del Estado de Washington en la Peninsula Olimpica durante 1980-98. Los halcones peregrinos se concentra- ron en la parte central del area, en donde la mayoria de pequenas aves marinas de Washington anidan. Durante nuestro estudio, los sitios de nidos ocupados se incrementaron de 3 a 24, las parejas en reprod- uccion de 2 a 17 y las parejas exitosas de 2 a 13. La media anual de la tasa de fracaso de anidacion vario de 0.8-2.5 juveniles. Las parejas exitosas produjeron un promedio de 1. 5-3.0 juveniles anualmente, lo cual represent© un incremeto significativo {P < 0.02) durante el period© de estudio, aproximandose asi a la poblacion estable y exitosa de la isla Langara, British Columbia. Esta poblacion marina de peregrinos produjo significativamente menos juveniles durante los ahos del Nino. El monitoreo cercano de los hal- cones peregrinos es necesario hasta que las poblaciones alcancen la capacidad de carga. [Traduccion de Cesar Marquez] The ecology and population status of the Pere- grine Falcon {Falco peregrinus) has received much 67 attention in the past several decades, following widespread population declines primarily related 68 Wilson et al. VoL. 34, No. 2 to use of DDT (Hickey 1969, Cade et al. 1988) and subsequent recovery (Federal Register 1999) due to restrictions placed on the use of this pesticide (Cade et al. 1988, Enderson et al. 1995), and be- cause of reintroductions of captive-bred birds into their former range (Enderson et al. 1995, Cade et al. 1996). In Washington, a population of pere- grines that has not been previously studied inhabits the remote and rugged outer coast of the Olympic Peninsula. The area is of interest not only because of the lack of published information on it, but also because it is a transition zone, resembling the Brit- ish Columbia coast more closely than the Oregon or California coasts. We report here on the results of long-term monitoring efforts of nesting pere- grines in this area. Study Area and Methods The study area was the outer coast of Washington’s Olympic Peninsula (Fig. 1). Between Neah Bay (48°21'N, 124°37'20'W) and Point Grenville (47°18'N, 124°16’45'Wh located 5.7 km south of the mouth of the Quinault River, 28 m^or islands and hundreds of smaller rocks and reefs occur within 3 km of shore. Most of the islands along this 130 km long coastal stretch are typical sea stacks with tall, rugged cliffs, but a few of them support vegetation domi- nated by salal {Gaultheria shallon) and salmonberry (Rubus spectabilis) . Several also have small stands of Sitka spruce {Picea sitchensis) . These islands are part of Washington Is- lands National Wildlife Refuge and support approximately 109 000 breeding pairs of seabirds (Speich and Wahl 1989). The mainland shoreline is characterized by rugged headlands with towering cliffs that rise out of the ocean, separated by beaches and river mouths. Much of this shoreline is part of Olympic National Park, whereas the marine waters surrounding this area are part of the Olym- pic Coast National Marine Sanctuary. From 1980-98, we monitored the area’s peregrine pop- ulation annually by determining the number of occupied sites (at least one adult present), breeding pairs (adult seen in incubating posture or eggs observed), successful pairs (young produced) and the number of young pro- duced by each successful pair. We searched for nest sites during April and May of each year, and revisited the sites with incubating birds until the breeding outcome could be determined. Because little was known about the area’s peregrines prior to this study, we initially used a combination of methods to search for nest sites and to determine the number of young produced. During 1980-88, data were collected by walking accessible beaches and making ob- servations from headland overlooks. Islands and main- land cliffs also were frequently surveyed from an inflat- able boat. Occasionally, we surveyed areas and nest sites with a Hughes 500D helicopter. Annual helicopter sea- bird surveys of all islands, rocks, sea stacks and mainland cliffs of the entire study area were conducted from 1984— 98 and contributed to our overall peregrine monitoring efforts. Figure 1. Locations of known Peregrine Falcon breed- ing territories on the outer coast of the Olympic Penin- sula, Washington during 1980-98. Because of the apparent increase in peregrine nesting and the difficulties in surveying the remote outer coast of Washington, we decided to monitor the species exclu- sively by helicopter starting in 1989. These surveys were conducted with a Hughes 500D or Bell 206 Jet Ranger with the passenger door removed. We generally conduct- ed two activity surveys during April and May when all known sites where peregrines had occurred previously June 2000 Washington Peregrine Falcons 69 were checked for incubating birds. During these flights, new areas with potential for peregrine breeding, but where birds had not been observed, also were searched. When peregrines were found, we classified the sites as being either occupied or having breeding pairs. In most cases, we could not confirm whether the birds were ac- tually incubating eggs, because the checks were brief in order to minimize disturbance. The activity surveys were followed up with two or more surveys during late May to July when the sites were checked for the number of young produced. Once the number of young was deter- mined, the sites were not checked again. Because the ages of the young varied during these final checks, our production estimates were not based on the number of young fledged, but on whether the breeding attempt had been successful. The surveys were flown on fair weather days. Observations were made with 7X binoculars and we took photographs of all nesting ledges. Photos were later enlarged and marked with the nest locations and were used as reference points during future surveys. El Nino events are known to cause widespread seabird breeding failures, lowered reproductive success and col- ony abandonment due to a collapse in the marine food chain (Wooster and Fluharty 1985, Wilson 1991). Along the eastern Pacific rim these phenomena manifest them- selves oceanographically in above normal sea surface temperatures, a depression of the thermocline and a rise m sea level (Hamilton and Emery 1985, Norton et al. 1985). To test the hypothesis that these events also affect marine peregrines, we compared breeding success of years under ENSO (El Nino Southern Oscillation) influ- ence, including 1981 which had a non-ENSO type warm water episode of similar magnitude, with breeding suc- cess during non-ENSO years when sea surface tempera- tures were normal. Wilson (1991) identified the 1981 warm event and determined that 1983, 1984 and 1988 were years when Washington outer coast seabirds were affected by warm episodes of ENSO-type origin. From monthly El Nino advisories produced by the National Ma- rine Fisheries Service, we concluded that 1993, 1997 and 1998 also were ENSO years. Of the warm water episodes that occurred during this study, the 1981 and 1988 events were of moderate intensity (Wilson 1991), while the re- maining episodes were severe. Trends in the data were determined with Spearman rank correlation analysis using SYSTAT 7.0 for windows (Wilkinson 1997). This program also produces a smooth curve for scatter plots by running along the x values and finding predicted values from a weighted average of near- by y values (Cleveland 1979). To aid in the interpretation of our data, we added these curves to data sets that showed significant trends. Results We found nesting peregrines throughout our study area, although 56% of the territories were located between the point on the coast west of the southern tip of Lake Ozette and 5 km south of the mouth of the Hoh River (Fig. 1). Sixty-six percent of known breeding pairs nested on islands, while the remainder occurred on mainland cliffs facing the Pacific Ocean. We documented 102 successful breeding attempts in at least 25 distinct territories (alternate nesting ledges in the same area were considered one territory). The number of peregrines increased substan- tially during our study with the largest increase oc- curring after 1988. Occupied sites increased from three in 1980 to 21 in 1998 with a peak of 24 in 1997 (Fig. 2A). This trend was significant (r^ = 0.95, AT = 19, P < 0.001). Breeding pairs increased from two in 1980 to 17 in 1998 (Fig. 2B), while the number of successful breeding pairs increased from two to 13 (Fig, 2C). The trends in both breed- ing pairs and successful breeding pairs were signif- icant (r^ = 0.98, N = 19, P < 0.001; and j; = 0.93, N= 19, P < 0.001, respectively). Because the sharp increase in sites after 1988 coincided with a change in study methods from a combination of methods to monitoring nest sites solely by helicopter, we also analyzed the 1980—88 and 1989—98 data sets separately. Breeding pairs and successful breeding pairs all showed significant positive trends during both time periods (1980-88: = 0.92, N = 9, P < 0.002; Eg = 0.61, N = 9, P < 0.05, respectively; 1989-98: r, = 0.95, 10, P < 0.001; and r, = 0.92, N = 10, P < 0.001, respectively). The failure rate of breeding pairs varied between 0-60% (x = 27%). Excluding the data from 1980- 82 due to small sample sizes, the failure rate showed no trend (Fig. 3A) . The number of young per breeding pair varied between 0.8-2. 5 young {x = 1.7 ± 0.5) and showed no trend (Fig. 3B). Per- egrine breeding success was influenced by El Nino events and a non-ENSO type warm episode that occurred during 1981 (Fig. 3C). The number of young per successful pair varied between 1. 5-3.0 young (x = 2.3 ± 0.4) and showed a significant positive trend for the 1980-98 period (r^ = 0.54, N = 19, P < 0.02). When only normal years were considered, the number of young per successful pair also increased significantly (r^ = 0.59, N = 12, P < 0.05). The difference of the two trend lines (Fig. 3C) showed how El Nino events influenced long-term monitoring of marine peregrines on the Washington coast. The moderate events of 1981 and 1988 only moderately depressed peregrine breeding success (Fig. 3C). During warm water years, successful pairs had smaller broods (Fig. 4) . The mean number of young per successful pair during warm water years was 2.2 ± 0.9 {N =41), compared to 2.6 ± 0.8 {N = 59) during normal years {t^g ^ “2.6, P = 0.01). 70 Wilson et al. VoL. 34, No. 2 Year Year Figure 2. Number of occupied sites (A), breeding pairs (B) and successful pairs (C) of Peregrine Falcons on the outer coast of the Olympic Peninsula, Washington during 1980-98. Figure 3. Percent failures (A), number young per breeding pair (B) and number young per successful pair (C) of Peregrine Falcons on the outer coast of the Olym- pic Peninsula, Washington during 1980-98. Solid black circles represent years under the influence of warm oce- anic conditions. The solid line represents all years, while the dashed line represents normal (non-ENSO) years. June 2000 Washington Peregrine Falcons 71 ENSO Years Non - ENSO Years Figure 4. Number of young in Peregrine Falcon broods during El Nino Southern Oscillation (ENSO) years com- pared to normal (non-ENSO) years on the outer coast of the Olympic Peninsula, Washington during 1980-98. The breeding season of Washington’s coastal peregrines was protracted, and it was not uncom- mon to have young fledge at some nests, while oth- er pairs were still brooding small, downy young. We estimated that fledging occurred as early as 2 June and as late as 20 July. Discussion Our results showed that the Washington outer coast peregrine breeding population experienced a m^or increase during 1980-98. Breeding success improved significantly with success poorest during years of warm oceanic conditions. Unfortunately, little published information on Washington coastal peregrines is available for com- parison with our study. According to Jewett et al. (1953), the species was a common permanent res- ident on the Washington outer coast, with inciden- tal nesting records for Carroll Island, Flattery Rocks and the Quillayute Needles. In 1957, Beebe (1960) searched Carroll Island, Cape Flattery and Neah Bay on the Washington coast and found no peregrines where they had been reported by Daw- son and Bowels (1909). C. Anderson and S. Her- man reported (Walton et al. 1988) no use of 14 historical nest sites by peregrines in 1976, but one new site was located. More recently, Paine et al. (1990) observed an increase in peregrine hunting activity on Tatoosh Island while studying the im- pacts of peregrines on the island’s seabird com- munity. The remoteness and ruggedness of the western Olympic Peninsula has undoubtedly pre- vented any detailed earlier work. This was recog- nized by Nelson (1969) who stated that the many nesting cliffs in Washington make a scientific check of these birds a m^or research problem. The distribution of known peregrine nest sites on the Washington outer coast appeared to be re- lated to the occurrence of islands, sea stacks and rocks, and major seabird colonies. Many of the is- lands and sea stacks with tall cliffs were located in the central portion of the coast, where we found most of the peregrine nest sites. Islands and sea stacks were less common in the other portions of the study area, where there were more beaches and fewer tall cliffs. Beebe (1969) found that, in coastal British Columbia, nesting peregrines preferred ar- eas where there was an abundance of small islands and sea stacks with seabird colonies. In our study area, 90% of an estimated population of 87 600 Cassin’s Auklets {Ptychoramphus aleutica), 98% of approximately 35 700 Leach’s Storm Petrels (Oceanodroma leucorhoa) and 51% of about 3900 Fork-tailed Storm Petrels {Oceanodroma furcata) nest within this area of concentrated peregrine ac- tivity (Speich and Wahl 1989). On the coast of Brit- ish Columbia, peregrines rely heavily on small al- cids and storm petrels {Oceanodroma) for prey (Beebe 1969). The protracted breeding season of Washington’s outer coast peregrines required several productiv- ity surveys because of the different stages of devel- opment of young at the various nest sites. We gen- erally had to schedule at least three flights, two weeks apart, in order to determine the number of young at successful sites. The timing of fledging of Washington coastal peregrines was almost identical to that of British Columbia peregrines (Campbell et al. 1990). The increase in the Washington outer coast nest- ing peregrine population observed during this study coincided with the widespread comeback of the species in western North America and else- where (Enderson et al. 1995, Federal Register 1999). The increase in Washington was remarkably similar to the recovery of peregrines observed in the Yukon and Colville River areas in Alaska (Am- brose et al. 1988, Enderson et al. 1995), all recov- ering naturally with no reintroductions. In Califor- nia, the breeding population increased from 38 pairs in 1981 to 113 pairs in 1992 with the help of released birds (Kirven and Walton 1992), and Ca- nadian F. p. anatum nest sites more than doubled between 1985/86 and 1990 (Holroyd and Banasch 1996) . In neighboring British Columbia, the coast- al F, p. pealei population was considered stable (Holroyd and Banasch 1996). The increase in Washington’s coastal peregrine population appears to have been natural because no captive-bred 72 Wilson et al. VoL. 34, No. 2 young were released in the area. We cannot rule out the possibility, however, that released birds from elsewhere contributed to the increasing Washington coast breeding population. Because predecline population data for Washington are lacking, we do not know to what extent the popu- lation has recovered. Because our study area was a very rugged stretch of coastline with many islands, sea stacks and main- land cliffs, some of our nest sites were difficult to find. A few of the nesting ledges were hidden in vegetation, under overhanging tree roots or in the backs of small cliff caves. Therefore, the possibility existed that we may have missed some nesting pairs altogether or found nesting pairs one or more years after their first breeding attempt. This may have resulted in underestimating the total number of nest sites, and underestimating or overestimat- ing the rate of breeding population increase. While we could not quantify how this affected our study, we feel that we were able to locate >90% of occupied sites or breeding pairs using our search methods. Because the 1980-88 and the 1989-98 data subsets both showed significant positive trends, and because we surveyed the entire study area each year during peregrine and seabird sur- veys (which were done by helicopter since 1984), we feel that the switch from a combination of methods to exclusively aerial surveys did not result in the observed increase in the peregrine popula- tion. Our mean of 1.7 young per breeding pair was well below Nelson’s (1990) comparable estimate of 2.3 young per pair. The 73% mean success rate of Washington coastal breeding peregrines was con- siderably lower than the 84% reported for Langara Island, British Columbia (Nelson 1990), perhaps reflecting in our study area a higher number of young recruits with a greater likelihood of nesting failure. These differences were not only due to higher breeding success of Langara Island birds, but may also have been related to the different methods of collecting data. The Langara Island nest sites were only visited during a 8-10 d period when nestlings were of banding age (Nelson 1990). We conducted breeding surveys at sites prior to hatching, thus documenting early failures. In sev- eral instances during subsequent surveys in the same year, adult falcons were absent at failed sites. Errors in our estimation of the number of breed- ing pairs on the Washington coast may have also occurred because we could not confirm if eggs were actually laid at all sites. We may also have missed some pairs that laid eggs late. We suggest caution when comparing our data on breeding pairs with other studies. There are few published studies of sufficient du- ration to detect trends in the number of peregrine young per successful pair on the west coast of North America. For Langara Island, Nelson (1990) reported a mean of 2.8 young per successful pair (range = 2.0-3. 3) during 1980-89. During the same time period, we found a mean of 2.1 young per successful pair (range = 1. 5-2.8). The pere- grine population on the Queen Charlotte Islands did not experience the same declines documented elsewhere (Beebe 1969), with the Langara Island population being stable and reproductively healthy during 1968-89 (Nelson 1990). Breeding success for the peregrines in our study area continued to improve to an average of 2.5 young per successful pair during 1993-98, perhaps reflecting a popula- tion with more mature members than during the early years. It was encouraging that this measure of reproductive success of Washington’s coastal pere- grines had approached that of the healthy Langara Island birds. The population increase in Washing- ton apparently is different from that in other west coast states. In California where DDT application was heavy through 1972, the reproductive success of peregrines during 1981-92 was below normal but increased due to the release of large numbers of captive-bred young (Kirven and Walton 1992) . Adverse effects of El Nino on peregrine breed- ing success have not been previously reported be- cause there are few studies of sufficient duration to include an adequate sample of El Nino years. On the Washington outer coast, numbers of Dou- ble-crested Cormorants {Phalacro corax auritus), Brandt’s Cormorants {P. penicillatus) and Common Murres {Uria aalge) were sharply reduced during warm water episodes (Wilson 1991). It was very likely that the smaller seabirds, which are part of the peregrine’s prey, were similarly affected during such years, thereby reducing prey available to suc- cessful pairs and limiting the number of young they produced. Given that the most spectacular in- stances of interannual variability in marine ecosys- tems are El Nino events (Cane 1983), these find- ings were not surprising. Future research on marine peregrines in the eastern subarctic Pacific Ocean must consider potential ENSO depressed breeding success since El Ninos occur regularly at intervals of 2-10 yr (Cane 1983). Data collected June 2000 Washington Peregrine Falcons 73 during severe El Nino and post-El Nino years rep- resent highly abnormal environmental conditions and should be used with caution. We believe the increase in number of pairs of peregrines in our study area, as well as the im- proved breeding success, was primarily due to the discontinued use of DDT and the resulting reduc- tion in DDE levels (a metabolite of DDT) in the peregrine’s prey (Cade et al. 1988, Peakall 1990, Enderson et al. 1995, Henny et al. 1996). Evidence exists that these pesticide levels in western Wash- ington have declined. Schick et al. (1987) docu- mented declines in DDE and PCB residues in shorebirds collected during winter and spring in four western Washington estuaries. Shorebirds are important prey species for falcons wintering on the Washington coast (Buchanan et al. 1986, Buchan- an 1996). Further evidence comes from nine ad- dled peregrine eggs collected from four nests in our study area between 1987-91. These eggs had DDE residue levels of :^10.8 ppm (x = 4.3 ± 2.8 wet weight, adjusted for moisture loss; Washington Department of Fish and Wildlife, unpubl. data). These levels were considered below concentrations known to affect peregrine productivity (Peakall 1976). Although the American Peregrine Falcon was re- cently removed from the federal Endangered Spe- cies List (Federal Register 1999) and its popula- tions are increasing in many areas, we do not know the current or historical carrying capacity for the species in our study area. We recommend that in- tensive survey efforts be continued until popula- tions have stabilized. Federal regulations under the Endangered Species Act require a minimum 5-yr monitoring period for the Peregrine Falcon after delisting (Federal Register 1999). Acknowledgments We thank J. Atkinson, E. Cummins, R. Harkins and R. Spencer for assistance in the field. K. McAllister helped with tabulating data. C. Henny, R. Knight, K, Kilbride, W. Koenig, T. Loughin, R.W. Nelson and D. Varland provid- ed many helpful comments on earlier versions of this manuscript. Special thanks also to E. Cummins, B. Hes- selbart, J. Kincheloe, K. Ryan, J. Takekawa andj. Welch for help in funding the flights. The U.S. Fish and Wildlife Service and the Washington Department of Fish and Wildlife paid for the aerial surveys. Literature Cited Ambrose, R.E., RJ. Ritchie, C.M. White, RE. Schempf, T. SWEM AND R. Dittrick. 1988. Changes in status of Peregrine Falcon populations in Alaska. Pages 73-82 in T.J. Cade, J.H. Enderson, C.G. Thelander and C.M White [Eds.], Peregrine Falcon populations: their management and recovery. The Peregrine Fund, Boi- se, ID U.S.A. Beebe, FL. 1960. The marine peregrines of the northwest Pacific coast. Condor 62:145-189. . 1969. The known status of the Peregrine Falcon in British Columbia. Pages 53-60 mJ.J. Hickey [Ed.], Peregrine Falcon populations: their biology and de- cline. Univ. Wisconsin Press, Madison, WI U.S.A. Buchanan, J.B. 1996. A comparison of behavior and suc- cess rates of Merlins and Peregrine Falcons when hunting Dunlins in two coastal habitats. J. Raptor Res 30:93-98. , S.G. Herman and T.M. Johnson. 1986. Success rates of the Peregrine Falcon (Falco peregrinus) hunt- ing Dunlin ( Calidris alpina) during winter. Raptor Res. 20:130-131. Cade, T.J., J.H. Enderson, C.G. Thelander and C.M. White [Eds.]. 1988. Peregrine Falcon populations: their management and recovery The Peregrine Fund, Boise, ID U.S.A. , M. Martell, P. Redig, G. Septon and H. Tor- DOFF. 1996. Peregrine Falcons in urban North Amer- ica. Pages 3-13 in D.M. Bird, D.E. Varland and J.J. Negro [Eds.], Raptors in human landscapes. Academ- ic Press, London, U.K. Campbell, R.W., N.K. Dawe, I. McTaggart-Cowan, J.M. Cooper, G.W. Kaiser and M.C.E. McNall. 1990. The birds of British Columbia. Royal British Columbia Mu- seum, Victoria, British Columbia, Canada. Cane, M.A. 1983. Oceanographic events during El Nino 222:1189-1195. Cleveland, W.S. 1979. Robust locally weight regression and smoothing scatterplots. J. Am. Stat. Assoc. 74:829- 836. Dawson, W.L. and J.H. Bowels. 1909. The birds of Wash- ington. Occidental Publ, Co., Seattle, WA U.S.A. Enderson, J.H., W. Heinrich, L. Kiff and C. White. 1995. Population changes in North American pere- grines. Trans. N. Am. Wildl. and Natur. Resour. Conf. 60: 142-161. Federal Register. 1999. Endangered and threatened wildlife and plants: final rule to remove the American Peregrine Falcon from the federal list of endangered and threatened wildlife, and to remove the similarity of appearance provision for free-flying peregrines in the conterminous United States. Federal Register 64: 46541-46558. Hamilton, K. and W.J. Emery. 1985. Regional atmospher- ic forcing of interannual surface temperatures and sea level variability in the Northwest Pacific. Pages 22- 30 in W.S. Wooster and D.L. Fluharty [Eds.], El Nino North: Nino effects in the Eastern Subarctic Pacific Ocean. Washington Sea Grant Program, Univ. Wash- ington, Seatde, WA U.S.A. Henny, C.J., W.S. Seegar and T.L. Maechtle. 1996. DDE 74 Wilson et al. VoL. 34, No. 2 decreases in plasma of spring migrant Peregrine Fal- cons. J. Wildl. Manage. 60:342-349. Hickey, J.J. [Ed.]. 1969. Peregrine Falcon populations: their biology and decline. Univ. Wisconsin Press, Mad- ison, WI U.S.A. Holroyd, G.L. and U. Banasch. 1996. The 1990 Cana- dian Peregrine Falcon {Falco peregrinus) survey./. Rap- tor Res. 30:145-156. Jewett, S.G., W.P. Taylor, W.T. Shaw and J.W. Aldrich. 1953. Birds of Washington state. Univ. Wash. Press, Seattle WA U.S.A. Kirven, M.N. and B J. Walton. 1992. The Peregrine Fal- con population recovery in California from 1981 to 1992. U.S. Bur. Land Manage. 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Field-Nat. 90:301-307. . 1990. Prospects for the Peregrine Falcon, Falco peregrinus, in the nineties. Can. Field-Nat. 104:168-173. Schick, C.T., L.A. Brennan, J.B. Buchanan, M.A. Finger, T.M. Johnson and S.G. Herman. 1987. Organochlo- rine contamination in shorebirds from Washington state and the significance for their falcon predators. Environ. Monitoring Assess. 9:115-131. Speich, S.M. and T.R. Wahl. 1989. Catalog of Washing- ton seabird colonies. U.S. Fish Wildl. Serv. Biol. Rep. 88, Washington, DC U.S.A. Walton, B.J., C.G. Thelander and D.L. Harlow. 1988. The status of peregrines nesting in California, Oregon, Washington, and Nevada. Pages 95-104 in T.J. Cade, J.H. Enderson, C.G. Thelander and C.M. White [Eds.], Peregrine Falcon populations: their management and recovery. The Peregrine Fund, Boi- se, ID U.S.A. Wilkinson, L. 1997. SYSTAT for windows: statistics, ver- sion 7.0. SYSTAT Inc., Evanston, IL U.S.A. Wilson, U.W. 1991. Responses of three seabird species to El Nino events and other warm episodes on the Washington coast, 1979-1990. Contior 93:853-858. Wooster, W.S. and D.L. Fluharty [Eds.]. 1985. El Nino North: Nino effects in the Eastern Subarctic Pacific Ocean. Washington Sea Grant Program, Univ. Wash- ington. Seatde, WA U.S.A. Received 30 July 1999; accepted 9 February 2000 J. Raptor Res. 34(2):75--84 © 2000 The Raptor Research Foundation, Inc. LANDSCAPE CHARACTERISTICS OF NORTHERN SPOTTED OWL NEST SITES IN MANAGED FORESTS OF NORTHWESTERN CALIFORNIA Lee B. Folliard^ and Ktrry P. Reese Department of Fish and Wildlife Resources, University of Idaho, Moscow, ID 83844 US. A. Lowell V. Diller Simpson Timber Company, RO. Box 68, Korbel, CA 95350 U.S.A. Abstract. — ^We investigated vegetative and topographic characteristics of forest landscapes surrounding Northern Spotted Owl {Strix occidentalis caurina) nest sites on managed timberlands in northwestern California. Nest sites occurred primarily in young (31-60-yr old) forests of redwood {Sequoia sempervirens) and Douglas-fir {Pseudotsuga menziesit). We compared 60 Northern Spotted Owl nest landscapes (0.8-km radius circle centered on the nest site) with 60 randomly selected landscapes. Vegetative type and age class were used to classify forest stands within the landscape. Landscape features differed between nest sites and random sites (Wilks’ F = 6.073, P < 0.001) suggesting that nest-site selection was correlated with landscape level features. Nest landscapes had greater amounts of forest in the 31-45 and 46-60 yr- old age classes, and a greater amount of total edge. In addition, nest sites were located lower on slopes. In our study area, dusky-footed woodrats {Neotoma fuscipes) were the major prey species. Edges may provide opportunities for owls to prey on woodrats that are abundant in early serai habitats. The coastal forests of the redwood zone have unique characteristics that contribute to rapid development of North- ern Spotted Owl habitat. These include coppice growth (i.e., vegetative reproduction) of redwoods and several hardwood species, favorable growing conditions and the occurrence of major prey species in young serai habitats. Despite differences in habitat types and age classes, Northern Spotted Owl nest- site selection in these young, managed forests showed some consistent patterns with other portions of the owls’ range. Key Words: Northern Spotted Owl, Strix occidentalis caurina; landscape pattern-, nest-site selection-, managed forest, redwoods; northwestern California. Caracteristicas paisajisticas de los sitios de anidacion de Strix occidentalis caurina en bosques manejados del noroeste de California Resumen. — Investigamos las caracteristicas topograficas y de vegetacion de los paisajes de bosques al- rededor de los sitios de nidos de Strix occidentalis caurina en bosques manejados del noroeste de Cali- fornia. Los sitios del nido fueron encontrados principalmente en bosques jovenes (31-60 anos de edad) de Sequoia sempervirens y en Pseudotsuga menziesii. Comparamos 60 paisajes de los sitios del nido (0.8 km de radio del circulo centrado en el sitio del nido) con 60 paisajes seleccionados al azar. El tipo de vegetacion y la edad por clase fueron utilizados para clasificar los arboles dentro de los paisajes. Las caracteristicas paisajisticas difirieron entre los sitios del nido y los sitios seleccionados al azar (Wilks’ F = 6.073, P < 0.001) lo que sugiere que la seleccion del sitio de anidacion fue correlacionada con las caracteristicas del nivel de paisaje. Los paisajes del nido tuvieron mas cantidad de bosque en los 31—45 y 46-60 anos de clases de edad y mayor cantidad de borde total. Adicionalmente, los sitios del nido fueron localizados en la parte baja de la ladera. En nuestra area de estudio Neotoma fuscipes fue la especie de presa mayor. Los hordes proveen muchas oportunidades para la depredacion de ratas las cuales son abundantes en este tipo de habitats. Los bosques costeros de maderas rojas poseen caracteristicas unicas que contribuyen al rapido desarrollo de habitat para Strix occidentalis caurina. Esto incluye el crecimiento del sotobosque (reproduccion vegetativa) de maderas rojas y especies de maderas duras, como tambien las condiciones favorables de crecimiento para la presencia de presas mayores en habitats jovenes. A pesar de las diferencias en tipos de habitats y de clases por edad, la seleccion de sitios de nido por ^ Present address: U.S. Fish and Wildlife Service, 2600 98‘^ Avenue Suite 100, Portland, OR 97266 U.S.A. 75 76 Folliard et al. VOL. 34, No. 2 parte de los buhos en estos bosques jovenes y manejados mostro algunos patrones de consistencia con otras porciones de su rango. [Traduccion de Cesar Marquez] The Northern Spotted Owl {Strix occidentalis caurina) is associated primarily with old forests throughout its range (Forsman et al. 1977, Gould 1977, Forsman et al. 1984, Solis and Gutierrez 1990). Loss of habitat, due to timber harvest, has been implicated as the mzqor threat to the subspe- cies’ continued existence (Forsman et al. 1984, Federal Register 1990). The Northern Spotted Owl is reported to be a forest-interior species that has a large home range that includes a significant amount of older forest (Thomas et al. 1990). Studies of the distribution of owl habitat at a landscape level consistendy indicate that larger amounts of old-growth forest surround Northern Spotted Owl activity centers than at random loca- tions (Ripple et al. 1991, Blakesley et al. 1992, Le- hmkuhl and Raphael 1993). In addition, their home-range size may be inversely related to the proportion of old-growth forest within the home range and the amount of old growth within home ranges is less variable than home range size itself (Carey et al. 1990). Generally, Northern Spotted Owls are less abun- dant in managed young forests (Forsman et al. 1977, Forsman 1988). However, they have been fre- quently reported in young forests within the coast- al redwood {Sequoia sempervirens) zone of Califor- nia that has litde or no old-growth habitat present (Thomas et al. 1990). Litde is known about the forest characteristics surrounding owl nest sites in these managed landscapes. Therefore, our objec- tive was to describe the vegetative and topographic characteristics of the forest landscape surrounding Northern Spotted Owl nest sites in managed for- ests of northwestern California. Specifically, we tested the null hypothesis that no differences in landscape-level variables existed between nest sites and random sites. Study Area The study area was located in Humboldt and Del Norte counties in northwestern California on forestlands owned by Simpson Timber Company (STC) (Fig. 1). In this region, STC manages approximately 1200 km^ of land parcels that vary in size from 16-200 km^. Most of the study area was located within 32 km of the Pacific Ocean. The study area was located within the Redwood Vegetation Zone (Mayer 1988) and corresponds to the Northern California Coast Range Physiographic Province (Thomas et al. 1990). The region has a maritime climate with mild winters and cool summers typified by valley fog and high humidity. Mean annual rainfall is 280 cm near the coast to 102 cm inland. Most precipitation occurs during winter months. Mean annual temperature is 11°C with little temperature fluctuation throughout the year (mean temperature in January = 8°C, July = 15°C) (Zinke 1988). Elevation ranges from near sea level to approximately 915 m. The dominant vegetation types were redwood, red- wood-Douglas-fir, Douglas-fir (Pseudotsuga menziesii) and oak spp.) woodlands (Zinke 1988). Oak wood- lands were predominately tanoak {Lithocarpus densiflorus) with California black oak {Quercus kelloggii) and Oregon white oak ( Quercus garryana) on drier sites. Forest zona- tion was produced by changes in climate, soil type and topography (Zinke 1988). Redwoods were limited to a coastal strip about 8-56 km wide that received adequate summer fog and mild temperatures (Fowells 1965). As conditions become drier away from the coast, Douglas-fir replaced redwood as the predominant conifer and hard- woods become more abundant. On most sites, the coni- fer forest types were associated with hardwood species such as tanoak, California bay {Umbellularia californica) , Pacific madrone {Arbutus menziesii) and red alder {Alnus rubra). Due to an extensive logging history, forests on STC lands were predominately even-aged young stands be- tween 30—60 yr of age. Approximately 1.0% of the area was old growth forest (>200-yr old), all of which oc- curred as isolated stands <40 ha in size. Variability in terrain, past harvest practices, reforestation practices and species composition influenced the structure of stands. These stands varied from young forest with little variation in tree size to areas having residual old trees scattered within young forest. Residual trees were larger remnant trees from the original forest stand. These residual trees increased variation in tree sizes and stand structure, but the density of these trees across the landscape was low. Methods We measured landscape characteristics around 60 Northern Spotted Owl nest sites located during 1990 and 1991. Nests were located from March-June following Forsman (1983). All nest sites included in this study were from different owl pairs. We defined the landscape as the mosaic of forest-cover types surrounding an owl nest with- in a 0.8 km-radius circle (203 ha) centered on the nest site. This plot size allowed minimal or no overlap among plots for pairs that nested in close proximity. In many cases, a larger circle would have encompassed the nest or roost area of other pairs of Northern Spotted Owls. This reduced the chance of assigning habitat character- istics to an owl site that may not have been available to those owls due to territoriality among sites. We plotted nest locations on 1988 color aerial photo- graphs and planimetric maps each having scales of 1. 12000. To reduce errors in area measurement from June 2000 Spotted Owl Nest Landscapes 77 Cresent City Pacific Ocean Eureka j — St udy Area California N I 1 1 0 10 20 Kilonneters ® Willow Creek Figure 1. Location of Northern Spotted Owl nest-site study area in the coastal redwood zone of northwestern California, 1990-91. 78 Folliard et al. VoL. 34, No. 2 slight deviations in scale on aerial photos, all sites were plotted on the planimetric maps with the plot boundary drawn around them (Avery and Berlin 1985:82). Maps were generated from a geographical information system (GIS) and overlaid with forest cover-type boundaries. STC maintains a current database of forest types and stand ages based on periodic timber inventories and har- vest dates. The combination of large-scale color air pho- tos and the GIS database increased the efficacy of our mapping technique. All forest information was updated to reflect recent timber harvesting. Within each landscape plot, we estimated the age clas- ses and species composition of all stands present. The dominant overstory vegetation was classified into five cov- er-types: redwood, redwood/Douglas-fir, Douglas-fir, hardwood and nonforest. Conifer cover types often in- cluded a minor component of hardwood species. We ini- tially classified seven stand age classes: 0-7, 8-30, 31—45, 46-60, 61-80, 81-200 and >200 yr of age. However, we collapsed the latter three classes into a single class of >60 yr of age because older stands were scarce as well as un- even in age and spatial distribution. Although the des- ignation of age classes was subjective, they were deter- mined from a biological and a forest management perspective as follows. The 0-7 yr class represented the period during which recently clearcut areas reestablished a dense cover of shrubs and seedlings and evidence (i.e., stick houses) of dusky-footed woodrats {Neotoma fuscipes ) , the owls’ primary prey, began to appear on the sites. The 8-30 yr class included a wider range of stand conditions but, in general, it supported woodrat populations (Sakai and Noon 1993, Hamm 1995). In addition, older stands in this age class were potentially reaching a stage of de- velopment suitable for owl foraging (Forsman et al. 1984). The 31-45 yr class was chosen to include a serai stage at which stands began to develop structural char- acteristics associated with owl use in other parts of their range. In general, this age class was suitable for roosting and foraging but, in some instances, it was suitable for owl nesting (Folliard 1993). The 46-60 yr age class was important to evaluate because most stands in this class were reaching an age and size permitting timber harvest, and they were generally suitable as nesting habitat for owls. The >60 age class was important to evaluate be- cause shorter harvest cycles will often preclude the de- velopment of this age class in future managed land- scapes. We mapped each landscape plot as polygons corre- sponding to these cover types and age classes. After map- ping all the polygons in the circular plot, we calculated the area of each polygon using a dot grid (Avery and Berlin 1985:85-86). Because a combination of aerial pho- tographs and GIS were used for polygon mapping, we did not have a complete GIS database to use for area calculations and other measurements. We estimated the amount of total edge, both low-contrast and high-con- trast, within the landscape plot by measuring the perim- eter of all polygon boundaries with a map wheel. We de- fined low-contrast edge as the juxtaposition of two distinct cover types or different age classes of the same cover type. We defined high-contrast edge as the bound- ary between stands s31-yr old and nonforest or stands 0-7-yr old. We considered low-contrast edge a measure of forest heterogeneity and high-contrast edge a measure of forest fragmentation. We recognize that low-contrast edge was the result of past timber harvest and was also a form of forest fragmentation. Total length of roads in the plot and the distance from a nest site to the nearest water source were also measured with a map wheel. We calcu- lated slope position of the nest site as the ratio between the distance from the nest site to the bottom of the slope and the total distance from the bottom of the slope to the ridgetop. To avoid bias in measurements, distances were measured along a line that passed through the nest site that was perpendicular to the ridgetop and bottom of the slope. Slope position varied from zero to one with zero being the bottom of the slope and one being the ridgetop. To test the null hypothesis that no differences in land- scape variables existed between nest and random sites, we selected a point at a random direction and distance (<19.2 km, based on the maximum spacing of habitat conservation areas for Northern Spotted Owls; see Thomas et al. 1990) from each nest site. Random points were rejected if they fell beyond STC ownership and could be no closer to a known owl site than the minimum distance observed between two nesting pairs (670 m) in our study area. Data from random plots were collected following the same procedure used at nest sites. Northern Spotted Owl pairs were considered repro- ductively successful if they fledged at least one owlet. To determine reproductive success of nesting Northern Spotted Owls, daytime visits were conducted at nest sites to search for fledged owlets. Minimum estimates of fledg- ing success were calculated by summing the total number of owlets fledged by all pairs and dividing by the number of pairs. It was possible that we did not find all fledged young at nest sites, and thus our estimates of productivity were minimum estimates. Statistical Anal-sses We calculated correlations among all the variables to reduce the dataset. Variables were considered correlated if the correlation coefficient was >0.80. If variables were correlated, the one that seemed most biologically rele- vant was retained for further analysis. We assessed all variables for normality within groups (i.e., nest and random landscape variables) using the Wilk-Shapiro test (Shapiro and Francia 1972). Variables that exhibited nonnormality were subjected to a square- root transformation to increase normality and homoge- neity among variances (Zar 1984:241). Following trans- formation, most variables were normal or near normal in distribution. To test for group differences between nest and random landscapes, we used multivariate analysis of variance (MAN OVA) (Wilkinson 1990) with site status (nest or random) as the independent variable. Although some of the dependent variables were not normally distributed, the MANOVA procedure (two-group case) is robust to deviation from this assumption (Seber 1984:113). A test for homogeneity of the covariance matrices (Morrison 1976:252) indicated that the null hypothesis of equality could not be rejected at the 5% level (x^ = 21.5, df = 15, P= 0.121). Following the MANOVA, univariate tests were per- June 2000 Spotted Owl Nest Landscapes 79 Table 1. Summary statistics for landscape variables measured within 203-ha circular plots centered on 60 Northern Spotted Owl nest sites and 60 random sites from northwestern California, 1990 and 1991. Variable Nest Landscapes Mean ± SD Random Landscapes Mean ± SD F P 0-7-yr-old forest, ha 17.2 25.8 23.3 ± 38.6 0.065 0.800 8-30-yr-old forest, ha 23.9 38.0 53.1 + 68.6 4.376 0.039 31-45-yr-old forest, ha 45.6 + 60.1 29.3 49.5 4.341 0.039 46-60-yr-old forest, ha 55.4 ± 63.7 33.9 56.4 5.314 0.023 >60-yr-old forest, ha 41.5 ± 45.7 40.7 48.8 0.122 0.728 Nonforest area, ha 19.7 ± 19.2 21.9 + 33.6 0.630 0.429 Total edge, km 8.1 -h 3.1 6.4 H- 2.9 9.273 0.003 High-contrast edge, km 4.2 -h 2.9 4.0 + 3.2 0.172 0.679 Position on the slope 0.35 0.23 0.52 0.28 14.034 <0.001 Distance to water, m 136.7 ± 96.7 190.9 141.0 4.711 0.032 Length of roads, km 3.7 2.1 4.8 -H 2.7 5.067 0.026 formed for each dependent variable to determine which variables contributed to group differences. Dependent variables that differed (P < 0.05) between nest and ran- dom sites were entered into a stepwise discriminant anal- ysis (Hintze 1997). We used a stepwise variable selection procedure with variables having a probability to enter the model of a = 0.15 and a probability a = 0.05 for removal from the model. Cohen’s kappa (Titus et al. 1984) was used to determine if the model classified groups signifi- cantly better than chance. A t-test was used to test for differences in landscape variables between reproductively successful and unsuccessful pairs. Results Landscape features differed between Northern Spotted Owl nest sites and random sites (Wilks’ F = 6.073, P 0.001). Subset^uent univanate tests showed differences between eight of the 12 depen- dent variables included in the MANOVA (Table 1). Random landscapes had more 8-30-yr old forest, while there was more forest in the 31-45 and 46- 60 yr age classes in nest landscapes. The amount of older forest age class (>60-yr old) was not dif- ferent between the groups. The amount of 0-7 yr age class, created by recent clearcut logging, was also not different. The >60-yr old forest age class was composed of three age classes, 61-80, 81-200 and >200 yr. We did not believe that grouping these classes biased the results, because the group means for these clas- ses were similar. The mean area of forest at nest and random sites, respectively, were 24.3 and 28.2 ha in the 61-80 yr class, 12.1 and 11.4 ha in the 81-200 yr class and 5.2 and 1.1 ha in the >200 yr class. Nest sites were lower on the slope {P — 0.001) and closer to water {P = 0.032) than ran- dom sites (Table 1 ) . The total length of roads was lower (P = 0.026) within nest landscapes. The amount of total edge in nest landscapes was higher (P = 0.003) than in random landscapes (Table 1). Total edge represents a combination of fragmen- tation and heterogeneity of the forested landscape. When considering just high-contrast edge, there was no difference (P = 0.679) between groups. This result was consistent with the lack of differ- ence in the amount of 0-7 yr age class. Variables significandy different between nest and random sites were entered into a stepwise discrim- inant analysis. Four variables were selected for the discriminant function model (Table 2) . The model had an overall correct classification rate of 72.5%, which was better than chance alone (Cohen’s kap- pa = 0.45, P < 0.001). Nest landscapes were cor- recdy classified 71.7% of the time, whereas 73.3% of random landscapes were correcdy classified. Of the 60 Northern Spotted Owl pairs studied, 47 pairs successfully fledged at least one owlet, eight pairs were unsuccessful and the reproductive status of two pairs was undetermined. Three pairs that were unsuccessful in 1990 successfully fledged young in 1991, but were excluded from this anal- ysis due to shifts in nest locations that would have required additional mapping of newly centered nest plots. Habitat variables that were significant in the discriminant analysis were tested for differenc- es between reproductively successful {N = 47) and unsuccessful pairs {N = 8), and no differences were found (Table 3). However, the small sample size of reproductively unsuccessful pairs probably limited our ability to detect differences. 80 Folliard et al. VoL. 34, No. 2 Table 2. Summary of stepwise discriminant analysis for comparison of 60 Northern Spotted Owl nest landscapes with 60 random landscapes in northwestern California, 1990 and 1991. Variable Step Entered Wilks’ Lamda P Coefficient'" Position on slope 1 0.894 <0.001 0.648 Total edge, km 2 0.812 <0.001 -0.739 31-45-yr-old forest, ha 3 0.719 0.027 -0.575 46-60-yr-old forest, ha 4 0.688 0.025 -0.501 Standardized canonical coefficients. Discussion We found significant differences in landscape features between Northern Spotted Owl nest and random landscapes, which suggested that owls se- lected nest areas based on habitat characteristics at scales larger than the forest stand. Other studies have evaluated Northern Spotted Owl habitat on a landscape level by focusing on the amount of ma- ture and old-growth forest across the landscape (Ripple et al. 1991, Lehmkuhl and Raphael 1993, Meyer et al. 1998) . Throughout most of its range, old-growth forest was an important indicator of site occupancy by Northern Spotted Owls (Forsman et al. 1977, Forsman et al. 1984), and increased amounts of older forest may contribute to in- creased reproductive success (Bart and Forsman 1992). Carey et al. (1990) found that Northern Spotted Owl home ranges contained more old growth than in the surrounding landscape, and there was a negative correlation between home range size and the proportion of old growth in the home range. Greater amounts of older forest types across the landscape presumably constitutes better habitat for Northern Spotted Owls, and manage- ment recommendations have been suggested based on this criteria (Ripple et al. 1991, 1997). Our study investigated correlative relationships between owl nest-site locations and specific char- acteristics of the surrounding forest environment. Although we did not demonstrate cause and effect relationships, description of the observed associa- tions provide a greater understanding of Northern Spotted Owl biology in this restricted portion of the range. Another limitation was the use of cir- cular plots to characterize owl habitat when actual home range configurations may exhibit different habitat patterns (Lehmkuhl and Raphael 1993). Furthermore, circular plots may include unused ar- eas. Information on owl home range size and con- figuration is lacking for the redwood zone. How- ever, we believe that all habitat in the 203-ha plots was likely to be used by owls because this size was not overly large compared to home ranges report- ed in California (Solis and Gutierrez 1990, Zabel et al. 1995), and because circles were centered around nest sites. In our study area, forest age classes of 31—45 and 46-60 yr were the most prevalent age classes in landscapes of nesting Northern Spotted Owls. We believe these mid-aged forest stands represented habitat used for nesting, roosting and foraging whereas stands <30 yr of age were generally lack- ing characteristics of owl nesting habitat. The ma- jority (53%) of nests were located in stands 35-60- yr old, while 30% were in stands 61-80-yr old and 17% in stands >80-yr old. No nests were found in Table 3. Univariate test statistics for landscape variables at nest landscapes compared between successful and un- successful Northern Spotted Owl pairs from northwestern California, 1990 and 1991. Variable Successful {N = 47) Mean ± SD Unsuccessful (AT= 8) Mean ± SD t P Total edge, km 8.0 ± 3.2 8.2 ± 2.9 0.25 0.805 31-45-yr-old forest, ha 44.5 ± 59.9 46.4 ± 62.7 0.06 0.950 46-60-yr-old forest, ha 62.6 ± 65.2 44.9 ± 61.9 1.02 0.314 Position on slope 0.33 ± 0.22 0.43 ± 0.33 1.02 0.312 June 2000 Spotted Owl Nest Landscapes 81 stands <35-yr old. Many of the stands in these age classes used by owls contained a component of re- sidual trees that added increased structure and complexity to the stands (Folliard 1993). Concen- trations of residual trees around nest areas may also contribute to higher reproductive success of owls on managed landscapes (Thome et al. 1999). Bias and Gutierrez (1992) found California Spot- ted Owl (5. 0 . occidentalis) roost and nest sites in pole-medium successional stage habitats with resid- ual old-growth trees present in the stands. They suggested that forest structure, not forest or tree ages per se, was important to California Spotted Owls. Although the amount of forest older than 60 yr was not different between groups at the land- scape level, these areas were used for nesting at the stand level (Folliard 1993). Because stands >60-yr old occurred at a low frequency across the land- scape and were scattered in distribution, there was little opportunity for owls to nest in and use a land- scape that encompassed a large proportion of this age class. Carey et al. (1990) found that the amount of old growth in Northern Spotted Owl home ranges was less variable than home range size itself. This im- plies that owls are maintaining home ranges of suf- ficient size to encompass some minimum amount of suitable habitat required for their life needs. The threshold of required habitat will likely vary depending on the spatial distribution of remaining forest and the availability of prey species. We found owls using small patches of older forest (>60-yr old) if the surrounding landscape had a high pro- portion of forested area >30-yr old. Small patches of older forest provided roosting and nesting areas, while the surrounding younger forest presumably provided foraging habitat. Young landscapes that lacked patches of older forest (>60-yr old) or re- sidual trees did not support nesting owls. The amount of total edge was an important var- iable distinguishing nest from random sites. Nest areas had more low-contrast edge, indicating that habitat pattern surrounding nest sites had more spatial heterogeneity due to a diverse matrix of dif- fering age classes and cover types. Our index of forest fragmentation (amount of high-contrast edge) and amount of recent clearcut area (0-7 yr age class) showed no difference between nest and random sites. Our findings were consistent with those of Meyer et al. (1998) who also found that the amount of clearcut area did not differ between owl and random sites within a 0.8-km radius circle. The increased spatial or forest heterogeneity (i.e., greater amounts of low-contrast edge) in nest areas may contribute to a higher abundance and diversity of prey species for owls on the study area. The abundance and diversity of available prey spe- cies may influence habitat use patterns of North- ern Spotted Owls (Carey et al. 1992). Studies in northwestern California indicate that woodrats are abundant in young serai stages (Raphael 1988, Sa- kai and Noon 1993, Hamm 1995). Hamm (1995) studied woodrats on our study area and found the highest densities in young stands 5-20-yr old. Most of these young stands where woodrats are abun- dant create low-contrast edge when adjacent to old- er forest age classes. Based on pellet analysis, wood- rats represented a significant component of the owls' diet on our study area (Diller unpubl. data). Although owls may not have foraged directly in such areas, they probably foraged near edges where older forest adjoins younger patches of prey habitat. In northwestern California, Northern Spotted Owl foraging locations have been found closer to edges where woodrats represent the ma- jor prey species (Zabel et al. 1995). In addition to edge providing foraging oppor- tunities, young forest patches likely provide a “res- ervoir” of prey that disperse into adjacent stands used by Northern Spotted Owls (Sakai and Noon 1993). Boxall and Lein (1982) found that Snowy Owls (Nyctea scandiaca) selected territories with higher amounts of edge habitat (fencerows and roadside ditches) that had a high abundance of prey. Although prey was abundant in edge habitats, they were usually not available to Snowy Owls until they moved into adjacent fields. The high amounts of total edge were at least partly related to the history of timber harvest in the area. Because most of the study area occurred in young and mid-aged forests, the owl sites were the result of past and ongoing timber harvests. Our results support the hypothesis that, in northern California, a certain degree of openness created by timber harvest may be beneficial to Northern Spot- ted Owls by providing younger serai habitat for ma- jor prey species such as woodrats (Sakai and Noon 1993). On our study area, Hamm (1995) found that woodrat abundance declined sharply in stands >30-yr old. However, Northern Spotted Owls may avoid areas that become overly fragmented by tim- ber harvest to reduce exposure to predators (Gu- tierrez 1985) or to lower interactions with potential 82 Folliard et al. VoL. 34, No. 2 competitors such as Barred Owls {Strix varia) (Dark et al. 1998). Northern Spotted Owl nest sites were most com- monly located on the lower portions of slopes and rarely near ridge tops. The tendency to locate nests on the lower portions of slopes has been docu- mented previously (Forsman et al. 1984, LaHaye 1988, Blakesley et al. 1992, Hershey et al. 1998). Northern Goshawks {Acdpiter gentilis ) , another for- est raptor, have also been observed nesting lower on slopes (Hayward and Escano 1989) . Lower por- tions of slopes may provide more favorable condi- tions for the development of large trees that pro- vide nest sites for owls (LaHaye 1988). However, this hypothesis does not adequately explain the phenomenon on our study area because even though Northern Spotted Owls nested lower on slopes, many nests were in relatively young trees. Most of the old growth forest has been previously harvested on our study area and across the land- scape there is no concentration of large trees lower on slopes. Although statistically different, the mag- nitude of difference in slope position between nest and random sites (0.35 vs. 0.52, respectively) may not be biologically significant; both could be viewed as midslope. The predictive capability of the discriminant model had a correct classification rate of 72.5%, which was significantly greater than chance. Mis- classification of sites, however, indicates that some nest and random areas had similar habitat char- acteristics. Explanations for the similarity of some nest and random sites are: (1) random sites may have been suitable as nest areas, but were not yet colonized by owls; (2) territoriality among owls could preclude areas from additional occupancy; and (3) quantification of owl sites based on vege- tative parameters alone may not represent the full suite of factors that influence site selection at a larger scale. Some measure of prey abundance in d i ff erent habitats may reveal further differences between nest and random areas. The associated prey base and its availability can be important de- terminants of suitable owl habitat (Carey et al. 1992) . In some areas, the density of raptors is large- ly determined by prey abundance, with the avail- ability of suitable nest sites having little influence on breeding densities (Korpimaki and Norrdahl 1991). The influence of prey abundance and avail- ability on Northern Spotted Owl occurrence in managed forests deserves further investigation. Our study occurred within the coastal Redwood Vegetation Zone, considered to be a unique por- tion of the Northern Spotted Owl range (Thomas et al. 1990). The coastal forests of redwood and Douglas-fir in northwest California have unique characteristics that contribute to the rapid devel- opment of Northern Spotted Owl habitat on man- aged landscapes. These characteristics include cop- pice growth (i.e., vegetative reproduction) of redwoods and several species of hardwoods and fa- vorable growing conditions that result in rapid growth of all vegetation. In addition, the mild coastal climate may reduce thermoregulatory de- mands on owls in young forests (Ting 1998). In spite of great differences in habitat types, age class distributions and management histories. Northern Spotted Owl nest-site selection in the redwood zone showed similarities with habitat se- lection in other portions of its range. Although our study area lacked significant amounts of old-growth forest. Northern Spotted Owls selected nest land- scapes with greater amounts of the oldest forest age classes available. This was consistent with other studies in Oregon and Washington (Ripple et al. 1991, Lehmkuhl and Raphael 1993, Meyer et al. 1998) and California (Hunter et al. 1995, Gutier- rez et al. 1998). Because our study area was pri- marily young and mid-aged forest stands, our re- sults suggested that forest age is probably less important than aspects of forest structure when identifying suitable habitat for Northern Spotted Owls. In addition to greater amounts of older for- est age class (46-60 yr) , the proportion of this age class in nest landscapes was similar among other studies. The mean area of forest stands >46-yr old in our study was 97 ha, which was 48% of the 203- ha circle. Hunter et al. (1995) reported a similar mean of 94 ha of mature and old-growth forest in 200-ha circular plots centered on Northern Spot- ted Owl nest sites. Lehmkuhl and Raphael (1993) found that Northern Spotted Owl home ranges on the Olympic Peninsula in Washington contained an average of 44% owl habitat (old-forest stands or 80-100-yr old stands with an overstory of remnant old trees). Northern Spotted Owl home ranges in southern Oregon contained from 27-75% old growth (Carey et al. 1990). Although the spatial scales varied among these studies, the similar pro- portions in suitable habitat may indicate a poten- tial threshold of suitable habitat to support owl oc- cupancy. Landscape features such as amount of older forest and the proportion of older forest June 2000 Spotted Owl Nest Landscapes 83 around owl sites are likely to be factors that influ- ence site selection on managed forestlands. The impacts of timber harvest on Northern Spotted Owl occupancy at the landscape or water- shed level depend on the extent and rate of har- vest in the area. Initially, when large amounts of mature forest are present, negative impacts from dispersed clearcutting are likely to be minimal. However, if harvest continues and larger areas be- come dominated by young forests, the suitability of the landscape as nesting habitat for Northern Spot- ted Owls will diminish. Landscapes extensively har- vested over the last 30 yr were the least used for nesting by Northern Spotted Owls in our study. Furthermore, prolonged timber harvest within Northern Spotted Owl territories may negatively influence reproductive success prior to affecting site occupancy (Thome et al. 1999). Our data suggested that, to maintain Northern Spotted Owls in managed forests of the redwood zone, >50% of the landscape or area surrounding nests should be in forests >45-yr old. In addition, clearcut harvesting on a landscape level scale should be distributed over time and space to pro- duce a mix of age classes that provide for Northern Spotted Owls and early serai prey species such as woodrats. Staggering harvest would reduce the oc- currence of large areas that are unsuitable for nest- ing. Clearcut harvesting should also retain green trees, particularly those with decadence or struc- tural deformities, to provide older forest structure in regenerating stands. Acknowledgments We thank Simpson Timber Company for financial sup- port of this project and acknowledge their commitment to continued wildlife research and management. Field assistance was provided by J. Beck, P. Birks, K. Hamm, R. Klug and B. Ross. R. Gutierrez, S. Horton and an anon- ymous reviewer provided helpful reviews that improved the manuscript. We also thank B. Dennis, University of Idaho, for statistical assistance. This is contribution No. 893 of the University of Idaho College of Forestry, Wild- life, and Range Experiment Station. Literature Cited Avery, T.E. and G.L. Berlin. 1985. Interpretation of ae- rial photographs. Burgess, Minneapolis, MN U.S.A. Bart, J. and E.D. ForsmAN. 1992. Dependence of North- ern Spotted Owls Strix ocddentalis caurina on old- growth forests in the western USA. Biol. Cons. 37:95- 100 . Bias, M.A. and R.J. Gutierrez. 1992. Habitat associations of California Spotted Owls in the central Sierra Ne- vada. J. Wildl. Manage. 56:584—595. 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Nat. 111:1-7. Wilkinson, L. 1990. SYSTAT: the System for Statistics. Systat Inc., Evanston, IL U.S.A. Zabel, C.J., K. McKelvey and J.P. Ward, Jr. 1995. Influ- ence of primary prey on home-range size and habitat- use patterns of Northern Spotted Owls (Strix occiden- talis caurina). Can. J. Zool. 73:433-439. Zar, J.H. 1984. Biostatistical analysis. Second ed. Prentice Hall, Englewood Cliffs, NJ U.S.A. Zinke, PJ 1988. The redwood forest and associated north coast forests. Pages 679—698 in M.G. Barbour and J Major [Eds.], Terrestrial vegetation of California. Univ. Calif., Davis, CA U.S.A. Received 19 July 1999; accepted 2 January 2000 J. Raptor Res. 34(2);85-92 © 2000 The Raptor Research Foundation, Inc. IDENTIFICATION OF INDIVIDUAL BARRED OWLS USING SPECTROGRAM ANALYSIS AND AUDITORY CUES Pamela L. Freeman Department of Zoology, North Dakota State University, Fargo, ND 58105 US. A. Abstract. — To determine if individual male Barred Owls (Strix varia) could be identified using spec- trogram analysis, I recorded vocalizations from mid-February through May at 17 different field locations in Minnesota. In 1997, 134 calls from seven locations were analyzed; in 1998, 531 calls from 15 locations were analyzed. The final four notes of the Legato hoot, which consisted of five-to-nine evenly accented notes, were used in the analyses. On each spectrogram, I measured 10 temporal and 12 frequency measures, then used stepwise logistic regression to select the seven most influential variables. A discrim- inate function analysis (DFA) separated and identified spectrograms from different locations in 1998 with an overall accuracy of 84.5%. Sortspects, a multimedia-based program, was developed and used to determine whether observers could discriminate unmeasured calls using only visual and auditory cues. Discrimination of 1997 calls (four locations with two nights each) was 100% for all four observers. The observers were able to correctly discriminate 1998 calls (15 locations, each with three nights) 38, 58, 76 and 87% of the time. Key Words: Barred Owl; Strix varia; vocalizations-, songs; individual identification. Identificacion individual de Strix varia mediante la utilizacion de analisis de espectogramas y senas auditivas Resumen. — Para determinar si un individuo macho de Strix varia puede ser identificado mediante un analisis de espectogramas, grabe sus vocalizaciones desde mediados de Febrero hasta Mayo 17 en dis- tintas localidades de Minnesota. En 1997, 134 vocalizaciones de siete localidades fueron analizadas; en 1998 531 vocalizaciones de 15 localidades fueron analizadas. Las cuatro notas finales de la secuencia de buhos ululando, la cual consistio de cinco a nueve notas igualmente acentuadas fueron utilizadas en el analisis. En cada analisis de espectograma tome las medidas temporales y 12 frecuencias, utilice una regresion logistica para selecionar las siete variables mas influyentes. Un analisis de funcion discrimi- natorio separo e identified los espectogramas con una exactitud del 84.5%. Mediante el desarrollo del “Sportspecs” un programa de multimedia se determind si los observadores podian discriminar las vocalizaciones no medidas mediante la utilizacidn de senas visuales y auditivas solamente. La discrimi- nacidn de las vocalizaciones de 1997 (cuatro localidades con dos noches cada una) fue del 100% para los cuatro observadores. Los observadores fueron capaces de discriminar correctamente las vocaliza- ciones de 1998 (15 localidades cada una y tres noches) 38, 58, 76 y un 87% del tiempo. [Traduccidn de Cesar Marquez] Acoustical identification of individuals in song- birds and seabirds has been documented exten- sively over the last several decades (Beer 1970, Falls 1982); however, only recendy has it been investi- gated in raptors (Eakle et al. 1989, Galeotti and Pavan 1991, Galeotti et al. 1993, Telford 1996, Ot- ter 1996, Appleby and Redpath 1997, Kuntz and Stacey 1997). Recognition of individuals by audi- tory cues is likely where vision is impaired due to darkness, topography, congested colony sites, or thick vegetation (Beer 1970, Falls 1982), and in species with repeated neighbor contact or long- term pair bonds (Falls 1982). Individual variation, therefore, may be particularly important in noctur- nal owls, especially in species that occupy large ter- ritories of varying topography and vegetation. Although the Barred Owl {Strix varia), an indi- cator species (U.S. Department of Agriculture 1985, 1986, 1987), is common throughout much of its range, its vocalizations have been little stud- ied. The ability to identify individuals using char- acteristics of calls potentially provides a new ap- proach to the study and census of this species. Voice analysis could potentially permit identifica- tion of individuals without having to capture or band birds. 85 86 Freeman VoL. 34, No. 2 I investigated the consistency and variability of Barred Owl vocalizations recorded at different lo- cations over two years, and determined whether human observers were able to distinguish between calls of birds recorded at different locations. Barred Owls are sedentary, long-lived and territo- rial (Nicholls and Fuller 1987, Mazur et al. 1998). Telemetry studies in Minnesota and Saskatchewan indicate that Barred Owls defend large (228-971 ha) exclusive areas (Nicholls and Warner 1972, Ma- zur et al. 1998) and spend little time outside them (Nicholls and Fuller 1987). Therefore, if the vo- calizations of individual Barred Owls are distinct, vocalizations recorded in one territory should be distinguishable from those recorded in other ter- ritories. Methods Sound Recording. The study was conducted at Itasca State Park in northcentral Minnesota, U.S.A. (47°12'N, 95°12'E). I recorded male Barred Owl vocalizations from mid-February through May in 1997 and 1998. To locate owls, I provoked responses by broadcasting conspecific calls. The tapes used for broadcasts contained 5-6-min segments, with calls every 25 sec. My goal was to maximize rather than standardize the number of record- ed vocalizations, so I used one to three different vocali- zations at each location: (1) Legato calls from a captive female recorded at the Raptor Research Center at the University of Minnesota, (2) Legato and Cook calls from a male recorded at Itasca State Park and (3) a short seg- ment of a pre-recorded male-female duet (National Geo- graphic Society 1983). Recordings of responses were made between 1600- 0730 H with a Sony TCM 5000 EV or Marantz PMD 221 tape recorder, and either a Sennheiser directional or a 45 cm parabolic microphone. To allow for comparisons of call structure under different conditions, I did not standardize for time of day, temperature, distance from the microphone or background noise level. However, calls were not recorded on nights with constant precipi- tation or wind speeds over 10 km/hr. After the initial recording, I used the same location for all subsequent recordings and noted the direction and distance of any individual that answered. Recording lo- cations ranged from 0. 4-4.0 km apart (x = 1.75 km). Since Barred Owls are highly sedentary, the likelihood of recording the same individual on more than one occa- sion at the same location was high. Neighboring birds were determined to be different individuals because they called simultaneously and consistently from a different location and direction than the one being recorded. Observer Discrimination. For all analyses, I used only calls with spectrograms that were clearly visible despite background noise, and which were recorded in response to broadcasts. Sortspects, a multimedia-based program developed by G. Nuechterlein at North Dakota State Uni- versity, was used to determine if observers could distin- guish calls from different locations using only visual and auditory cues. The 15 locations in 1998 were each represented by three computer “folders” (45 folders total). Each folder represented a different recording night at a location and contained the sounds and spectrograms of three random- ly selected calls from that night. The Sortspects program randomly and anonymously displayed the spectrograms and played the sounds within each folder. All 45 folders were simultaneously visible, and the goal of the observer was to place the calls into like groups using visual and auditory clues. The observer was not provided any quan- titative measurement. Once sorting was finished, I checked the identities and scored the trial. In 1998, the maximum score for an observer was 45 correct. Sample sizes were smaller in 1997 with only four locations and two recording nights per location (eight folders). The maximum score for an observer was 8 correct. Four observers were used in the Sortspects discrimi- nation process. To assess if spectrogram experience would be necessary for this procedure, I used two expe- rienced and two inexperienced spectrogram readers. For each Sortspects task, a Monte Carlo simulation with 10000 trials was run to determine the probability of ran- domly attaining scores using the Sortspects program. Points for the Monte Carlo were assigned in the same fashion as in the Sortspects program. Spectrogram Measurements. The most common vocal- ization heard in response to broadcast calls was the male’s Legato hoot, five to nine evenly accented hoots ending with a “hoo-aw” (Freeman 1999). For analyses, I randomly selected complete Legato hoots from each re- cording session at each location and analyzed spectro- grams and spectra (filter bandwidth = 88 Hz) using CA- NARY 1.2 software (Cornell Laboratory of Ornithology, Ithaca, New York U.S.A) . All but two of the 22 variables studied were taken from the final four notes of the Legato vocalization. I focused on this segment of the call because it showed more con- sistency within a call sequence and more variability be- tween locations than did the introductory notes. CA- NARY provided quantitative measurements of the last four notes from each spectrogram, which included peak amplitude frequency of each note from the spectrum (FTRl, FTR2, FTR3 and FTR4) and spectrogram (FGRl, FGR2, FGR3 and FGR4); duration of each note (Dl, D2, D3 and D4); length of intervals between notes (II, 12 and 13) measured from the end of one note to the beginning of the next note; and peak amplitude interval (PIl, PI2, and PI3) measured between peak amplitude of successive notes (Fig. 1). Other measurements from the call were peak amplitude frequency of the last four notes together from the spectrum and spectrogram (FTRALL4 and FGRALL4) and peak amplitude of the entire call from the spectrum and spectrogram (FTROV and FGROV). In 1997, I analyzed 134 calls from seven locations. The number of recording nights per location varied from 1- 6 with a total of 4-59 (x = 19.1) spectrograms per loca- tion. In 1998, the number of successful recording nights per location varied from 3-6 with 29-42 (x = 35.4) spec- trograms per location, for a total of 531 calls in 1998. Statistical Analyses. The 22 spectrogram variables were subjected to univariate and multivariate analyses. 1 first June 2000 Individual Identification of Barred Owls 87 A) S B) Figure 1. Spectrogram and spectrum of a Legato call from a male Barred Owl (Location K) showing examples of measurements, (a) Spectrogram measurements: D = note duration, I = note interval, PI = peak amplitude interval and FGR = peak amplitude frequency, (b) Spectrum of the entire call showing FTR = peak amplitude frequency. conducted a separate analysis of variance (ANOVA) on each variable to determine if significant differences ex- isted within and between locations. Variables were also plotted against recording dates to determine if charac- teristics of vocalizations changed over the breeding sea- son. To reduce the number of variables for the discriminate analysis, I performed a stepwise logistical regression (SLR) with the 15 locations from 1998. The SLR provid- ed the concordant values and indicated the accuracy with which the regression model fit the data, or in this case, how well it could predict the location. The seven vari- ables that were meaningful in identifying locations were then subjected to a Fisher’s quadratic discriminate func- tion analysis (DFA). A cross-validation technique was used to \^idate the model. The parameters from the 1997 data were not directly analyzed by a DFA owing to small sample sizes. Results Observer Discrimination. Although spectro- grams of Legato calls were easily recognizable in overall structure, there were differences between spectrograms from different locations. Examples of differences in note shape c