Rev. Bíol. Trop., 41 (1): 143-148, 1993

Nest site charaderistics and reproductive success of burrowing owls (Strigiformes: Strigidae) in , Mexico

Ricardo Rodríguez-Estrella' and AlfredoOrtega- Rubio' Instituto de EcologíaA.P . 18-845. México 11800 D.E México. 1 Present address: Centro de Investigaciones Biológicas de Baja CaliforniaSur, Apartado Postal 128, La Paz 23000 B.C.S., México.

(Rec. 23-Vll-1991. Acep. 26-VIII-1992)

Abstract: The burrowing owl, Athene cunicularia,is a threatened redspecies in most of its North American distribu­ tíon. Nest-site characteristícs and reproductive success were compared for two breeding seasons in the southem , Durango, Mexico. Reproductive success is highly correlated with the presence of the Prosopis­ Hilaria grassland association. The following factors were not associated with nesting success: burrow type, distance to the nearest adjacent nest, soil texture and number of perches.

Key words: Athene cunicularia, burrowing owls, nesting habitat characteristics, México, reproductive success.

Studies on nest-site selection by birds of MATERIAL AND METHODS prey indicate tilat raptors select nest sites accor­ ding 10 physical characteristics of the habitat. Study aTea: We worked in the Mapimí such as 1Opography and perch availability. and Biosphere Reserve (260 29'-260 52' N, 1030 according to biotic features. such as ecosystem 58'-103032' W). In Mapimí, 9 vegetationtypes maturity. prey abundance, and interspecific have been recognized by Martinez and MoreDo competition (Southern and Lowe 1968. Bock (1977), with Larrea tridentata. Fouquieria and Lepthien 1976, Janes 1985). Burrowing splendes, Prosopis glandulosa, Jatropha dioi­ owIs (Athene cunicularia) in Canada and the ca, Agave sp., Opuntia spp. and Hilaria muti­ United States of America generally nest in and, . ca as dominant species. open country with grasslands (Coulombe 1971, The study area Hes between 1,000 and 1,350 Martín 1973, Green and Anthony 1989). m elevation. Mean monthly temperature varíes Population declines have occurred in recent between a mínimum of 11°C in January­ decades through much of the owl's range (Zam February and a maximum of 28°C during sum­ 1974, Powers and McIntosh 1975, Collins mero Precipitation is highly variable, with an 1979) apparently due to habitat destruction or annual mean of about 230 mm (Barbault and modification and to the control of burrowing Halffter 1981). About 80% of the precipitation mammals (Best 1969, Butts 1973). falls during fue summer (June tú September). In Mexico, no information is available re­ Livestock grazing is the principal human acti­ garding the status, breeding ecology, nest-site vity in the area, but this activity apparently has characteristics or habitat use of this species. not yet strongly modified the habitat (Barral This study considers nest-site characteristics 1988). and reproductive success during two breeding Metbods: OwIs and their burrows were 10- seasons in the southern portion of the cated by intensively searching on an area of Chihuahuan desert, Mexico. 20,000 ha from March through July in 1985 144 REVISTADE BIOLOGIATROPICAL and 1986. Surveys were inüially random and RESULTS stand condition maps of the reserve were poste­ riorly used to garantee that all potential bree­ We found 29 nesting paírs in 1985 and 23 ding habitats had been searched. Every nes! bu­ pairs in 1986 in the Mapimí desert region. rrow occupied by a breeding pair was visited at Nesting densitieswere 0.15 pairs/km2 and 0.12 least once a week. For each nest we recorded pairs/km2, respectively (p > 0.05, x2 test). the kind of burrow used, the surrounding vege­ Although we probably did not find every nes­ tation type, soil texture, number of suitable per­ ting pair, we applied the sarne searching effort ches within 40 m of theburrow, distance to per­ in eachyear. manent water, distance ID the nearest occupied Nesting success was similar in both years, burrow and the number of young fledged. We 55% in 1985 and 65% in 1986 (x2 = 2.84; df= classified the surrounding vegetation types into 1; P > 0.05; Table 1). However, productivity 7 minor vegetal associations dominated by dif­ was slightly but not significantly higher in 1986 ferent species. Soils were classified according ID (2.19 young per successful nest, 1.52 per at­ texture. Burrow types were classified according tempt) compared to 1985 (1.63 young per suc­ to the animal species having constructed them. cessful nest, 0.90 young per attempt) (p > 0.05; We determined the number of young at each t-test). Nest faBure was due to abandonment nest-burrow during the post-fledgingperiodo (37.9% in 1985, 21.7% in 1986), predation by We tested the nature of spacing between coyotes (3.5% in 1985), predation by badgers nest-burrows using a "nearest-neighbor test" of (4.3% in 1986), and human interference (3.5% dispersion (Clark and Evans 1954). Mean dis­ in 1985, 8.7% in 1986). Of the burrows occupied tances from one active nest to the next nearest in 1985, 16 (55.2%) were agron occupied in were calculated and compared with those of a 1986. Individual owls were not banded, so we random distribution. do not know if pairs present at the sarne site both A Principal Component Analysis (PCA) years were the sarneindividuals. (Conner and Adkisson 1977, Morrison 1981) was performed in order ID identify the characte­ TABLE 1 ristics of the owl' nests most strongly correlated Summary of the burrowing owl produclivity with used nests. We have chosen this analysis in Ihe Mapimídes ert, México because PCA is a multivariate technique that elucidates underlying factors without any a 1985 1986 Bothyears priori assumptions. The variables chosen for No. nesting auempts 29 23 52 No. successful nests 16 16 32 further analyses were those which were most No. fledglings 26 35 61 readily interpretable in a biological sense.· A Fledglings/successful 1.63 2.19 1.91 correlation analysís was then performed betwe­ nest en nest site-characteristics, successful nests and Fledglings/attempt 0.9 ± 1.0 1.52 ± 1.3 1.17 ± 1.1 number of fledglings. A x2 test of association between the number of fledglings and the va­ Tables 2 and 3 show the main features of the riables most correlated was then applied. habitat surrounding the nests.

TABLE2

BUTroWingowl nest-site characteristics inthe MapimíBiosphere Reserve, Durango, México, 1985-1986. Themean and standardare deviatíon presenred for each variable

1985 1986 Bothyears (N=29) (N=23) (N=52)

Mean;!;SD Mean;tSD Mean;tSD Range

Distance to nearest adjacent nesta l.l±l.l 1.1 ±0.9 1.1 ± 1.0 0.03-4.1 Distance to wate� 4.1±3.2 3.4± 1.7 3.8± 2.6 0.05-12.5 Number of perches 11.8±5.1 11.8±4.9 11.8± 4.9 4-20 a Distances are expressed in km. RODRIGUEZ-ESTRELLA, & ORTEGA-RUBIO: Nest site suceess of owIs 145

TABLE3

ThItassocialion between some burrowing owl nest-site characteristics andprodu.ctillUy in thltMapimi Biosphltre Reserve

Number of nests Numberof fledglings N % N % Vegetation type Lorrea 3 8.3 2 3.3 F ouquieria-Larrea 5 13.9 9 14.7 Lorrea-Prosopis-Agave-Fouquieria 2 5.6 3 4.9 Prosopis-Larrea 7 19.4 12 19.7 Prosopis 6 16.7 3 4.9 Prosopis�HiJaria 12 33.3 30 49.2 Fouqueria-Prosopis-Larrea 2.8 2 3.3 Total 36 100.0 61 100.0

Soiltexture Oay 13 36.1 22 36.1 Oay-sand 18 50.0 33 54.1 Sand 5 1,3.9 6 9.8 Total 36 100.0 61 100.0

Burrowtype Badger 6 16.7 14 22.9 Fox 8 22.2 17 27.9 Kangaroo-rat 15 41.7 15 24.6 Coyote 1 2.8 O 0.0 Deserttortoise 6 16.7 15 24.6 ------Total 36 100.0 61 100.0

Wben we perfonned a PCA foe 1985 and 1986 TABLE4 habitat characteristics, the results for both years Results ofthlt Principal Component Analysis performed where similar, sowe applied a including all PCA on alll/ariableslor burrowing owl Mst-siles. habitat characteristics for both years 1985-1986. Factors with weights >± 0.5 are in bold Thefirst three factors explained a1168%of the toIal variance in both years. We chose the factorshaving Variable Factor high weightitlg values for burrows used as nests 2 3 (Thble 4).Thevegetation type, the burrow typeand the todistance water, which were highly correlated A Vegetation type 0.78 0.10 -0.06 in the first factor, were chosen foe further analysis B -0.05 because they were the most important biological Burrowtype 0.51 0.63 factors. In addition, a correJation analysis showed e Distanceto wa ter 0.78 -0.15 -0.06 that the vegetation type was the only one factor most correlatedwith nesting success (r = 0.5746). D Nearest adjaeent nest 0.17 -0.87 -0.11 Mosi burrow-nests were under grassland E Soil -0.02 0.11 ·0.85 Prosopis- Hilaría (33%) and Prosopis-Larre a texture vegetal associations (19%) (Table 3). Knowing F Number of perches -0.16 0.24 0.73 that the vegetation type seems 10 be the main factor associatedwith the reproductive success, Cumulative varianee 25.8 47.0 68.6 and taking into account that there are not signi­ expIained ficant differences between the data of 1985 and 1986, we combined the data (Table 3), and 146 REVISTA DE BIOLQGIA1ROPICAL

found that nests located at the Prosopis­ 100 tendence of owls to nest in "playas" with Hilarla grassland vegetation producedalmost the Prosopis-Hila.ria association seems to been­ 50% ofthe totalfledglings, which is highly sig­ hancedby the availabilityof burrows, soiltexture, nificant(x2 =7.62; df= 1; P < 0.01). numberof perches, low OO8tpredation and availa­ These owls used five kinds of burrows. bility oC preys. All these Cactors may act impro­ These apparently were constructed by kanga­ ving the reproductive success oC burrowing owls roo-rats (Dipodomys merriami. D. nelsoni) in theProsopis-Hilaria association. (41.7%), foxes (Urocyon cinereargenteus) These owls generally use abandoned bu­ (22.2%), badgers (Taxidea taxus) (16.7%), de­ rrows oC any marnmal for nesting (Coulombe sert-tortoises (Gopherus flavomarginatus) 1971, Rich 1984, Green and Anthony 1989). At (6.7%), and coyotes (Canis latrans) (2.8%). Mapimí,they nest in a widevariety oC marnmal Sorne of the kangaroo-rat burrows could also burrows, chiefly those of foxes and kangaroo­ have been constructed by ground squirrels rats (or squirrels). They also nest in desert tor­ (Spermophilus spilosoma). Kangaroo rat bu­ toise burrows. Kangaroo rat and tortoise bu­ rrows were most frequently used in both years rrows in Mapimí concentrate mainly on the ba­ (12 of 29 nests in 1985 and 8 oC 23 in 1986). A jadas and playas (Grenot and Serrano 1981, difCerencein thenumber of fledglingsproduced Morafka et al. 1981); badger and fox burrows in each kind of used burrows was found (x2 = distribute in playas and hilIs (J. Herrera pers. 9.22; df = 3; p < 0.05; Table 3), aU the comm.). Probably, owl nest distribution in burrow-nests having produceda similarpropor­ Mapimí is not related to the distribution oC tion of fledglings. mammal burrows, as found in other studies Differences between yearswere not signi­ (Green and Anthony 1989) as any kind of bu­ ficant for distance to water (t-test =0.9737; rrowis used. P > 0.05), and for the nearest adjacent nest In contrast with populations oC the United (t-test =0.0839; P > 0.05). Most nests were States (Thomsen 1971, Butts 1973, Martin over 3 km from water but the distance lo 1973, Oreen and Anthony 1989) the Mapimí permanent water ranged from 50 10 12 500 owls didnot oftennest close 10 otherowl nests. m (i =3806 ± 2625 m). The number of per­ We Observed many apparently suitable unused ches ranged from 4 to 20 (x = 11.8 ± 4.9) burrows between nest sites. This availability oC (Table 2). Nest-burrows were most fre­ nest-burrows could explain the wider nesting quently in clay and clay-sand soils (x2 = distribution and the regular spacing between 11.2; df =2; p < 0.01; comparing the three nestsin Mapimí. kinds of soil) (Table 3). Soil texture has a significant effect on the The mean distance between adjacent Qwl longevity oC a burrow (Morafka el al. 1981, nests were over 1 km, but ranged from 30 to 4 Oreen and Anthony 1989), and at Mapimí a 167 m (x 1287= ±98 m). The distribution of cláy-sand soil texture seems to be the principal active nests in both years indicate a tendency factor influencing burrow re-use. Oreen and toward regular spacing of breeding pairs Anthony (1989) present similarfindings related (c1985 = 3.04; c1986 = 3.29; P < 0.01; see to soiltexlure andburrow re-use. However, bu­ Clark and Evans 1954). rrowre-use in Mapimímay be also related 10 a good production oC fledglings in the previous year (or years) (14 oC 21 nest-burrows success.. fuI in 1985 were re-used in 1986), and to the DISCUSSION sma1l percentageof nest predation. Preys of this owl in Mapimí weremainly in­ As in other North American deserts sects and small marnmals (Rodríguez-Estrella, (: Coulombe 1971, Thomsen 1971; unpubl. data).These preyswere specially abun­ : Martín 1973; Oregon: Oreen and dant in the Prosopis-Hilaria association Anthony 1989), burrowing owls in the Mapimí (Rodríguez-Estrella, unpubl. data, Grenot and desert nest in open habitats, called "playas". Serrano 1981). They particularly nested where elevated per­ Nesting success in Mapimí (62%) was ches were availableánd their nests weré asso­ I ciated mainly with a mixtureof grassland vege­ slightly higher than the 54% in California I I tation and sparse trees. (Thomsen 1971) and the 53% in Oregon RODRIGUF.Z-ESTRELLA.ORI'BGA-RUBIO: Nest lite succes. eLowla 147

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