Chaco Owl (Strix chacoensis)

CHACO OWL (Strix chacoensis) Overview Chaco Owl is a medium-sized owl that is distributed through a 766,000 km2 area of southern South America, from the Chaco region of southern Bolivia (Santa Cruz) south western Paraguay and to Argentina, south to Córdoba and Buenos Aires provinces. This owl inhabits both dense and open forests and shrubs on either hilly, rolling and flat areas. Its diet include small mammals, birds, reptiles, insects, scorpions and centipedes. Chaco Owl is mainly nocturnal and crepuscular, and rests during the day in tree cavities, dense brush, isolated trees or even on the ground. This owl primarily is resident, but some individuals may move seasonally among different habitats. Although considered "scarce", it could be locally common. Although Chaco Owl is not considered to be threatened on the global level, local populations of this species may be vulnerable to habitat changes.

Identification Summary Wood owls (Strix) are medium sized to large owls with large, rounded heads without ear-tufts. Chaco Owl is a medium sized wood owl, with feathered tarsi, and toes feathered on basal half. The upperparts are gray brown, distinctly barred paler (whitish and buff), especially on the head and nape; the underparts are barred black and white. Chaco owl is very similar to Rufous-legged Owl (Strix rufipes); see Similar Species. Similar Species Chaco Owl is very similar to Rufous-legged Owl (Strix rufipes), but these two species are mostly allopatric. Apparently, both species do not differ markedly in body size, but Chaco Owl is much paler than Rufouslegged Owl, and buff tones the plumage are reduced or lacking; the facial disc is off white (not orange brown or ochre) with darker, more pronounced concentric lines (König and Weick 2008); and the two species differ vocally (Straneck and Vidoz 1995). Chaco and Rufous-legged owls also differ ecologically: Chaco Owl occurs in the dry forest Chaco of Bolivia, Paraguay and Argentina (Mazar-Barnett and Pearman 2001), while Rufous-legged Owl inhabits mainly the southern temperate rainforest region of Chile and Argentina and the sclerophyllous forest in Central Chile (Martínez and Jaksic 1996, Figueroa et al. 2013). Vocalizations The song of the male Chaco Owl is a croaking or deep grunting and is described as a "rather froglike crococro craorr-craorr craorr-craorr, with emphasis on the first craorr"(König and Weick 2008). The trisyllabic introductory note is very quiet. When the bird is excited, it may utter additional double notes in sequence after the introduction. The female has a very similar but slightly higher-pitched song; single "craorr" calls and a harsh, drawn-out shriek also may be given (König and Weick 2008). Additional audio recordings of vocalizations of Chaco Owl can be heard at xeno-canto. Nonvocal Sounds None reported.

Detailed Description (appearance) The following plumage description is based on Cherrie and Reichenberger (1921), Dabbene (1926), and König and Weick (2008): Adult: Sexes similar. Upperparts dusky brownish black, narrowly barred with white and yellowish buff; crown and nape rather finely barred light and dark with some pale orange buffish bars. Facial disc pale grayish white with narrow darker concentric lines, whitish eyebrows not very prominent; rim of disc inconspicuous. Underparts off white, barred dark brown. Wing coverts coarsely blotched with some pale orange buff on dark grayish brown. Flight feathers boldly barred dark grayish-brown and orange buff; rectrices dark grayish brown with a few narrow pale buffish bars. Tarsi and upper half of toes feathered very pale orange buff or cream colored with few darker bar or spots. Juvenile: Downy chick whitish. Mesoptile very fluffy, pale grayish brown, often with buffish suffusion; facial disc pale grayish. Bare Parts Iris: blackish brown Bill and cere: Yellowish Toes: grayish-brown/yellowish; claws reddish horn with darker tips Bare parts color data from König and Weick (2008) and personal observations in the field. Measurements Total length: 35-38 cm (Weick 2006, König and Weick 2008) Linear measurements Taken from Cherrie and Reichenberger 1921, Wheeler 1938, and Straneck and Vidoz (1995) Male (n = 3, including the type; Dabbene 1926; Wheeler 1938) Wing length: 251-265 mm Tail length: 146-150 mm Tarsus length: 50-51 mm Culmen from base: 32-33 mm Culmen from cere: 20 mm Female (n = 2; Wheeler 1938) Wing length: 281-291 mm Tail length: 162-182 mm Tarsus length: 50-53 mm Culmen from base: 32-33 mm Culmen from cere: 20-22 mm In addition, Straneck y Vidoz (1995) provide the following mean mearsurements: 2 females, wing 286 mm, tail 162.5 mm, tarsus 51.5 mm, culmen from cere 21.0 mm; 1 male + 2 unsexed birds, wing 290 mm, tail 152 mm, tarsus 54 mm, culmen from cere 22.3 mm; and 2 unsexed birds, wing 252.5 mm, tail 142.5 mm; tarsus 45.5 mm, culmen from cere 20 mm. Mass: One male and one female captured by Kratter et al. (1993) weighted 350 g and 375 g, respectively. Weick 2006 and König and Weick (2008), apparently based on Steinberg (1999), give the following weights: male: 360-435 g, female 420-500 g. Measurements of wing length, tail length, and mass suggest strong reverse sexual dimorphism in body size. Molts Not described.

Geographic Variation Monotypic. However, specimens from southern localities (La Pampa) tend to be darker in plumage (Dabbene 1926). Systematics The species was described as Strix chacoensis by Cherrie and Reichenberger (1921) based on an individual captured at Fort Wheeler, Chaco Boreal, Paraguay. Dabbene (1926), however, suggested, based on plumage coloration patterns, that chacoensis was only a geographic variety of Strix rufipes (Rufous-legged Owl). After Dabbene (1926),chacoensis was classified by most authors as a subspecies of rufipes (e.g., Meyer de Schauensee 1970, Short 1975, Clark et al. 1978, Fjeldsa and Krabbe 1990). Later, Straneck and Vidoz (1995) based on differences in body size and vocalizations again confirmed to distinctiveness of chacoensis, and recognized it as a separate species. Chaco Owl appears to be more closely related to Strix hylophila (Rusty-barred Owl) than toStrix rufipes (König and Weick 2008, Remsem et al. 2013).

Distribution The Americas Chaco Owl is endemic to southern South America. Its distributional range comprise an area of 766,000 km2 including the Chaco region of southern Bolivia (Santa Cruz) south to western Paraguay and to Argentina, south to Córdoba and Buenos Aires provinces (Ochoa de MásRamón 1983, König and Weick 2008, BirdLife International 2008). Its elevational range is poorly known. In Argentina it ranges from 500 m in Mendoza to 1300 m in Salta (König and Weick 2008, Santander et al. 2012). Outside the Americas Endemic to southern South America. Habitat Chaco Owl inhabits both dense and semi-open vegetation on either hilly, rolling and flat areas (Cracraft 1985). Known habitats for this species include woodland with a dominance of lower (1-2 m tall) and open shrubs interspersed with small (2-7 m) trees (paloblancoAspidosperma quebracho, Chorisia sp., Prosopis sp.) and columnar cacti (Trichocereussp.) 4-8 m tall in the Bolivian Chaco (Kratter et al. 1993), tall dry forest (10-15 m tall), scrubby woodland, and uniform, xerophytic, spiny forest in the Paraguayan Chaco (Zyskowski et al. 2003, Smith et al. 2006, Smith and Clay 2008), homogeneous red quebracho (Schinopsis balansae) forest in the Argentine Chaco (Bodrati 2005), and shrubs and open forest of algarrobo (Prosopis flexuosa) and chañar (Geoffroea decorticans) in the Monte ecoregion of Mendoza (Santander et al. 2012). In Telteca, Chaco Owl have been observed in mature forests with trees >100 years old (F. Martínez, unpublished data). Historical changes None reported. Fossil history None reported.

Life History Food The diet of Chaco Owl includes small mammals, birds, reptiles, and diverse invertebrates. Stomachs of two individuals in the Bolivian Chaco contained scorpions, beetles, large spiders, centipedes, and a blind

snake (Leptotyphlops sp.; Kratter et al. 1993). Santander et al. (2012), based on an examination of 121 pellets, reported the following prey items: small mammals (39.1% of all individual prey), scorpions (28.9%), insects (27.3), birds (2,5%) and reptiles (2.2%). Small mammal prey included both rodents (e.g., Eligmodontiasp.) and marsupials (e.g. Thylamys pallidior). Behavior Little known. Apparently is an owl mainly nocturnal, with crepuscular vocal activity (Kratter et al. 1993). During daytime it rest in tree cavities (Bodrati 2005), dense brushes, isolated trees, and sometimes on the ground (Santander, unpublished data). It wait for prey from a perch (Kratter et al. 1993). Based on identified prey, Santander et al. (2012) suggested that Chaco Owls move throughout all locally available habitats searching for prey. This species is considered a resident (Lopez-Lanúz et al. 2008), but some individuals may move seasonally among different habitats (Bodrati 2005). Territoriality Apparently Chaco Owl is strongly territorial; it responds rapidly to playback (Bodrati 2005). Estimates based on habitat availability suggest that owl pairs in the Monte region could maintain a territory of 89.6179 ha (n = 3; F. Martínez, unpublished data). Sexual Behavior Presumably Chaco Owl is monogamous; often a male and a female are observed together (Dabbene 1926, Kratter et al. 1993, Straneck and Vidoz 1995, Bodrati 2005). Social and interspecific behavior Chaco Owl is solitary or in pairs (e.g., Kratter et al. 1993, Straneck and Vidoz 1995, Bodrati 2005). Predation There are no reports of predation on Chaco Owl. Reproduction The breeding biology of Chaco Owl is little known; probably is similar to that of other species of Strix (König and Weick 2008). This species probably nests in tree cavities or perhaps in holes on the ground. Females in captivity lay 2-3 white eggs with incubation lasting 35 days (Pitterman 2005); egg hatching is asynchronous and young leave the nest at 5 weeks (Pittermann 2005). Populations and Demography The global population size of Chaco Owl has not been estimated (BirdLife International 2013). In Argentine it is considered a "scarce" species (Giacomelli 1923, López-Lanúz et al. 2008), but it can be locally common (Bodrati 2005, Smith et al. 2006). There is no information related to topics such as age at first breeding, life span and survivorship, dispersal, or population regulation for Chaco Owl.

Conservation Conservation status Population trend appears to be stable. The IUCN Red List conservation status of Chaco Owl is listed as Least Concern (BirdLife International 2013). At a local scale, however, it is listed as "vulnerable" (e.g., López-Lanúz et al. 2008, Trejo et al. 2012). In Argentina, its distribution includes at least three protected areas larger than 3000 ha (López-Lanúz et al. 2008). The Convention of International Trade in Endangered Species (CITES) includes all Neotropical species of owl, including Chaco Owl, on Appendix II (i.e., international commercial trade in this is controlled in order to avoid harvesting incompatible with the survival of the species).

Effects of human activity on populations Lopez-Lanúz et al. (2008) consider Chaco Owl to be moderately sensitive to anthropogenic habitat change. Habitat conversion and fragmentation probably is the main threat to this species (Marks et al. 1999). Mortality caused by collision with vehicles could be an additional threat (F. Martinez, unpublished data).

Priorities for Future Research Urgent research must be addressed on population size, habitat use, breeding biology, food habits and threats. Bioindicator power and functional role in ecosystems also must be evaluated in the medium and long-term.

References BirdLife International. 2013. Species factsheet: Chaco Owl Strix chacoensis. Downloaded from BirdLife International on 04 August 2013. Bodrati, A. 2005. Notas sobre la avifauna del Parque Nacional Chaco, el Parque Provincial Pampa del Indio y otros sectores de la provincia de Chaco, Argentina. Nuestras Aves 49:15–23. Cherrie, G.K., and E.M.B. Reichenberger. 1921. Descriptions of proposed new birds from Brazil, Paraguay and Argentina. American Museum Novitates number 27. Clark, R. J., D. G. Smith and L. H. Kelso. 1978. Working bibliography of owls of the World. National Wildlife Federation Scientific and Technical Series 1:1-319. Cracraft, J. 1985. Historical biogeography and patterns of differentiation within the South American avifauna: areas of endemism. Ornithological Monographs 36:49–84. Dabbene, R. 1926. Sobre la distribución de la lechuza Strix rufipes. Hornero 3:405-407. Figueroa, R. A., S. Alvarado, E. S. Corales, D. González-Acuña, R. Schlatter and D.R. Martínez. 2013. Buhos de Chile. Pp. 163-267 en Búhos Neotropicales: Diversidad y Conservación (Enriquez P, Ed.). Ecosur, México. In press. Fjeldsa, J. and N. Krabbe. 1990. Birds of the High Andes. Apollo Books, Svendborg. Giacomelli, E. 1923. Catálogo sistemático de las aves útiles y nocivas de la provincia de la La Rioja. Hornero 3: 66-84. Hardy J. W., B. B. Coffey Jr and G. B. Reynard. 1989. Voices of the New World owls. ARA Records, Gainesville, Florida. IUCN. 2012. IUCN Red List of Threatened Species (ver. 2012.1). Available at:http://www.iucnredlist.org. (Accessed: 19 June 2012). König, C. and Weick, F. 2008. Owls of the World. Second Edition. Christopher Helm, London. Kratter, A.W., T.S. Sillet, R.T. Chesser, J.P. O'Neill, T.A. Parker III, and A. Castillo. 1993.Avifauna of a Chaco locality in Bolivia. Wilson Bulletin 105: 114–141. López-Lanús, B., P. Grilli, E. Coconier, A. Di Giacomo and R. Banchs. 2008. Categorización de las aves de la Argentina según su estado de conservación. Aves Argentinas/ AOP y Secretaría de Ambiente y Desarrollo Sustentable. Buenos Aires, Argentina.

Marks, J. S., R. J. Cannings and H. Mikkola. 1999. Family Strigidae (typical owls). Pp. 76-242 In J del Hoyo, AD Elliott & I Sargatal (Eds.), Handbook of birds of the World. Vol. 5. Lynx Edicions, Barcelona. Martínez, D. R. and F. M. Jaksic. 1996. Habitat, abundance and diet of Rufous-legged Owls (Strix rufipes) in temperate forest of southern Chile. Ecoscience 3:259-263. Mazar Barnett, J. and M. Pearman. 2001. Lista comentada de las aves argentinas. Lynx Edicions, Barcelona, Spain. Meyer de Schauensee, R. 1970. A guide to the birds of South America. Livingston Publishing Co., Wynnewood, Pennsylvania. Ochoa de MásRamón, D. 1983. Lista de aves del Nordeste de San Luis. Hornero (número extraordinario):77-87. Pittermann, W. 2005. Der Chacokauz. Gefiederte Welt 6: 170–173 Remsen, J. V. Jr, C. D. Cadena, A. Jaramillo, M. Nores, J. F. Pacheco, J. Pérez-Emán, M. B. Robbins, F. G. Stiles, D. F. Stotz and K. J. Zimmer. 2013. A classification of the bird species of South America. American Ornithological Union, U.S.A. www.museum.lsu.edu/~Remsen/SACCBaseline.html. Santander, F., S. Alvarado, P. Ramírez, R. A. Figueroa and L. Muñoz. 2012. First report on the diet of the Chaco Owl (Strix chacoensis) in the Monte Desert, Argentina. Journal of Raptor Research 46:409–411. Short, L. L. 1975. A zoogeographic analysis of the South American Chaco avifauna. Bulletin of the American Museum of Natural History 154:163-352 Smith, P. and R. Clay. 2008 Birding Paraguay, the ‘forgotten heart’ of South America. Neotropical Birding 3: 53-58 Smith, P., A. Betuel, H. Batjes, S. Centrón and H. del Castillo. 2006. An avifaunal inventory of Parque Nacional Teniente Agripino Enciso, Departamento Boquerón, northern Paraguay. Fauna Paraguay Technical Publication. Steinberg, R. 1999. Der Gran Chaco Rotfußkauz. Society for the Conservation and Research of Owls Magazine 2:45–52 Straneck, R. J. and F. Vidoz. 1995. Sobre el estado taxonómico de Strix rufipes (King) y deStrix chacoensis (Cherrie y Reichenberger) (Aves: Strigidae). Notulas Faunísticas 74:1-5. Trejo, A., M. S. Bó and L. Biondi. 2012. Búhos de Argentina: estado de conservación y prioridades de investigación. Ornitologia Neotropical 23: 221–228. Weick, F. 2006. Owls (Strigiformes) - Annotated and Illustrated Checklist. Springer, Berlin Heidelberg, New York. Wheeler, L. 1938. A new wood owl from Chile. Field Museum of Natural History Zoological Series 20:479483. Zyskowski, K., M. B. Robbins, A. T. Peterson, K. S. Bostwick, R. P. Clay and L. A. Amarilla. 2003. Avifauna of the northern Paraguayan Chaco. Ornitologia Neotropical 14: 247–262. Recommended Citation SANTANDER, F., R.A. FIGUEROA, F. MARTÍNEZ AND S. ALVARADO. 2013. Chaco Owl (Strix chacoensis), Neotropical Birds Online (T. S. Schulenberg, Editor). Ithaca: Cornell Lab of Ornithology; retrieved from Neotropical Birds Online: http://neotropical.birds.cornell.edu/portal/species/overview?p_p_spp=214616