Western North American Naturalist

Volume 63 Number 4 Article 16

12-3-2003

Nest-site selection by Western Screech-Owls in the Sonoran Desert, Arizona

Paul C. Hardy University of Arizona, Tucson

Michael L. Morrison University of California, Bishop

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Recommended Citation Hardy, Paul C. and Morrison, Michael L. (2003) "Nest-site selection by Western Screech-Owls in the Sonoran Desert, Arizona," Western North American Naturalist: Vol. 63 : No. 4 , Article 16. Available at: https://scholarsarchive.byu.edu/wnan/vol63/iss4/16

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NEST~SITE SELECTION BY WESTERN SCREECH~OWLS IN THE SONORAN DESERT, ARIZONA

Paul C. Hardyl,2 and Michael L. Morrison3

Key words: Gila Woodpecker, Gilded Flicker, nest-site selection, Otus kennicottii, saguaro cacti, Sonoran Desert, West­ ern Screech~Owl.

The Western ScreecbOwl (Otus kennicottii) valley includes the creosote (Larrea triden­ is a small, nocturnal, secondary caVity-nesting tata)~white bursage (Ambrosia dumosa) series, bird that is a year-round resident throughout and in runnels and washes, the mixed-scrub much of western North America and Mexico series (Turner and Brown 1994). Temperatures (Marshall 1957, Phillips et al. 1964, AOU 1983, for the Arizona Upland and Lower Colorado Johnsgard 1988, Cannings and Angell 2001). River valley regions average 29°C-=30°C in the Several aspects of Western Screech~Owl biol­ summer and 11°C=12°C in the winter; mean ogy, including taxonomy (Miller and Miller annual precipitation is 20-30 em (Sellers et al. 1951, Marshall 1957), distribution (Marshall 1985). Our study site was in northeastern 1957, Johnsgard 1988), food habits (Brown et al. BMGR in the Sauceda and Sand Tank moun­ 1986), and breeding density (Johnson et al. tain ranges at 366=853 m elevation. 1981), have been studied (see Cannings and We searched for nests in association with Angell [2001] for a review), but no quantitative conducting point~count surveys (see Hardy information on nest-site selection by the [1997] and Hardy et al. [1999] for details) for species has been published. screech~owls and Elf Owls (Mic1"(.lthene whit­ During 1995 and 1996 We studied nest~site neyi) in 1995 and 1996. We conducted surveys selection by Western Screech~Owls (hereafter, along 6 point transects consisting of 10 sta­ screech-owls) in the Sonoran Desert ofArizona tions each (60 stations total) spaced at 0.8~km using a multi-scaled approach. At the scale of intervals. We used compass triangulation data the nesting area, we assessed whether features from surveys to identifY areas of concentrated of cavities, trees, and surrounding vegetation singing activity where we later conducted nest were related to nest~site selection. At the scale searches. We did nest searches during the of the nest tree, we examined Whether owls nestling period (mid-April through late May). selectively choose cavities from those available. Nestling screech-owls were quite vocal and OUr study took place on the Barry M. Gold~ could be heard up to 50 m away. water Air Force Range (BMGR) in southwest­ We centered a 25~m-radius plot (0.2 ha) on ern Arizona. The BMGR occupies approxi­ each nest. Because we did not measure terri~ mately 10,900 km2 ofunpopulated land, one of tory or home range size, we refer to selection the largest and best~preserved regions of at the scale of the nesting area rather than at native desert remaining in the U.S. Two major the scale ofthe territory or home range. vegetation types are found on BMGR: the Ari­ We surveyed vegetation around each nest zona Upland and the Lower Colorado River along eight 25-m point-intercept transects valley subdivisions ofthe Sonoran Desert. The (Bonham 1989). We spaced intercept points at Arizona Upland includes the paloverde (Cer­ 5~m intervals at which we collected informa­ cidium spp.)-mixed-cacti scrub series and tion on each plant species in 4 vertical height paloverde-catclaw (Acacia greggii) xeroripar­ classes: 0 m (ground level), >0 m to 1.0 m ian associations. The Lower Colorado River (understory), > 1.0 m to 2.5 m (mid~canopy),

lWI1dlife and Fisheries Science Program, School ofRenewable Natural Resources, University ofArizona, Thcson, AZ 85721. 2Present address: Box 4276, Quincy, CA 95971. 3Corresponding author. White Mountain Research Station, University ofCalifornia, 3000 East Line Stree~ Bishop, CA 93514.

., 533 534 WESTERN NORTH AMERICAN NATURALIST [Volume 63 and >2.5 m (overstory). We identified live and nest tree, we measured the same features as dead shrubs, trees, and most cacti to species. described above for the surrounding vegeta­ For each we determined the number ofinter­ tion, nest tree, and nest cavity. If a tree had cepts in each height class with the aid of a multiple potential cavities, we randomly graduated extendable pole. We divided this selected 1 for comparison. To facilitate infer­ number by total intercept points (40 per nest) ences about the selection ofnest cavities from to estimate percent cover by height class and nest trees, we measured the height, orienta­ percent cover ofperennial vegetation. tion, location, and diameter (as described Within each 25-m-radius plot, we recorded above) of all potential nest cavities in the nest the number oftrees and cacti>2.5 m tall with tree itself average stem diameter > 15 em (Goad and We used univariate, paired tests to deter­ Mannan 1987) and the number of cavities mine which variables differed between used within these structures. Following Goad and and potential nest sites. For continuous vari­ Mannan (1987), we placed saguaro cacti (Car­ ables We used paired t tests. For categorical negiea gigantea) into the following structure variables we conducted G4ests for homogene­ categories: (1) <3 m tall with no branches, (2) ity (Zar 1996:489). We used Raleigh's test (Zar ~3 m tall with no branches or branches <6 em 1996:615) to determine ifnest cavity and poten­ in length, (3) >4 m tall with branches 6 em to tial cavity orientations were nonrandom. We 1 m in length; (4) >5 m tall with 1~2 branches set alpha at 0.1. We recognize, however, that > 1 m in length, (5) >5 m tall with 3 or more our multiple statistical tests are not indepen­ branches>1 m in length, or (6) >2 m tall with dent. We chose not to apply experiment-wise a broken top. The number ofsaguaros per plot adjustment of error (e.g., Bonferonni proce­ c1id not include a nest saguaro. .. dures; Winer et al. 1991:158-166) because we Features ofthe nest tree we measured were are in the exploratory phase of this research, (1) height, (2) diameter at breast height (dbh); where an increase in the level of significance (3) number ofcavities per woodpecker species is not of major importance. In this phase it is (excluding nest cavity), and (4) structure cate­ more important to examine and interpret vari­ gory (if a saguaro, described above). We mea­ ations in data than to worry about specific sured height as indicated above and dbh with alpha levels. Additionally, over-reliance on sta~ a diameter tape. tistical hypothesis testing can often obscure We measured the following features of the potentially meaningful biological relationships nest cavity, height, compass orientation ofopen­ (e.g., Johnson 1999). ing, location (branch or stem), and diameter To examine nest~cavity selection within nest (whether excavated by Gila Woodpecker [Melan­ trees, we compared features of nest cavities erpes uropygialis] or Gilded Flicker [Colaptes with those of potential cavities within nest chrysoidesJ). We used cardboard disks attached trees. We calculated the mean height and ori~ to an extendable pole to estimate the excavat­ entation (compass direction) for all potential ing woodpecker species following Kerpez and cavities in each nest tree and then paired Smith (1990). We did not measure inner dimen­ these means with nest cavity heights and ori­ sions ofcavities due to the dangers ofclimbing entations used by the birds using paired t saguaros, and hole size is correlated with inside tests, G-tests, and Raleigh's tests. area (McAuliffe and Hendricks 1988, Ketpez In 1996 we located 12 screech-owl nests: and Smith 1990). 10 in saguaro cavities and 2 in mesquite tree For each nest tree, we located the nearest (Prosopis spp.) cavities. We searched for nests potentially Suitable, but unused, tree (McCal­ too late in 1995, finding fledglings but no lum and Gehlbach 1988) meeting the follow­ nests. In 1996, the 3rd year of a prolonged ing requirements: positioned >50 m from the drought, very few screech-owls nested (Hardy nest tree (to avoid plot overlap), possessing at 1997, Hardy et al. 1999). A nesting Gilded least 1 potentially suitable cavity ~3.2 m high Flicker concurrently occupied 1 nest in saguaro; (the lowest height we recorded for a screech­ all other nest saguaros were occupied solely owl nest), and unoccupied by a woodpecker. by screech-owls. When necessary, we examined cavities using a The 2 nests in mesquite were in trees 6.5 m mirror attached to an extendable pole to deter­ and 7.8 m tall, with cavities at 2.1 ill and 2.8 m mine occupation. When we located a potential in height, respectively. Because only 2 screech- 2003] NOTES 535

owl nests were in trees, we limited detailed differ from the height ofpotential cavities (t = analyses below to nests located in saguaros. 0.69, 9 df, P ~ 0.51). Mature ironwoods (Olneya linearis), mature Finding that screech-owls nested in mes­ paloverde, and the 2 tallest structure classes of quites is significant, because other than in saguaros were found in significantly lower mesic riparian areas (Bendire 1892), screech~ densities at screech~owl nest sites than at owls have not been documented to nest in tree potential sites (Table 1). The cover ofoverstory cavities in the Sonoran Desert (Bent 1938, perennial vegetation was also significantly lower Johnsgard 1988). Hardy (1997) and Hardy et around screech~owl nests (Table 1). Note that, al. (1999) found that Elf Owls nested exclu­ except for cavity height, the variance (standard sively in saguaros and suggested this was due error) associated with each variable describing to the cooler and more stable microclimate of potential nest sites was greater than that for saguaro cavities relative to tree cavities. Unlike used nest sites (Table 1). ElfOwls, screech"owls might not be limited to Nests in saguaros ranged in height from saguaros because they have a wider thermo­ 4.7 m to 10.9 m with a mean of 8.1 m (sx ~ neutral zone (Ligon 1969). They are probably 0.64, n = 10) and ranged in dbh from 45 cm limited more by the presence of structures to 59 cm with a mean of 52.9 em (sx = 1.29, large enough to house cavities than by micro­ n = 10). They had a significantly greater dbh climatic factors in the Sonoran Desert. than potential nest saguaros (Table 1). The Unlike ElfOwls, which seem to prefer Gila number ofpotential cavities within nest s~guaros Woodpecker cavities (Hardy 1997, Hardy et al. ranged from 2 to 15 with a mean of 5.1 (sx = 1999), screech~owls nested nearly exclusively 1.26, n ~ 10) and did not significantly differ in Gilded Flicker cavities. The larger body size from the number of potential cavities within of Western Screech~Owls might limit them potential nest saguaros (Table 1). However, the from entering Gila Woodpecker cavities, or the number of Gilded Flicker cavities was greater much smaller interior of Gila Woodpecker within nest saguaros than within potential nest cavities (Kerpez and Smith 1990) may con­ saguaros (t = 3.06, 9 df, P = 0.009). strain the large brood sizes characteristic of Nine ofthe 10 screech-owl nests located in the species (Johnsgard 1988). The fact that saguaros were in cavities excavated by Gilded screech~owls selected saguaros oflarge diame~ Flickers, and these cavities were used signifi­ ter for nesting suggests that cavity volume is a cantly out of proportion to their potential consideration. In Oregon and Washington, availability (Fisher's exact test, n ::;:: "10, P ~ Western Screech-Owls used cavities in trees 0.015). The 1 screech~owl nest not in an obvi~ with a minimum dbh of30.5 cm (Thomas et al. ous Gilded Flicker cavity was intermediate in 1979). size between a Gila Woodpecker and Gilded Although the total number of cavities (re­ Flicker cavity (vertical diameter = 6.2 em, hori~ gardless of excavating species) did not differ zontai diameter == 7.1 cm). Nest cavity height between nest saguaros and potential" nest ranged from 3.2 m to 8.6 m with a mean of6.2 saguaros, we found significantly more Gilded (sx ::::: 0.55, n == 10) and did not significantly Flicker cavities in nest saguaros than in poten~ differ from potential cavity height (Table 1). The tial saguaros. Thus, having alternate Gilded orientation of nest cavities was random (r = Flicker cavities may be important in nest-site 0.09, P > 0.50, n = 10) and the location of:p.est selection by screech-owls. Other studies have cavities did not significantly differ from that shown that the number of alternate suitable expected based on location of potential cavi­ cavities surrounding nests is an important fac­ ties (Fisher's exact test, n :::: 10, P = 0.89). tor in nest-site selection by cavity-nesting birds Cavity selection within nest saguaros was (e.g., Swallow et al. 1986, Martin and Roper similar to that within the nesting area. Screech­ 1988). Alternate cavities in the vicinity of the owl nests were again located in Gilded Flicker nest provide birds with renesting sites in case cavities more frequently than expected based of nest failure (Rendell and Robertson 1994) on potential availability (Fisher's exact test, P = or nest usurpation, and in Eastern Screech­ 0.012, n = 10). The location of nest cavities Owls alternate cavities may be lised as roost, did not differ from that expected based on the food storage, and alternative nest sites (Gehl~ location of potential cavities (Fisher's exact bach 1995). Alternatively, if flickers tend to test, P = 0.98, n = 10), and the height did not make multiple cavities once they find a suitable 536 WESTERN NORTH AMERICAN NATURALIST [Volume 63

TABLE 1. Features of surrounding vegetation, saguaros, and cavities at nest sites of Western Screech-Owls (n = 10) and at unused but potential nest sites, Barry M. Goldwater Air Force Range, Arizona, 1995-1996. Potential Category Used Variable x Sx x Sx pa Surrounding vegetationb Class 5c saguaro densityd 0.3 0.14 0.8 0.23 0.082 Class 4C saguaro density 0.6 0.21 1.3 0.26 0.081 Ironwood density 0.8 0.54 3.0 0.69 0.019 Paloverde density 1.2 0.71 4.2 1.36 0.034 Perennial vegetation % cover understorye 31.2 6.33 38.8 7.27 0.444 % cover mid-canopyf f).8 2.11 12.7 3.44 0.527 % cover overstoryg 1.4 0.76 4.2 1.30 0.079 Saguaro Total cavities 5.1 1.26 3.7 1.46 0.474 Height(m) 8.1 0.64 8.0 0.66 0.840 DBH(cm) 52.6 1.41 46.4 2.12 0.024 Cavity height (m) 6.2, 0.55 5.8 0.41 0.609 aSignmcance ofpaired t test bFeatures ofsurrounding vegetation measured within 25-m-radiu5 plot centered on nest saguaro ·Class 5 saguaros: >5 m tall with atleast 3 branches>1 m long; class 4 saguaros: >5 m tall with 1-2 branches>1 m long dNumber ofstructures (e.g., cavities, trees) within 25-m-radius plot e>0-1m tall f>1-2.5m tall g>2.5mtall

cactus, then the presence of multiple cavities ing potential nest sites was greater than that could play no role in nest-site selection by associated with used sites. This higher vari­ owls. Another possibility is that saguaros with ance could also reflect that owls are selecting more flicker cavities may be mOre likely to for a narrower subset of conditions than those have owl nests simply because ofhigher avail­ available to them. Eastern Screech-Owls and ability of suitable cavities. Experimental eval­ Flammulated Owls (Otus jlammeolus) also seem uation of these ideas would be necessary to to prefer nest sites with open subcanopy space resolve the primary reasons for using struc­ and sparse shrub cover (McCallum and Gehl­ tures with multiple cavities. bach 1988, Robbins et al. 1989, Belthoff and Other than their apparent preference for Ritchison 1990, Gehlbach 1995). McCallum Gilded Flicker cavities, screech-owls did not and Gehlbach (1988) suggested that the flight select cavities based on orientation, height, or behavior of these species near the nest is re~ location (branch or stem). These findings are lated to their preference for open vegetation consistent with previous studies of nest-site around the nest. Our observations ofthe flight selection (Gehlbach 1995). Although the direc­ behavior of screech~owls,although not quanti~ tion a cavity entrance faces may affect the micro" fred (unpublished data), are consistent with climate within the' cavity, our results suggest those described by McCallum and Gehlbach that entrance orientation is oflittle importance (1988). to screech-owls in our study area, despite ex­ treme summer temperatures. Similarly; Duley (1979) and Belthoffand Ritchison (1990) found We thank Luke Air Force Base for funding that Eastern Screech-Owl nest cavities were this project via their enVironmental program, randomly oriented. Flight. We also thank the School ofRenewable Screech-owl nesting areas were character~ Natural Resources at the University ofArizona ized by lower cover of perennial vegetation, for financial and logistical support. We thank saguaros, ironwood, and paloverde than poten­ field assistants T. Abeloe and G. Downard for tial sites. We also found that the variance (stan­ their outstanding work, and S. DeStefano, R.W dard error) associated with variables describ- Mannan, E Gehlbach, J. Marks, J. Belthoff, 2003] NOTES 537 and G. Hayward for reviewing the manuscript KERPEZ, TA., AND N.S. SMITH. 1990. Nest-site selection and providing many constructive comments. and nest-cavity characteristics of Gila Woodpeckers and Northern Flickers. Condor 92:193-198. LIGON, J.D. 1969. 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