Burrowing Owl Pair Count Report 2017

Western Riverside County Multiple Species Habitat Conservation Plan Biological Monitoring Program

2015–2017 Western Burrowing Owl (Athene cunicularia hypugaea) Pair Count Report

Five Burrowing Owl nestlings at a burrow at the San Jacinto Wildlife Area. Photo was taken by a Bushnell trail camera.

9 July 2018 2015–2017 Western Burrowing Owl Pair Count Survey Report

TABLE OF CONTENTS INTRODUCTION ...... 1

GOALS AND OBJECTIVES ...... 1

METHODS ...... 2

PAIR COUNT SURVEYS ...... 2

HABITAT ASSESSMENT ...... 5

TRAINING ...... 6

DATA ANALYSIS ...... 6

RESULTS ...... 7

2015 PAIR COUNT RESULTS ...... 7

2016 PAIR COUNT RESULTS ...... 9

2017 PAIR COUNT RESULTS ...... 9

HABITAT ASSESSMENT ...... 10

DISCUSSION ...... 12

LAKE SKINNER/DIAMOND VALLEY LAKE CORE AREA ...... 13

SAN JACINTO WILDLIFE AREA/MYSTIC LAKE/LAKE PERRIS CORE AREA ...... 14

TRAIL CAMERAS ...... 14

HABITAT ASSESSMENT ...... 16

RECOMMENDATIONS ...... 19

ACKNOWLEDGEMENTS ...... 20

LITERATURE CITED ...... 20

LIST OF TABLES Table 1. Summary of Burrowing Owl pair count surveys within occupied Core Areas from 2015–2017 ...... 7 Table 2. Presumed outcomes of active Burrowing Owl burrows observed during 2015– 2017 pair count surveys ...... 9 Table 3. Mean ± SE distances to the nearest road, tree, potential non-nesting satellite burrow, and potential perch for nesting burrows and nearby, unused sites in 2016 and 2017...... 10 Table 4. Mean ± SE number of potential burrows and perches within 10 m and 50 m of nesting burrows and nearby, unused sites in 2016 and 2017 ...... 10 Table 5. Ground cover types, represented by the first foliar hits along transects, within 50 m of nesting burrows and nearby, unused sites in 2016 and 2017 ...... 12

Western Riverside County MSHCP ii Biological Monitoring Program 2015–2017 Western Burrowing Owl Pair Count Survey Report

Table 6. Summary of Burrowing Owl productivity (fledglings/breeding pair) reported within the range of the species...... 13 Table 7. Summary of reported burrow densities at various distances from nesting burrows within the range of Burrowing Owls...... 17

LIST OF FIGURES Figure 1. Burrowing Owl burrows monitored during pair count surveys from 2015-2017 in the Lake Skinner Core Area...... 3 Figure 2. Burrowing Owl burrows monitored during pair count surveys from 2015-2017 in the San Jacinto Wildlife Area ...... 4 Figure 3. Locations of Burrowing Owl burrows monitored during pair count surveys that produced fledglings from 2015-2017 in the Lake Skinner Core Area...... 8 Figure 4. Locations of Burrowing Owl burrows monitored during pair count surveys from 2015-2017 in the San Jacinto Wildlife Area...... 11 Figure 5. Two adult Burrowing Owls at the North Natural 7 burrow in Johnson Ranch, photographed using a Bushnell trail camera. Note the alphanumeric auxiliary leg bands whose characters are clearly discerned...... 15 Figure 6. A male Burrowing Owl delivering a San Diegan tiger whiptail (Aspidoscelis tigris stejnegri) to an active nesting burrow in the San Jacinto Wildlife Area...... 15 Figure 7. A Common Raven inspecting an active Burrowing Owl burrow at the San Jacinto Wildlife Area. Photos and videos from trail cameras allow us to notify land managers of threats to Burrowing Owls by potential nest predators...... 16

Western Riverside County MSHCP iii Biological Monitoring Program 2015–2017 Western Burrowing Owl Pair Count Survey Report

NOTE TO READER: This report is an account of survey activities conducted by the Biological Monitoring Program for the Western Riverside County Multiple Species Habitat Conservation Plan (MSHCP). The MSHCP was permitted in June 2004. Reserve assembly is ongoing and is expected to take 20 or more years to complete. The Conservation Area includes lands acquired under the terms of the MSHCP and other lands that have conservation value in the Plan Area (called public or quasi-public lands in the MSHCP). In this report, the term “Conservation Area” refers to these lands as they were understood by the Monitoring Program at the time the surveys were conducted. The Monitoring Program monitors the status and distribution of the 146 species covered by the MSHCP within the Conservation Area to provide information to Permittees, land managers, the public, and the Wildlife Agencies [i.e., the California Department of Fish and Wildlife (CDFW, formerly California Department of Fish and Game) and the U.S. Fish and Wildlife Service]. Monitoring Program activities are guided by defined conservation objectives for each Covered Species, other information needs identified in MSHCP Section 5.3 or elsewhere in the document, and the information needs of the Permittees. A list of the lands where data collection activities were conducted from 2015–2017 are included in Section 7.0 of the Western Riverside County Regional Conservation Authority (RCA) Annual Report to the Wildlife Agencies. The primary author of this report was the 2017 Avian Program Lead, Nicholas Peterson. This report should be cited as: Biological Monitoring Program. 2018. Western Riverside County MSHCP Biological Monitoring Program 2015–2017 Western Burrowing Owl (Athene cunicularia hypugaea) Pair Count Report. Prepared for the Western Riverside County Multiple Species Habitat Conservation Plan. Riverside, CA. Available from http://wrc-rca.org/about- rca/monitoring/monitoring-surveys/. While we have made every effort to accurately represent our data and results, it should be recognized that data management and analysis are ongoing activities. Any reader wishing to make further use of the information or data provided in this report should contact the Monitoring Program to ensure that they have access to the best available or most current data. Please contact the Monitoring Program Administrator with questions about the information provided in this report. Questions about the MSHCP should be directed to the Executive Director of the RCA. Further information on the MSHCP and the RCA can be found at www.wrc-rca.org. Contact Information: Executive Director Monitoring Program Administrator Western Riverside County Western Riverside County MSHCP Regional Conservation Authority Biological Monitoring Program Riverside Centre Building 4500 Glenwood Drive, Bldg. C 3403 10th Street, Suite 320 Riverside, CA 92501 Riverside, CA 92501 Ph: (951) 248-2552 Ph: (951) 955-9700

Western Riverside County MSHCP iv Biological Monitoring Program 2015–2017 Western Burrowing Owl Pair Count Survey Report

INTRODUCTION The Western Burrowing Owl (Athene cunicularia hypugaea) is one of 45 bird species covered by the Western Riverside County MSHCP (Dudek & Associates 2003). Burrowing Owls are a Species of Special Concern in California (Gervais et al. 2008) and a National Bird of Conservation Concern (USFWS 2002). Burrowing Owls are found within California throughout the Central Valley, from Redding south to the Grapevine, east through the Mojave Desert, west to San Jose and San Francisco, within the outer coastal foothills area, and within the Sonoran Desert (Grinnell and Miller 1944). Owls inhabit the central portion of the Plan Area within the open lowlands (Garrett and Dunn 1981), and their overall distribution is scattered outside of the montane areas (Dudek & Associates 2003). The MSHCP identifies grasslands, agricultural fields, and playas and vernal pools as owl habitat within the Plan Area (Dudek & Associates 2003). Burrowing Owls in California tend to breed from February through August (Thomsen 1971; Poulin et al. 2011) and they typically nest in abandoned California ground squirrel (Otospermophilus beechyi) burrows, but may also use burrows previously occupied by small mammals, badgers (Taxidea taxus), or marmots (Marmota spp.). Owls will also use pipes, culverts, and nest boxes for nesting where natural burrows are scarce (Robertson 1929). Young are typically present as early as mid-April (Poulin et al. 2011) and will emerge from burrows about 14 d post-hatching (Zarn 1974). At approximately six weeks old, young owls fly well (Zarn 1974) and by 44–52 days post-hatching, they fledge (i.e., leave immediate vicinity of burrow) (Landry 1979; Todd et al. 2007). Western Burrowing Owls may attempt a second brood if the first nesting attempt fails early in the season (Thomsen 1971; Butts 1973; Wedgwood 1976); otherwise, pairs produce a single brood each year (Poulin et al. 2011). Pairs may use satellite burrows throughout the breeding season to protect themselves or their fledglings from predators or inclement weather (Gleason 1978; Rich and Trentlage 1983). The Western Riverside County MSHCP identifies seven species objectives for owls, one of which requires documentation of ≥120 owls within the Plan Area, with no fewer than five breeding pairs in any one of five listed Core Areas (Dudek & Associates 2003). The following summarizes Biological Monitoring Program and Management Program efforts from 2015–2017 within the two Core Areas where breeding Burrowing Owls were observed on conserved lands during that time. Goals and Objectives 1. Determine whether Burrowing Owl Core Areas support a minimum of five breeding pairs of owls. a. Observe active burrows or burrow colonies periodically throughout the breeding season to determine whether owls were breeding, and if so, how many pairs were using each Core Area. 2. Determine whether Burrowing Owls appear to be selecting for certain habitat characteristics within the landscape.

Western Riverside County MSHCP 1 Biological Monitoring Program 2015–2017 Western Burrowing Owl Pair Count Survey Report

a. Collect habitat data at a micro- and macrohabitat scale around nesting burrows and nearby, unused locations to identify Burrowing Owl habitat and site preferences.

METHODS Pair Count Surveys Before conducting pair count surveys, biologists visited all artificial burrows, and any natural burrows occupied within the previous two years, to determine whether burrows were currently being used by owls (i.e., “active”). We briefly inspected the burrow entrances, looking for recent owl sign such as fresh pellets, whitewash, feathers, or prey remains. We also made note of whether owls were seen or heard during the pre- survey visit. These visits were brief in duration to minimize disturbance of owls and did not occur more than seven days before the planned date of the pair count surveys. Burrows that appeared active were then included during pair count surveys within the following seven days. We collected additional data related to Burrowing Owl pair status during banding efforts, pre-survey site visits, and unrelated work occurring near active burrows. We collected pair data within the San Jacinto Wildlife Area (WA)/Mystic Lake Core Area in 2015 and 2017, and the Lake Skinner/Diamond Valley Lake Core Area each year from 2015 to 2017 (Figs. 1 and 2). Specifically, we collected data at the San Jacinto WA within the San Jacinto WA/Mystic Lake Core Area, and El Sol, Johnson Ranch, Skunk Hollow, and the Southwestern Riverside County Multi-Species Reserve within the Lake Skinner/Diamond Valley Lake Core Area. We did not conduct pair count surveys if maximum wind speeds were >20 km/h, or during periods of precipitation or fog. The ideal conditions for conducting these surveys are temperatures >20 °C, wind <12 km/h, and cloud cover <75%. We conducted pair count surveys from 15 min before apparent sunrise until 1000 h (Conway et al. 2008). For pair count surveys, I assigned each participant a vantage point from which to observe one or more apparently active burrows. When possible, I assigned two observers to simultaneously collect data on each active burrow, although the number of participants each year usually precluded this. Vantage points were at least 50 m apart and were at least 50 m from active burrows (Conway and Simon 2003). We coordinated count times of Burrowing Owls, with the first count occurring 15 minutes before apparent sunrise, subsequent counts occurring exactly every 30 minutes thereafter, and the final count occurring at 1000 h (Conway et al. 2008). In addition to making observations during these assigned count times, participants recorded any noteworthy owl activity that occurred between coordinated count times, such as a female appearing from the burrow or prey deliveries by a male. Observers ultimately assessed how many owls were at each burrow and they recorded these data on their data sheet. Observers also recorded data on the behavior of the owls throughout the observation period, the time the behavior occurred, and any direction that owl(s) may have flown to or from. These data were useful in determining how many owls were being observed, whether the owls could be classified as a breeding pair, the approximate nesting stage when applicable, and whether owls were using burrows being observed by multiple people.

Western Riverside County MSHCP 2 Biological Monitoring Program Western Riverside County

LAKE SKINNER

Existing Conservation Land ") ")

") ") (!!( !( ") (!")!( !( !( (! !( (! (!(! ")!((!")!(") (!")(!")") (!")!((!")")!( !( (!(!(!")!( !(!( (!")") (!") (!(! (! !( (!")!( (! !( (!(!")")!(

Legend Source: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID, !( Burrow monitored in 2017 IGN, and the GIS User Community ") Burrow monitored in 2016 (! Burrow monitored in 2015 Date: 17 April 2018 Water Bodies UTM Nad 83 Zone 11 km Contact: Nicholas Peterson Existing Conservation Land I 0 0.375 0.75 1.5 2.25 3 MSHCP Biological Monitoring Program

Figure 1. Burrowing Owl burrows monitored during pair count surveys from 2015-2017 in the Lake Skinner Core Area. Western Riverside County

Existing Conservation Land

Source: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID, Legend IGN, and the GIS User Community !( Burrow monitored in 2017 (! Burrow monitored in 2015 Date: 17 April 2018 Water Bodies UTM Nad 83 Zone 11 km Contact: Nicholas Peterson Existing Conservation Land I 0 0.375 0.75 1.5 2.25 3 MSHCP Biological Monitoring Program

Figure 2. Burrowing Owl burrows monitored during pair count surveys from 2015-2017 in the San Jacinto Wildlife Area. 2015–2017 Western Burrowing Owl Pair Count Survey Report

I provided observers with enough information on Burrowing Owl breeding biology to determine nesting stage based upon the behavior of the owls. For example, females exclusively incubate and brood the young, while males are responsible for hunting. Males will typically return to the burrow carrying prey in their talons, at which point they will transfer the prey to their bill, and then give the prey to the female, who will either feed the young or consume the prey herself. Females begin hunting as the young are able to thermoregulate. By two weeks of age, nestlings will often wait at the burrow entrance for their parents to return with food (Poulin et al. 2011). In 2017 we incorporated the use of trail cameras (Bushnell Trophy Cam HD) in our collection of pair data. I placed one camera per active burrow and I deployed each camera for approximately 24 hours at a time. Cameras were 1–2 m from the front of burrow entrances and were positioned on the ground, usually strapped to a wooden stake that was already present as a perching site. We anticipated that cameras would provide us with information that would be impossible to obtain with human observers, including leg band data, prey items, and the presence of potential nest predators. The cameras are motion-activated and I set them to take bursts of three photos, followed immediately by 10 seconds of video, at intervals of no less than one minute. Nighttime photos were illuminated with infrared light so did not produce any visible flash. Finally, I used Picture Information Extractor (Picmeta Systems) to view and name the photo and video files. Habitat Assessment Beginning in 2016, we conducted habitat assessments to collect data on the habitat surrounding nesting burrows used by Burrowing Owls on Conserved Lands within the Plan Area. These data are intended to provide information for land managers who wish to improve habitat for Burrowing Owls or determine whether a site may be suitable for translocation of individuals. We collected these data following the failure or fledging of young at burrows. We collected data at every nesting burrow we monitored in 2016 and 2017 and we also collected data from one random point per occupied burrow. Such random points were exactly 100 m from the associated burrow and at least 100 m from any other active burrow or random point. We then used these paired data to determine whether owls were selecting for any of the habitat characteristics that we measured. For artificial burrows with two entrances, we determined whether both entrances appear to have been used by the owls (e.g., feathers, whitewash, or pellets were present at both entrances). If this was the case, our survey point was the approximate midpoint between the two entrances. If only one of the entrances appear to have been used (e.g., the second entrance was covered by spider webs, clogged with dirt, or otherwise does not exhibit use by owls as described above), then that entrance was our survey point. At each burrow/random point we determined the distance (m) to the nearest road, tree, potential non-nest satellite burrow, and potential observation perch (Green and Anthony 1989; Stockrahm 1995; Restani 2002; Thiele et al. 2013). We defined trees as a woody plant with a single main stem at least 7.6 cm in diameter at breast height and growing more than 3.9 m tall (Petrides and Petrides 1998). Potential non-nest satellite

Western Riverside County MSHCP 5 Biological Monitoring Program 2015–2017 Western Burrowing Owl Pair Count Survey Report burrows had an entrance with a minimum diameter of approximately 11–15 cm and a maximum diameter of 76 cm (Martin 1973; Butts and Lewis 1982; Haug 1985; MacCraken et al. 1985; Poulin et al. 2005). Finally, we defined potential observation perches as discrete features up to 2 m tall, including shrubs, stakes, fence posts, etc. We did not consider fence lines as perches because they could not be defined as discrete features. We considered clumps of shrubs whose canopies overlapped to be one observation perch. Next, we counted the number of potential non-nest satellite burrows and observation perches within 10 and 50 m of each burrow/random point (Haug 1985; Haug and Oliphant 1990; Restani 2002; Crowe and Longshore 2013). Collecting these data enabled us to quantify the microhabitat (10 m) and macrohabitat (50 m) around each burrow/random point as it related to the number of potential burrows and perches. Finally, we collected data along 50-m transects that radiated from the burrow/random point in each of the four cardinal directions (Green and Anthony 1989). At 5-m intervals, beginning 5 m from the burrow/random point, we recorded maximum vegetation height (cm) and we also made a tally indicating the type of ground cover (bare ground, litter, forb, grass, shrub, or tree) based on the first foliar hit. We defined forbs as herbaceous, non-grasslike plants, and shrubs as woody plants usually growing with several strong stems and less than about 3.9 m tall (Petrides and Petrides 1998). Finally, we recorded the total length (cm) of shrub canopy cover along each transect. We did this by making note of the total length of each 50-m tape that is covered, either partially or entirely, by the canopy of shrubs. Training Observers participating in pair counts were familiar with the ecology of Burrowing Owls within western Riverside County. Additionally, they had experience conducting habitat assessments and non-breeding or breeding season surveys. Breeding surveys conducted for the Biological Monitoring Program qualified as experience, as did the collection of data during our triannual surveys at active burrow sites. Observers were also familiar with the appropriate state and federal statutes pertaining to Burrowing Owls, scientific research, and conservation. For a list of participating individuals and agencies see the Acknowledgements section at the end of this report. Data Analysis I used one-tailed t-tests on our data pertaining to 1) the distances from survey points to the nearest road, tree, potential non-nesting satellite burrow, and perch, 2) the maximum vegetation height within 50 m of survey points, and 3) the length of shrub canopy cover along our 50-m-long transects. I hypothesized that nesting burrows would be farther from roads and trees, and closer to satellite burrows and perches, than nearby, unused sites (Dechant et al. 1999; Restani 2002; Poulin et al. 2005; Berardelli et al. 2010; Crowe and Longshore 2013; Thiele et al. 2013). I also hypothesized that the maximum vegetation heights within 50 m of nesting burrows would be shorter than unused sites (Butts and Lewis 1982; MacCracken et al. 1985; Green and Anthony 1989; Dechant et al. 1999; Thiele et al. 2013). Finally, I hypothesized that the amount of shrub canopy cover within 50 m of nesting burrows would be less than unused sites (Green and Anthony 1989; Dechant et al. 1999; Crowe and Longshore 2013; Thiele et al. 2013).

Western Riverside County MSHCP 6 Biological Monitoring Program 2015–2017 Western Burrowing Owl Pair Count Survey Report

I used a chi-squared test of independence between two samples on data pertaining to the distribution of 1) potential satellite burrows and perches within 10 m and 50 m of survey points, and 2) ground cover types within 50 m of survey points. I hypothesized that nesting burrows would have more potential burrows and perches within 10 m and 50 m than unused points (Desmond et al. 1995; Restani 2002; Poulin et al. 2005; Berardelli et al. 2010). I also hypothesized that nesting burrows would have more open habitat including forbs and bare ground, and less shrub coverage, than unused sites (MacCracken et al. 1985; Rich 1986; Green and Anthony 1989; Dechant et al. 1999; Crowe and Longshore 2013; Thiele et al. 2013).

RESULTS 2015 Pair Count Results We monitored 28 active burrows within the Lake Skinner/Diamond Valley Lake Core Area during the 2015 breeding season, during which time we detected a maximum of 11 owl pairs. The number of unpaired adult owls generally remained stable as the breeding season progressed, with a minimum of zero in April to a maximum of four in May (Table 1). Nine (32%) of the 28 burrows within the Lake Skinner/Diamond Valley Lake Core Area showed evidence of nesting. Six of these burrows produced at least nine fledglings (i.e., at least 1.5 fledglings per breeding pair) (Fig. 3). The remaining three burrows were used as nesting sites based upon the presence of nestlings, but we were ultimately unable to determine the outcome of the nesting attempts and thus classified them as “unknown” (Table 2). We were unable to confirm nesting at the remaining 19 (68%) burrows (Table 2). These burrows were occupied either by solitary adult owls that never interacted with other owls, owl pairs that never behaved in a way that indicated they were nesting, or solitary juvenile owls. Finally, of the nine burrows that showed evidence of nesting, two (22%) were artificial burrows and seven (78%) were natural burrows.

Table 1. Summary of Burrowing Owl pair count surveys within occupied Core Areas from 2015–2017. Months during which we did not conduct surveys are indicated with “NS.” Number observed by month Core Area Year Adult Owls Mar. Apr. May Jun. Jul. Aug. Pairs 6 11 10 10 NS NS 2015 Apparently solitary 2 0 4 2 NS NS individuals Lake Skinner / Pairs 9 8 9 8 NS NS Diamond 2016 Apparently solitary 2 4 0 1 NS NS Valley Lake individuals Pairs 9 5 10 10 6 7 2017 Apparently solitary 5 5 0 0 1 1 individuals Pairs NS NS NS NS 1 NS San Jacinto 2015 Apparently solitary NS NS NS NS 0 NS WA / Mystic individuals Lake including Pairs 0 1 1 1 1 1 Lake Perris 2017 Apparently solitary 0 0 0 0 0 0 individuals

Western Riverside County MSHCP 7 Biological Monitoring Program Western Riverside County

LAKE SKINNER

Existing Conservation Land

(!") !( (!")!( !( !( (! (! ") ") (!!( ") ") (!")")!(

Legend Source: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID, !( Burrow that fledged young in 2017 IGN, and the GIS User Community ") Burrow that fledged young in 2016

(! Burrow that fledged young in 2015 Date: 17 April 2018 Water Bodies UTM Nad 83 Zone 11 km Contact: Nicholas Peterson Existing Conservation Land I 0 0.375 0.75 1.5 2.25 3 MSHCP Biological Monitoring Program

Figure 3. Locations of Burrowing Owl burrows monitored during pair count surveys that produced fledglings from 2015-2017 in the Lake Skinner Core Area. 2015–2017 Western Burrowing Owl Pair Count Survey Report

Table 2. Presumed outcomes of active Burrowing Owl burrows observed during 2015–2017 pair count surveys. We were unable to determine the outcome of apparent nesting attempts within the “Unknown” category. Total Observed Burrow Status Burrows Core Area Year Monitored No Nesting Unknown Fledged Lake Skinner / 2015 28 19 3 6 Diamond 2016 26 12 5 9 Valley Lake 2017 19 10 3 6 San Jacinto 2015 1 0 1 0 WA / Mystic Lake including 2017 1 0 0 1 Lake Perris

2016 Pair Count Results We monitored 26 active burrows within the Lake Skinner/Diamond Valley Lake Core Area during the 2016 breeding season, during which time we detected a maximum of nine owl pairs. The number of unpaired adult owls generally remained stable as the breeding season progressed, with a minimum of zero in May to a maximum of four in April (Table 1). Fourteen (54%) of the 26 burrows within the Lake Skinner/Diamond Valley Lake Core Area showed evidence of nesting. Nine of these burrows produced at least 23 fledglings (i.e., at least 2.6 fledglings per breeding pair) (Fig. 3). The remaining five burrows were used as nesting sites based upon the presence of nestlings, but we were ultimately unable to determine the outcome of the nesting attempts and thus classified them as “unknown” (Table 2). We were unable to confirm nesting at the remaining 12 (46%) burrows (Table 2). These burrows were occupied either by solitary adult owls that never interacted with other owls or owl pairs that never behaved in a way that indicated they were nesting. Finally, of the 14 burrows that showed evidence of nesting, six (43%) were artificial burrows and eight (57%) were natural burrows. 2017 Pair Count Results We monitored 19 active burrows within the Lake Skinner/Diamond Valley Lake Core Area during the 2017 breeding season, during which time we detected a maximum of 10 owl pairs. The number of unpaired adult owls generally decreased as the breeding season progressed, from a maximum of five in March and April to a minimum of zero in May and June (Table 1). Nine (47%) of the 19 burrows within the Lake Skinner/Diamond Valley Lake Core Area showed evidence of nesting. Six of these burrows produced at least 10 fledglings (i.e., at least 1.7 fledglings per breeding pair) (Fig. 3). The remaining three burrows were used as nesting sites based upon the presence of nestlings, but we were ultimately unable to determine the outcome of the nesting attempts and thus classified them as “unknown” (Table 2). We were unable to confirm nesting at the remaining 10 (53%) burrows (Table 2). These burrows were occupied either by solitary adult owls that never interacted with other owls or owl pairs that never behaved in a way that indicated they were nesting.

Western Riverside County MSHCP 9 Biological Monitoring Program 2015–2017 Western Burrowing Owl Pair Count Survey Report

Finally, of the nine burrows that showed evidence of nesting, three (33%) were artificial burrows and six (67%) were natural burrows. We monitored one active natural burrow within the San Jacinto WA/Mystic Lake Core area in 2017 (Fig. 4). We documented the presence of a breeding pair in April and May and the pair hatched five young sometime between the deployment of a trail camera on 15 May and 13 June. All five nestlings fledged and persisted at the site through at least October 2017. Habitat Assessment There were no significant differences in the distances to the nearest road, tree, or potential non-nesting satellite burrow in 2016 and 2017 when I compared used burrows to nearby, randomly-selected unused sites (Table 3). In both years, however, nesting burrows tended to be closer to the nearest potential perch site than were unused sites. In 2016, nest burrows were a mean (± SE) distance of 3.2 ± 1.3 m from the nearest potential perch, whereas unused sites were 16.9 ± 4.5 m from the nearest perch site (t = -2.95, df = 9, p = 0.008). In 2017, nest burrows were 7.0 ± 6.0 m from the nearest potential perch, while unused sites were 58.4 ± 9.6 m from the nearest perch site (t = -4.55, df = 18, p = 0.0001).

Table 3. Mean ± SE distances to the nearest road, tree, potential non-nesting satellite burrow, and potential perch for nesting burrows and nearby, unused sites in 2016 and 2017. Asterisk (*) indicates a significant difference between used and unused burrows within a given year. Distance (m) to the nearest: Year Site use Road Tree Burrow Perch Used 247.8 ± 58.7 417.7 ± 68.0 18.4 ± 5.0 3.2 ± 1.3* 2016 Unused 256.0 ± 52.8 447.4 ± 63.3 19.4 ± 5.0 16.9 ± 4.5 Used 162.9 ± 59.5 225.5 ± 68.4 29.5 ± 18.0 7.0 ± 6.0* 2017 Unused 164.5 ± 61.5 395.3 ± 105.9 50.8 ± 13.3 58.4 ± 9.6

Nest burrows in 2016 had more potential satellite burrows within 10 m (2 = 32.67, df = 1, p < 0.00001) and 50 m (2 = 17.04, df = 1, p < 0.0001) than did unused sites. These burrows also had more perches within 10 m (2 = 4.76, df = 1, p = 0.029) and 50 m (2 = 19.56, df = 1, p < 0.00001) than did unused sites. Similarly, nest burrows in 2017 had more burrows within 10 m (2 = 24.0, df = 1, p < 0.00001) and 50 m (2 = 52.8, df = 1, p < 0.00001), and more perches within 10 m (2 = 10.0, df = 1, p = 0.002), than did unused sites. The number of perches within 50 m, however, did not differ between the two groups in 2017 (2 = 2.37, df = 1, p = 0.124) (Table 4).

Table 4. Mean ± SE number of potential burrows and perches within 10 m and 50 m of nesting burrows and nearby, unused sites in 2016 and 2017. All data are significant within each category and year, except for number of perches within 50 m in 2017. Within 10 m Within 50 m Year Site use N burrows N perches N burrows N perches Used 4.0 ± 1.9 1.1 ± 0.3 20.8 ± 6.8 9.6 ± 3.0 2016 Unused 0.5 ± 0.2 0.3 ± 0.2 13.8 ± 4.4 4.8 ± 1.5 Used 4.5 ± 1.2 1.0 ± 0.1 18.0 ± 4.9 6.2 ± 2.3 2017 Unused 0.9 ± 0.6 0.0 6.6 ± 3.8 4.6 ± 2.5

Western Riverside County MSHCP 10 Biological Monitoring Program Western Riverside County

Existing Conservation Land

Source: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID, Legend IGN, and the GIS User Community !( Burrow that fledged young in 2017

(! Burrow that failed in 2015 Date: 17 April 2018 Water Bodies UTM Nad 83 Zone 11 km Contact: Nicholas Peterson Existing Conservation Land I 0 0.375 0.75 1.5 2.25 3 MSHCP Biological Monitoring Program

Figure 4. Locations of Burrowing Owl burrows monitored during pair count surveys from 2015-2017 in the San Jacinto Wildlife Area. 2015–2017 Western Burrowing Owl Pair Count Survey Report

Maximum vegetation height (mean ± SE) within 50 m of nest burrows in 2016 was shorter than unused sites (1.0 ± 0.2 cm and 2.2 ± 0.5 cm, respectively) (t = -2.40, df = 697, p = 0.008). Vegetation height in 2017 was substantially taller than in 2016, but nest sites were still surrounded by shorter vegetation than unused sites (16.6 ± 1.2 cm and 22.2 ± 1.5 cm, respectively) (t = -2.94, df = 764, p = 0.002). Ground cover, recorded as first foliar hit, differed between nest and unused sites in both 2016 and 2017. Nest sites in 2016 tended to have less grass and shrub cover than expected compared to unused sites, contributing to an overall significant difference in ground cover between nest and unused sites (2 = 11.89, df = 5, p = 0.036). Shrubs were also less abundant than expected near nest sites in 2017, as was forb cover, contributing to the overall difference in ground cover between nest and unused sites (2 = 12.87, df = 4, p = 0.012). Bare ground was the most commonly encountered cover type in 2016 and grass was the most common in 2017 (Table 5).

Table 5. Ground cover types, represented by the first foliar hits along transects, within 50 m of nesting burrows and nearby, unused sites in 2016 and 2017. Cover types differed overall each year between site use categories; see text for details. Ground cover Bare Year Site use ground Litter Grass Forb Shrub Rock Used 232 184 10 37 3 5 2016 Unused 225 177 27 31 9 7 Used 78 124 145 42 6 1 2017 Unused 64 115 137 65 18 1

Finally, there were no significant differences in 2016 and 2017 in the amount of shrub canopy coverage within 50 m of nest and unused sites, as measured along each of the four cardinal directions. In 2016, nest sites had a mean (± SE) of 102.1 ± 58.4 cm of shrub canopy coverage and unused sites had 262.0 ± 149.5 cm of canopy coverage (t = - 1.00, df = 14, p = 0.17). Shrub canopy coverage increased somewhat in 2017, with nest sites having 388.0 ± 273.3 cm of canopy coverage within 50 m and unused sites having 775.7 ± 483.6 cm (t = -0.70, df = 18, p = 0.25).

DISCUSSION We documented at least five breeding pairs of Burrowing Owls using the Lake Skinner/Diamond Valley Lake Core Area each year from 2015 to 2017, but we did not meet this threshold in any of the remaining four Core Areas during that time. Further, the largest breeding population we documented during that period was in May and June of 2017, when we observed 11 pairs on conserved land, which falls far short of the goal of 120 individuals mentioned in Objective 2 of the species account. Overall, then, we have failed to demonstrate that Objective 2 is being met, which requires that Core Areas should support a combined total breeding population of approximately 120 owls, with no fewer than five pairs in any one Core Area.

Western Riverside County MSHCP 12 Biological Monitoring Program 2015–2017 Western Burrowing Owl Pair Count Survey Report

Lake Skinner/Diamond Valley Lake Core Area We detected a consistent number of pairs of owls within the Lake Skinner/Diamond Valley Lake Core Area from 2015–2017 (maximum of 11, 9, and 10 in 2015, 2016, and 2017, respectively). The number of occupied burrows, however, declined from 28 in 2015 to 19 in 2017. The sites we surveyed within this Core Area are actively managed for Burrowing Owls but we noticed a substantial increase in vegetation cover in 2017 following a relatively wet winter preceding the breeding season. The increased vegetation cover may have made the area less appealing to breeding Burrowing Owls, which generally prefer areas with low, sparse vegetation (James et al. 1991; Clayton and Schmutz 1999). Additionally, it is possible that there are additional owls on Conserved Land within the Core Area that were not detected by our biologists and thus were not included during pair counts. As a result, then, our data should be interpreted as the minimum number of owls in the Lake Skinner/Diamond Valley Lake Core Area. Apparent productivity, which was the minimum number of fledglings we observed per breeding pair, varied across years, from a low of 1.5 fledglings/pair in 2015 to a high of 2.6 fledglings per pair in 2016. These data fall within the range of Burrowing Owl productivity reported by investigators throughout the range of the species (Table 3).

Table 6. Summary of Burrowing Owl productivity (fledglings/breeding pair) reported within the range of the species. Productivity Location Source 0.9–1.5 Durango, Mexico Rodríguez-Estrella and Ortega-Rubio 1993 1.3–1.5 Pampas Region, Argentina Martínez et al. 2017 1.5 Riverside County, CA This study (2015) 1.5–2.0 Santa Clara County, CA Trulio and Chromczak 2007 1.6–2.4 British Columbia, Canada Mitchell et al. 2011 1.7 Riverside County, CA This study (2017) 2.2–3.4 Oakland, CA Thomsen 1971 2.5 Imperial Valley, CA Rosenberg and Haley 2004 2.5–2.6 Doña Ana and Sierra counties, NM Berardelli et al. 2010 2.6 Buffalo Gap, SD Griebel and Savidge 2007 2.6 Riverside County, CA This study (2016) 2.6–3.5 Little Missouri National Grassland, ND Davies and Restani 2006 3.4 Little Missouri National Grassland, ND Restani 2002 4.9 Albuquerque, NM Martin 1973

We were unable to determine the outcome of 11 nests within the Lake Skinner/Diamond Valley Lake Core Area from 2015 to 2017. Each of these burrows contained nestlings but we were unable to confirm fledging at the location. The burrows could have fledged young without our knowledge or they could have failed due to depredation or abandonment by one or both adults. Young owls tend to occupy satellite burrows near natal burrows at about 7–8 weeks of age (Green 1983). If this had occurred with the young at any of the 11 burrows of “unknown” outcome, we likely would have observed their presence during the pre-survey visits to the sites, or during the pair count surveys. More likely, these burrows failed due to desertion (i.e., abandonment) or depredation, which are known to be two of the most common causes of nest failure in Burrowing Owls (Green and Anthony 1989). Because we typically visited these burrows just once or twice per month during the breeding season, during which time an owl

Western Riverside County MSHCP 13 Biological Monitoring Program 2015–2017 Western Burrowing Owl Pair Count Survey Report carcass could have been completely consumed by a predator or scavenger, we cannot conclude with certainty whether any burrows that failed did so due to abandonment or depredation. San Jacinto Wildlife Area/Mystic Lake/Lake Perris Core Area Despite having an abundance of apparently suitable breeding habitat for Burrowing Owls within this Core Area, we have detected just two breeding pairs from 2015–2017. We cannot overlook the possibility that additional breeding pairs exist without our knowledge, but given the number of biologists and birders visiting the area it is unlikely that there are many such pairs. The pair that produced five fledglings in 2017 apparently fledged all of the young that we ever observed above ground. All five fledglings and both adults remained within 50 m of the natal burrow through the end of 2017. The site can be characterized as a large (55 m × 70 m, or 0.385 ha) dirt platform containing numerous burrows created via erosion. Trail Cameras The use of trail cameras in 2017 was a cost-effective way of monitoring burrows and it provided us with data that we were unable to obtain with our prior method of using human observers. I typically deployed cameras for 24-hour increments, which far exceeded the four or five hours that we typically observed burrows using biologists. Any owls at the burrow would certainly be active at some point during the 24-hour camera deployment, whereas those same owls could remain below ground and go undocumented during our four- or five-hour observation periods using biologists. Additionally, the trail cameras provided enough detail for us to be able to identify individual owls who were fitted with colored alphanumeric auxiliary leg bands (Fig. 5). This allowed us to make accurate counts of the number of owls at each burrow. The photos also confirmed that owls occasionally perched on artificial perches installed by land managers. Finally, the cameras allowed us to document prey items including a centipede (Class Chilopoda), crickets (Family Gryllidae), kangaroo rats (Dipodomys spp.), a San Diegan tiger whiptail (Aspidoscelis tigris stejnegri) (Fig. 6), and toads (Family Bufonidae), and the presence of potential nest predators such as Common Ravens (Corvus corax) and coyotes (Canis latrans) (Fig. 7), all of which are important information for our land managers.

Western Riverside County MSHCP 14 Biological Monitoring Program 2015–2017 Western Burrowing Owl Pair Count Survey Report

Figure 5. Two adult Burrowing Owls at the North Natural 7 burrow in Johnson Ranch, photographed using a Bushnell trail camera. Note the alphanumeric auxiliary leg bands whose characters are clearly discerned.

Figure 6. A male Burrowing Owl delivering a San Diegan tiger whiptail (Aspidoscelis tigris stejnegri) to an active nesting burrow in the San Jacinto Wildlife Area.

Western Riverside County MSHCP 15 Biological Monitoring Program 2015–2017 Western Burrowing Owl Pair Count Survey Report

Figure 7. A Common Raven inspecting an active Burrowing Owl burrow at the San Jacinto Wildlife Area. Photos and videos from trail cameras allow us to notify land managers of threats to Burrowing Owls by potential nest predators. Habitat Assessment Nest burrows in our study were an average of 247.8 m and 162.9 m from the nearest road in 2016 and 2017, respectively. These distances did not differ significantly from unused sites and were similar to those documented in Colorado (x̅ = 105.3–114.5 m; Plumpton and Lutz 1993), North Dakota (x̅ = 0.28 km; Restani 2002), and southern Canada (x̅ = 146.9 m; Scobie et al. 2014). Crowe and Longshore (2013) reported that burrows in their study in the eastern Mojave Desert were an average of 842 m from the nearest road. The roads near our study sites are infrequently traveled, which may partially explain why Burrowing Owls do not appear to be selecting for sites that are significantly farther from roads than what is otherwise available. Further, vehicles on the roads near our site are generally traveling at slow speeds and thus may not be eliciting a negative behavioral response from the Burrowing Owls (Scobie et al. 2014). The nearest visible tree was an average of 417.7 m and 225.5 m from nest burrows in 2016 and 2017, respectively, and these distances did not differ significantly from unused sites. The amount of tree cover was found to affect the likelihood of site selection by Burrowing Owls in a South Dakota study (Thiele et al. 2013). Specifically, with 0% tree cover within 800 m, the probability of a site being selected by nesting Burrowing Owls was >80%; however, when tree cover increased to 3.5%, that probability fell below 50%. Thiele et al. (2013) theorized that sites with more tree cover were avoided as nesting sites because nearby trees could provide perch and nest sites for larger raptors that prey upon Burrowing Owls. Unpublished data from local land managers indicate that Burrowing Owls in western Riverside County are preyed upon by

Western Riverside County MSHCP 16 Biological Monitoring Program 2015–2017 Western Burrowing Owl Pair Count Survey Report at least two species of raptors that can nest in trees, including Barn Owls (Tyto alba) and Great Horned Owls (Bubo virginianus). Potential satellite burrows in our study were an average of 18.4 m and 29.5 m from nest burrows in 2016 and 2017, respectively, and these distances did not differ significantly from unused sites. This is substantially farther than nearest burrows reported by Crowe and Longshore (2013), which were an average of 2.9 m from nest burrows in their study area in the eastern Mojave Desert. The close proximity of satellite burrows to nest burrows may increase the likelihood of owls being able to escape predators (Desmond and Savidge 1999; Holmes et al. 2003; Martin 1973), and this may be particularly critical to young owls that are less than two weeks post-fledging (Todd et al. 2003). Nest burrows in our study had significantly more available burrows within 10 m and 50 m than did unused sites in both 2016 and 2017. This is consistent with previous investigations that collected data on burrow densities within 25 m and 75 m of nest burrows and unused sites (Plumpton and Lutz 1993; Poulin et al. 2005). Another pattern in our data that is also supported by other investigations is that burrow density decreased as the distance from the nest burrow increased (Table 7). The importance of nearby satellite burrows was investigated by Crowe and Longshore (2013), who reported that an increased number of potential satellite burrows within 5 m of nest burrows was the best determinant of nest success. Similarly, Lantz et al. (2007) found that the likelihood of a site being used by nesting Burrowing Owls increased by 5% for each additional unused burrow within 30 m of the nest burrow. As discussed earlier, nearby satellite burrows can provide refuge from predators, for both young and adult owls (Desmond and Savidge 1999; Holmes et al. 2003; Martin 1973), provide shelter from inclement weather and sites for food caching (Haug 1985; Poulin et al. 2001 Lantz et al. 2007), and they can also serve as roost sites for dispersing young (King and Belthoff 2001; Todd 2001). The use of nearby satellite burrows may also reduce ectoparasite infestations in nest chambers by reducing crowding of family groups (Green and Anthony 1997). Finally, Ronan and Rosenberg (2014) reported that young owls spent 79% of their time within 20 m of the natal site prior to fledging, further providing evidence for the importance of a high density of satellite burrows near nest burrows.

Table 7. Summary of reported burrow densities at various distances from nesting burrows within the range of Burrowing Owls. Radius (m) No. of burrows Density (burrows/ha) Source 10 4.0 127.39 This study (2016) 4.5 143.31 This study (2017) 25 --a 107–144 Plumpton and Lutz 1993 30 29.0 102.58 Lantz et al. 2007 39.0 137.81 Restani 2002 50 18.0 22.93 This study (2017) 20.8 26.50 This study (2016) 38.2–47.6 48.66–60.64 Berardelli et al. 2010 75 19.7 11.15 Poulin et al. 2005 74.8 42.33 Desmond et al. 1995 a Data on the number of burrows were not provided, but densities were.

Western Riverside County MSHCP 17 Biological Monitoring Program 2015–2017 Western Burrowing Owl Pair Count Survey Report

The nearest discrete perches were an average of 3.2 m and 7.0 m from nest burrows in 2016 and 2017, respectively, and these were significantly closer than nearest perches to unused sites in both years. Generally speaking, nest burrows in our study had wooden stakes installed as perch sites by land managers, and these were usually installed within 5 m of burrow entrances, which likely accounts for the significant differences we saw both years. Our unused sites were an average of 16.9 m and 58.4 m from the nearest perch in 2016 and 2017, respectively, which may better represent what is naturally available in the landscape. For example, Poulin et al. (2005) reported that perches were an average of 51 m from burrows used by juvenile Burrowing Owls in their Saskatchewan study area, where dedicated artificial perches were not provided, and this distance was not significantly different from unused sites. Nest burrows in our study had significantly more perches within 10 m in 2016 and 2017 when compared to unused sites, and significantly more within 50 m in 2016 versus unused sites. Few investigators have reported on perch density near Burrowing Owl nest burrows, but Martínez et al. (2017) found that >75% of nests in their study had at least one perch within 20 m, suggesting that perch presence may play a role in site selection. Perches near nest burrows may provide several benefits to Burrowing Owls, including allowing owls to detect potential predators earlier (Andersson et al. 2009; Scobie et al. 2014) and improving hunting success during sit-and-wait foraging (Bellocq 1987). Finally, some investigators have suggested that Burrowing Owls may ultimately be selecting for the presence, rather than the number, of perch sites when choosing nest sites (Rodríguez-Estrella and Ortega-Rubio 1993; Scobie et al. 2014). The average maximum vegetation height within 50 m of nest burrows in our study was 1.0 cm and 16.6 cm in 2016 and 2017, respectively, and this was significantly shorter than vegetation surrounding unused sites for both years. The meteorological winter of 2015–2016 was drier than normal, with the study area receiving rainfall amounts that were approximately 7.62–8.89 cm below the historical average. The winter of 2016– 2017, however, was wetter than normal, with the area receiving rainfall amounts that were approximately 8.89–16.51 cm above the historical average (NOAA 2018). These variations in rainfall over two consecutive winters likely account for the different vegetation heights that we observed in 2016 and 2017. Regardless, the vegetation heights we observed both years are generally similar to those reported by other investigators. Butts and Lewis (1982) and Green and Anthony (1989) reported that Burrowing Owl nest burrows in their studies were surrounded by vegetation that was less than 10 cm tall. Plumpton and Lutz (1993) also found that Burrowing Owls preferred sites with significantly shorter vegetation than unoccupied sites, and the average vegetation at these sites did not exceed 8 cm in height. Young owls may also select roosting burrows whose entrances are surrounded by short vegetation (Poulin et al. 2005). McCracken et al. (1985) reported that owls in their study area nested in areas with slightly taller vegetation, averaging 12.8 cm tall. Finally, Thiele et al. (2013) found that owls in their study preferred sites with low visual obstruction (i.e., low vegetation). Low vegetation height may be preferred because of the benefits it provides for visual hunters such as Burrowing Owls (Johnsgard 2002). Additionally, hunting techniques used by Burrowing Owls, which include pursuit of prey on the ground, flying from perches to the ground, and

Western Riverside County MSHCP 18 Biological Monitoring Program 2015–2017 Western Burrowing Owl Pair Count Survey Report hovering, are all improved when vegetation height is relatively low (Butts 1973; Clayton and Schmutz 1999; Johnsgard 2002; Poulin et al. 2011). Nest burrows in our study tended to have less shrub cover than unused sites in both 2016 and 2017. They also had less grass cover in 2016 and less forb cover in 2017 than unused sites. We did not detect any differences in the amount of bare ground at used sites versus unused sites, but bare ground was surpassed by grasses and litter as the most common cover types in 2017, likely due to the above-average rainfall the area received the preceding winter (NOAA 2018). Previous investigations of the effects of ground cover on nest site selection have provided inconsistent results. Green and Anthony (1989) and Plumpton and Lutz (1993) found that Burrowing Owls preferred less grass cover and more bare ground at nest sites, while MacCracken et al. (1985) reported that nest sites in their study tended to have less bare ground than unused sites and equal amounts of grass and forb cover. Finally, Thiele et al. (2013) found that Burrowing Owls were most likely to nest where the amount of bare ground and forb cover was relatively high. Less vegetative density may benefit foraging owls because dense vegetation can impede the movement of small mammal (Rickard and Haverfield 1965) and arthropod prey (Tester and Marshall 1961; Gano and Richard 1982). Dense vegetation, especially grasses, can also make predation by Burrowing Owls more difficult by providing cover for potential prey (Southern and Lowe 1968; Wakeley 1978; Bechard 1982). We did not find that the amount of shrub canopy cover, measured along 50-m transects, differed between used and unused sites. Previous investigations have reached different conclusions about the role of shrub cover near Burrowing Owl sites. In the eastern Mojave Desert, where vegetation cover is generally low, Crowe and Longshore (2013) reported that Burrowing Owls preferred nest sites that had more creosote bush (Larrea tridentata) cover within 50 m of burrows. They theorized that since the landscape generally consisted of open habitat, owls did not need to select for areas with reduced cover, and that creosote bushes near nest burrows may ultimately serve as perch sites from which owls can hunt and defend against potential predators. Conversely, Green and Anthony (1989) and Lantz et al. (2007) reported that nesting Burrowing Owls selected sites with less shrub volume. This may indicate that Burrowing Owls are making a trade-off by selecting for areas with increased horizontal visibility (i.e., fewer shrubs) over areas with more potential perch sites (i.e., more shrubs) (Green and Anthony 1989). Recommendations Future Surveys I recommend continuing to survey for Burrowing Owl pairs using biologists, rather than trail cameras, for the early portions of the breeding season (i.e., March and perhaps April). During this time period we have seen that owl pairs have not necessarily selected a nesting burrow and may be moving around frequently. In such cases, a stationary trail camera will be unable to document which burrows are being used by a pair of owls. Later in the breeding season, after nesting burrows have been selected, the Program should continue using trail cameras to monitor burrows. Burrows should be monitored on a monthly basis at minimum, but biweekly deployments are preferred so that we miss fewer events at burrows such as appearance of nestlings and departure of fledglings.

Western Riverside County MSHCP 19 Biological Monitoring Program 2015–2017 Western Burrowing Owl Pair Count Survey Report

I also recommend we continue collecting habitat data at nesting burrows, and nearby unused sites for comparison. Such information is difficult to obtain for southern California in general, and western Riverside County specifically, so the data we collect will be especially useful to local land managers who are conserving habitat for Burrowing Owls. Conservation and Management Our results, and those by previous investigators, indicate that Reserve Managers in western Riverside County should continue to keep vegetation short (<10 cm) in areas being managed for Burrowing Owls. Reserve Managers may also want to focus on increasing the number of potential satellite burrows within 5 m of potential nest burrows, although the ideal number of such burrows is currently unknown. The Playa west of Hemet, a designated Core Area for Burrowing Owls, would benefit from active vegetation management, either grazing or mowing. The topography of the site seems conducive for the species, but current vegetation height and density are likely preventing owls from occupying the site. Core Area Definitions Our Program biologists detected a breeding pair of owls on Conserved Land approximately 440 m north of the boundary of the Santa Ana River Core Area, as defined in the MSHCP. We have since included this area in our searches for Burrowing Owls, and recommend officially expanding the boundary of this Core Area in the MSHCP.

ACKNOWLEDGEMENTS We thank the land managers in the MSHCP Plan Area, who in the interest of conservation and stewardship facilitate Monitoring Program activities on the lands for which they are responsible. Funding for the Biological Monitoring Program is provided by the Western Riverside Regional Conservation Authority and the California Department of Fish and Wildlife. Program staff who participated in 2015, 2016, or 2017 were Jess Burton, Tara Graham, Jen Hoffman, Cristina Juran, Michelle Mariscal, Lynn Miller, Bob Packard, Nicholas Peterson, Karen Riesz, Esperanza Sandoval, Ana Sawyer, and David Tafoya. Additional observers included Valerie Carlisle (CDFW), Betsy Dionne (RCA), Gerald Esra (CDFW Natural Resources Volunteer Program), Kim Klementowski (Center for Natural Lands Management), Christine Moen (Riverside County Parks), Ashley Ragsdale Aguiluz (volunteer and former MSHCP biologist), and Jonathan Reinig (RCA). Finally, we thank Kim Klementowski for allowing us to use five of her trail cameras during the 2017 season.

LITERATURE CITED Andersson M, Wallander J, Isaksson D. 2009. Predator perches: A visual search perspective. Functional Ecology 23:373–379. Bechard MJ. 1982. Effect of vegetative cover on foraging site selection by Swainson’s Hawk. Condor 84:153–159.

Western Riverside County MSHCP 20 Biological Monitoring Program 2015–2017 Western Burrowing Owl Pair Count Survey Report

Bellocq MI. 1987. Habitat selection and differential predation by Athene cunicularia on rodents of agrarian ecosystems. Revista Chilena de Historia Natural 60:81–86. Berardelli D, Desmond MJ, Murray L. 2010. Reproductive success of Burrowing Owls in urban and grassland habitats in southern New Mexico. Wilson Journal of Ornithology 122:51–59. Butts KO. 1973. Life history and habitat requirement of Burrowing Owls in western Oklahoma. Master’s Thesis. Oklahoma State University, Stillwater. Butts KO, Lewis JC. 1982. The importance of prairie dog towns to Burrowing Owls in Oklahoma. Proceedings of the Oklahoma Academy of Science 62:46–52. Clayton KM, Schmutz JK. 1999. Is the decline of the Burrowing Owls Speotyto cunicularia in prairie Canada linked to changes in Great Plains ecosystems? Bird Conservation International 9:163–185. Conway CJ, Garcia V, Smith MD, Hughes K. 2008. Factors affecting detection of Burrowing Owl nests during standardized surveys. Journal of Wildlife Management 72:688–696. Conway CJ, Simon J. 2003. Comparison of detection probability associated with Burrowing Owl survey methods. Journal of Wildlife Management 67:501–511. Crowe DE, Longshore KM. 2013. Nest site characteristics and nesting success of the Western Burrowing Owl in the eastern Mojave Desert. Journal of Arid Environments 94:113–120. Davies JM, Restani M. 2006. Survival and movements of juvenile Burrowing Owls during the postfledging period. Condor 108:282–291. Dechant JA, Sondreal ML, Johnson DH, Igl LD, Goldade CM, Rabie PA, Euliss BR. 1999 (revised 2002). Effects of management practices on grassland birds: Burrowing Owl. Northern Prairie Wildlife Research Center, Jamestown, ND. Desmond MJ, Savidge JA. 1999. Satellite burrow use by Burrowing Owl chicks and its influence on nest fate. Studies in Avian Biology 19:128–130. Desmond MJ, Savidge JA, Seibert TF. 1995. Spatial patterns of Burrowing Owl (Speotyto cunicularia) nests within black-tailed prairie dog (Cynomys ludovicianus) towns. Canadian Journal of Zoology 73:1375–1379. Dudek & Associates. 2003. Western Riverside County Multiple Species Habitat Conservation Plan (MSHCP). Final MSHCP, volumes I and II. Prepared for County of Riverside County Transportation and Lands Management Agency by Dudek & Associates, Inc. Approved June 17, 2003. Gano KA, Rickard WH. 1982. Small mammals of a bitterbrush-cheatgrass community. Northwest Science 56:1–7. Garrett K, Dunn J. 1981. Birds of southern California: Status and distribution. Los Angeles Audubon Society.

Western Riverside County MSHCP 21 Biological Monitoring Program 2015–2017 Western Burrowing Owl Pair Count Survey Report

Gervais JA, Rosenberg DK, Comrack LA. 2008. Burrowing Owl (Athene cunicularia). In Shuford WD, Gardali T, editors. California Bird Species of Special Concern: A ranked assessment of species, subspecies, and distinct populations of birds of immediate conservation concern in California. Studies of Western Birds 1. Western Field Ornithologists, Camarillo, California, and California Department of Fish and Game, Sacramento. Gleason RS. 1978. Aspects of the breeding biology of Burrowing Owls in southeastern Idaho. Master’s Thesis. University of Idaho, Moscow. Green GA. 1983. Ecology of breeding Burrowing Owls in the Columbia basin, Oregon. Master’s Thesis, Oregon State University, Corvallis. Green GA, Anthony RG. 1989. Nesting success and habitat relationships of Burrowing Owls in the Columbia Basin, Oregon. Condor 91:347–354. Green GA, Anthony RG. 1997. Ecological considerations for management of breeding Burrowing Owls in the Columbia Basin. Journal of Raptor Research Report 9:117–121. Griebel RL, Savidge JA. 2007. Factors influencing Burrowing Owl reproductive performance in contiguous shortgrass prairie. Journal of Raptor Research 41:212– 221. Grinnell J, Miller AH. 1944. The distribution of the birds of California. Pacific Coast Avifauna 27. Cooper Ornithological Club, Berkeley, California. Reprinted by Artemisia Press, Lee Vining, California; April 1986. Haug EA. 1985. Observations on the breeding ecology of Burrowing Owls in Saskatchewan. Master’s Thesis, University of Saskatchewan, Saskatoon. Haug EA, Oliphant LW. 1990. Movements, activity patterns, and habitat use of Burrowing Owls in Saskatchewan. Journal of Wildlife Management 54:27–35. Holmes AL, Green GA, Morgan RL, Livezey KB. 2003. Burrowing Owl nest success and burrow longevity in north central Oregon. Western North American Naturalist 63:244–250. James PC, Ethier TJ, Fox GA, Todd M. 1991. New aspects of Burrowing Owl biology. In Holroyd GL, Burns G, Smith HC, editors. Proceedings of the 2nd Endangered Species and Prairie Conservation Workshop. Provincial Museum of Alberta Natural History Occasional Paper no. 15. Johnsgard PA. 2002. North American owls: Biology and natural history. Second edition. Smithsonian Institution Press, Washington D.C. King RA, Belthoff JR. 2001. Post-fledging dispersal of Burrowing Owls in southwestern Idaho: Characterization of movements and use of satellite burrows. Condor 103:118–126. Landry RE. 1979. Growth and development of the Burrowing Owl. Master’s Thesis. California State University, Long Beach.

Western Riverside County MSHCP 22 Biological Monitoring Program 2015–2017 Western Burrowing Owl Pair Count Survey Report

Lantz SJ, Conway CJ, Anderson SH. 2007. Multiscale habitat selection by Burrowing Owls in black-tailed prairie dog colonies. Journal of Wildlife Management 71:2664–2672. MacCracken JG, Uresk DW, Hansen RM. 1985. Burrowing Owl foods in Conata Basin, South Dakota. Great Basin Naturalist 45:287–290. Martin DJ. 1973. Selected aspects of Burrowing Owl ecology and behavior. Condor 75:446–456. Martínez G, Baladrón AV, Cavalli M, Bó MS, Isacch JP. 2017. Microscale nest-site selection by the Burrowing Owl (Athene cunicularia) in the Pampas of Argentina. Wilson Journal of Ornithology 129:62–70. Mitchell AM, Wellicome TI, Brodie D, Cheng KM. 2011. Captive-reared Burrowing Owls show higher site-affinity, survival, and reproductive performance when reintroduced using a soft-release. Biological Conservation 144:1382–1391. NOAA. 2018. Advanced hydrologic prediction service: Quantitative precipitation estimates. Available from https://water.weather.gov/preip/ (accessed March 2018). Petrides GA, Petrides O. 1998. A field guide to western trees: Western United States and Canada. Houghton Mifflin Company, Boston. Plumpton DL, Lutz RS. 1993. Nesting habitat use by Burrowing Owls in Colorado. Journal of Raptor Research 27:175–179. Poulin RG, Todd LD, Dohms KM, Brigham RM, Wellicome TI. 2005. Factors associated with nest- and roost-burrow selection by Burrowing Owls (Athene cunicularia) on the Canadian prairies. Canadian Journal of Zoology 83:1373–1380. Poulin R, Todd LD, Haug EA, Millsap BA, Martell MS. 2011. Burrowing Owl (Athene cunicularia). In Poole A, editor. The Birds of North America Online. Cornell Lab of Ornithology, Ithaca, New York. Available from http://bna.birds.cornell.edu.bnaproxy.birds.cornell.edu/bna/species/061 (accessed August 2014). Poulin RG, Wellicome TI, Todd LD. 2001. Synchronous and delayed numerical responses of a predatory bird community to a vole outbreak on the Canadian prairies. Journal of Raptor Research 35:288–295. Restani M. 2002. Nest site selection and productivity of Burrowing Owls breeding on the Little Missouri National Grassland. 2001 Annual Report. Prepared for Dakota Prairie Grasslands, USDA Forest Service, Billings, MT. Rich T. 1986. Habitat and nest-site selection of Burrowing Owls in the sagebrush steppe of Idaho. Journal of Wildlife Management 50:548–555. Rich T, Trentlage B. 1983. Caching of long-horned beetles by the Burrowing Owl. Murrelet 64:25–26.

Western Riverside County MSHCP 23 Biological Monitoring Program 2015–2017 Western Burrowing Owl Pair Count Survey Report

Rickard WH, Haverfield LE. 1965. A pitfall trapping survey of darkling beetles in desert steppe vegetation. Ecology 46:873–875. Robertson JM. 1929. Some observations on the feeding habits of the Burrowing Owl. Condor 31:38–39. Rodríguez-Estrella R, Ortega-Rubio A. 1993. Nest site characteristics and reproductive success of Burrowing Owls (Strigiformes: Strigidae) in Durango, Mexico. Revista de Biología Tropical 41:143–148. Ronan NA, Rosenberg DK. 2014. Response of Burrowing Owls to experimental removal of satellite burrows. Journal of Wildlife Management 78:1115–1119. Rosenberg DK, Haley KL. 2004. The ecology of Burrowing Owls in the agroecosystem of the Imperial Valley, California. Studies in Avian Biology 27:120–135. Scobie C, Bayne E, Wellicome T. 2014. Influence of anthropogenic features and traffic disturbance on Burrowing Owl diurnal roosting behavior. Endangered Species Research 24:73–83. Southern HN, Lowe VPW. 1968. The pattern of distribution of prey and predation in Tawny Owl territories. Journal of Animal Ecology 37:75–97. Stockrahm DMB. 1995. Distribution of the Burrowing Owl (Athene cunicularia) in Billings County, North Dakota. Unpublished report. Moorhead State University, Moorhead, MN. Tester JR, Marshall WH. 1961. A study of certain plant and animal inter-relationships of a native prairie in north-western Minnesota. Occasional Papers of the Minnesota Museum of Natural History 8:1–51. Thiele JP, Bakker KK, Dieter CD. 2013. Multiscale nest site selection by Burrowing Owls in western South Dakota. Wilson Journal of Ornithology 125:763–774. Thomsen L. 1971. Behavior and ecology of Burrowing Owls on the Oakland Municipal Airport. Condor 73:177–192. Todd LD, Poulin RG, Brigham RM, Bayne EM, Wellicome TI. 2007. Pre-migratory movements by juvenile Burrowing Owls in a patchy landscape. Avian Conservation and Ecology – Écologie et conservation des oiseaux 2:4. Available from http://www.ace-eco.org/vol2/iss2/art4/ (accessed August 2014). Todd LD, Poulin LG, Wellicome TI, Brigham RM. 2003. Post-fledging survival of Burrowing Owls in Saskatchewan. Journal of Wildlife Management 67:512–519. Trulio LA, Chromczak DA. 2007. Burrowing Owl nesting success at urban and parkland sites in northern California. California Burrowing Owl Symposium 1–15.

Western Riverside County MSHCP 24 Biological Monitoring Program 2015–2017 Western Burrowing Owl Pair Count Survey Report

USFWS (U.S. Fish and Wildlife Service). 2002. Birds of Conservation Concern. U.S. Department of the Interior, Fish and Wildlife Service, Administrative Report, Arlington, VA. Available from http://www.fws.gov/midwest/wind/references/BCC2002.pdf (accessed August 2014). Wakeley JS. 1978. Factors affecting the use of hunting sites by Ferruginous Hawks. Condor 80:316–326. Wedgwood JA. 1976. Burrowing Owls in south-central Saskatchewan. Blue Jay 34:26– 44. Zarn M. 1974. Burrowing Owl. Report no. 11. Habitat management series for unique or endangered species. Bureau of Land Management, Denver, CO.

Western Riverside County MSHCP 25 Biological Monitoring Program