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There is very limited, and frequently outdated, information on the status, natural history, and distri- bution of Neotropical owls (Marks et al. 1999, Trejo et al. 2012, Rangel-Salazar and Enríquez 2015), even though this information is essential for evaluating population fluctuations, response to environmental changes, and threats (Rangel-Salazar and Enríquez 2015, Trejo and Bó 2015). Relatively better known are Neotropical owls inhabiting open and semiopen lands, such a the Burrowing Owl (Athene cunicularia) (Trejo et al. 2012, Trejo and Bó 2015, Rangel-Salazar and Enríquez 2015), a widely distributed species from southern Canada and the United States of America to southern Argentina and Chile (Marks et al. 1999, Poulin et al. 2020). In Argentina, the Burrowing Owl is found in the southern, central, and northern prov- inces, and it is considered a species of "least concern” (Trejo and Bó 2015, MAyDS and AA 2017). Currently, detailed information about its regional status is lim- ited. In those areas where it has been recently stud- ied, such as agroecosystems in the Pampas of central Argentina, the Burrowing Owl seems to be thriving (Sánchez et al. 2008, Pairo et al. 2017, Rebolo-Ifrán et al. 2017). The species has also successfully colo- nized several urban areas, benefitting from artificial night lighting, habituation to human’s presence, in- creased foraging efficiency, and availability of nesting substrates and prey (Sánchez et al. 2008, Cavalli et al. 2014, 2018, Rebolo-Ifrán et al. 2017; Baladrón et al. 2020, Rodríguez et al. 2021). In contrast, some pop- ulations in Canada and in the United States are de- clining and considered threatened due to habitat loss and fragmentation, and lack of burrows (Holroyd et al. 2001, Conway 2018, Poulin et al. 2020). A matter of concern is the fact that, by the 1920s, the Burrowing Owl was extirpated from Tierra del Fuego island in the southernmost province of Argentina, possibly due to sheep ranching practices; this population never re- covered, and it is now considered extinct (Reynolds 1935, Iriarte et al. 2019). Thus, collecting baseline information about the presence of this species in its southernmost continental range of distribution may contribute to better understanding regional varia- tions in its ecology and conservation status (Sexton et al. 2009). This is particularly important in Santa Cruz province, southern continental Patagonia, where anthropogenic activities such as the use of baits with strychnine and carbofuran for predator control, di- rect persecution, hold-leg traps, mining, and oil extraction are known to negatively affect several species of raptors (Olrog 1979a, De Lucca and Saggese 1989, Travaini et al. 2000, García Brea et al. 2010).
Historically, the Burrowing Owl was documented in Santa Cruz province by Scott and Sharpe (1915), based on observations made in the Río Chico Depart- ment during the late nineteenth century. Almost a hundred years after these initial observations, and without additional published records for this prov ince, Navas and Bó (1997) reported that the Bur- rowing Owl was present in northeastern Santa Cruz, based on eleven skulls recovered from open petrole- um pits, where these and many other birds died (Tu- baro et al. 2000). Subsequently, Darrieu et al. (2008) included the species in their list of Santa Cruz prov- ince avifauna; however, other ornithological surveys of Santa Cruz province did not include the Burrowing Owl (Zapata 1967, Olrog 1979a, Chébez et al. 1988, De Lucca and Saggese 1992, Travaini et al. 2004, Im- berti 2005, Roesler et al. 2014). Even in the absence of detailed records, and possibly due to these few ex- isting early reports, several ornithological reference books have included most or the entire Santa Cruz province within the distribution range of this species (e.g., Olrog 1979b, 1984, Narosky and Yzurieta 1987, Fjeldsá and Krabbe 1990, Canevari et al. 1991, Clark 1997, Mazar-Barnet and Pearman 2001, De la Peña 2019). Despite all listed above, the current distribu- tion range, breeding areas and overall status of the Burrowing Owl in this province is unknown. There- fore, our goal was to investigate and determine the past and present status and distribution range of this owl in Santa Cruz province in southern continental Patagonia, Argentina. We also discuss potential fac- tors that might influence its demography there. To the best of the author’s knowledge, this is the first analysis of Burrowing Owl’s population at the southern lim- it of its Pan-American distribution.
study area and methods
Study area
Santa Cruz province is in the southernmost con tinental end of Argentina, separated from Tierra del Fuego Island, insular Patagonia (Argentina and Chile) by the Strait of Magallanes. After Buenos Aires province, it is the second largest political geographic unit of this country, and the province with the lowest number of inhabitants/km2. Several major ecore- gions and their respective ecotones can be found in Santa Cruz (Burkart et al 1999, Coronato et al. 2008). In the West, the orography is dominated by the An des mountains, home of the Magallanes and Valdivian forest ecoregions. East of the Andes, mainland San ta Cruz is characterized by a succession of plateaus and lowlands where other major ecoregions are the Patagonian Steppe (central, east, and north) and the Patagonian grasslands (south). In the East, along the Atlantic coast, the Mar Argentino ecoregion covers its entire extent; cliffs of variable height and sand/ pebbled beaches can be found there. For a detailed physical depiction of Santa Cruz phytogeography, hy- drography, orography, and climate see Paruelo et al. (1998) and Coronato et al. (2008).
Human population in Santa Cruz province is scarce, mostly concentrated in a few cities and towns, with an average population density of 1.1 inhabitants/ km2 (INDEC 2012). Approximately 50% of Santa Cruz human population is found in a few coastal cities such as Río Gallegos and Caleta Olivia, with more than 50 000 inhabitants each. Most towns have less than 10 000-20 000 inhabitants. Private ranches dominate the area, and the landscape has been strongly modi- fied by a hundred-year long tradition of sheep ranch- ing, which was considered as one of the most impor- tant economic activities in the province. In addition, mining, gas, and petroleum extractions are other im- portant components of Santa Cruz economy. Fishing, timber industry, and tourism are additional econom- ic activities, although more seasonal in character. Only 8% of Santa Cruz province is legally protected as national and provincial parks, mainly localized in the west and along the Atlantic coast; the remainder of the land is privately owned (Di Giacomo 2007).
Burrowing Owl records and analysis
To understand the distribution and the status of Burrowing Owl in Santa Cruz we combined a series of approaches to collect data on the species. First, we conducted a detailed literature review to identify pre- viously published and historical records of the species in Santa Cruz province. When possible, we contacted the authors to ask them for additional information and to corroborate some of the information there- by indicated. We also searched for other Burrowing Owl records in online databases (EcoRegistros and eBird). We also searched for photos and comments about this species in Argentinian Facebook pages and groups where raptors, including Burrowing Owl, were commonly featured. The authors of these records, photos and posts were also contacted, when possible, to obtain additional information (see below). We also contacted wildlife photographers, park rangers, orni- thologists and birdwatchers who have visited Santa Cruz in the last two decades or are residents there. We concluded our search for records on this species on July 31, 2020. Additionally, we included data gath- ered during a recent ornithological trip in Santa Cruz (January 2020, R. Montero).
Overall, for each record we aimed to obtain infor- mation on date, location (locality and department), georeferences, number and age (adults or juveniles), presence of burrows, and evidence of reproduction (observation of eggs, nestlings and/or fledglings/ju- veniles) when available. For all collected records we identified the main ecoregion of Santa Cruz province where observations were made following Burkart et al. (1999) and Coronato et al. (2008). Reports of Burrowing Owls observed in areas inside or in the imme- diate periphery of houses and other buildings within city limits were classified as urban. We considered the southern seasons as follows: summer (21 Decem- ber-March 20); fall (21 March-20 June); winter (21 June-20 September); and spring (21 September-20 December). To avoid duplicating and overestimating the number of records, Burrowing Owls observed in several successive days (< 3 weeks) and in the same location ca. 500 m radius from its first observation and in the same year were considered as a single report. Burrowing Owls were considered resident when seen repeatedly during the year and in different seasons. Local citizens and other contributors were kindly asked to contribute with photographs of the species, when available, as indisputable evidence of their observations.
results
The analyses of previously published records of Burrowing Owls allowed us to identify all historical reports on the species and to discover and to correct erroneous reports for Santa Cruz province (Table 1).
Table 1: Published records of Burrowing Owl in Santa Cruz. Those that were validated, accepted or confirmed are indicated with an asterisk (*)
We obtained a total of 32 new, unpublished re cords of Burrowing Owls in Santa Cruz province, 18 of them were supported by photographic evidence in the period Sep 2012-Jul 2020 (Table 2). We did not identify through our search any other unpublished observations on the species before this period. Subsequent follow up and communication with contributors al- lowed us to obtain additional information for 19 re cords, including those supported by evidence (Table 2). These records of Burrowing Owls were collected in the province of Santa Cruz from a thorough assess- ment of website databases: 11 on Facebook, four on EcoRegistros and 17 on eBird.
Table 2: Records of Burrowing Owl in Santa Cruz province, 2012-2020. Georeferences included only when available. Those with photographic evidence available are indicated with an asterisk (*)
Burrowing Owls were observed in urban and non-urban open land areas in or nearby Caleta Olivia city, respectively (Table 2, Figs. 1a and 1b), Patagoni- an Steppe ecoregion. Burrowing Owls were observed perching on or standing nearby houses, sides of ur- ban lagoons, wire fence posts, yards, piles of earth, and roadsides and highways. Burrowing Owls were also observed along the Atlantic coast, Mar Argentino ecoregion, in non-populated areas north of the Caleta Olivia city, such as in the areas called La Lobería and Playa Acina (Table 2, Fig. 1c). Burrowing Owls were observed in seven opportunities on the coastal sand dunes and on the beach alongside the strips of land (approximately 100 m wide) between the National Highway 3 (RN3) and the sea (Table 2).
Figure 1: Photographic evidence of Burrowing Owl in Santa Cruz province, southern continental Patagonia, Argentina. a) Burrowing Owls within an urban setting of Caleta Olivia city, May 30 2020, photo by S. Herrera; b) Juveniles at their burrow entrance, Caleta Olivia city, Nov 28 2018, photo by M. Barrientos Vidal c) Perching on a sand dune by the Atlantic coast shore, Playa Acina, 23 Jan 2020, photo by R. Montero d) Only record of the species for southwestern Santa Cruz, Río Bote, 11 Oct 2017, photo by R. Taggiasco.
All adults, except when noted
Only 2 records do not belong to northeastern Santa Cruz province, in or nearby areas of Caleta Ol ivia city, Deseado Department. One of these records comes from Río Bote, near the city of Calafate, Lago Argentino Department, southwestern Santa Cruz. One Burrowing Owl was observed there and photo- graphed in October 2017 (Table 2, Fig. 1d). The other record would be the southernmost of the species in Santa Cruz and consists of one individual observed in Río Gallegos city, December 14, 2019 (Table 2).
Burrowing Owls were recorded yearlong and dur- ing all the months; the highest number of records (for all years combined) was in spring (n = 14), followed by summer (n = 7), fall (n = 6), and winter (n = 5). Re cords almost doubled in numbers during the period of spring-summer when compared with the number of records during the period fall-winter. Moreover, citizen scientists, birdwatchers, and ornithologists that we consulted consider the species to be a year- long resident (Table 2).
Most observations reported Burrowing Owls as being seen alone or in pairs, less commonly they were observed in small groups of 3-4 or more indi- viduals (Table 2), mainly adults. Fledglings and juve niles were observed in the months of November (Fig. 1b), March, April, May, and July at different locations. Most reports also indicated the presence of nearby active burrows, in some cases used for many years (M. Barrientos Vidal, pers. comm.) (Fig. 1a).
discussion
Although the Burrowing Owl was included in the Santa Cruz province by many previously published ornithological reference books and field guides (see introduction for a selected list), we established that most historical reports for this province were inexact or erroneous, except for a few (Table 1). The alleged presence of this species in the Parque Nacional Bosques Petrificados de Jaramillo (APN 2018; see Table 1) needs confirmation, as it has not been reported there by several ornithological surveys (Chébez et al. 1988, De Lucca and Saggese 1992, Imberti pers. comm., Saggese et al. unpublished data).
Validated records during the period 2012-2020 (Table 2) are the only ones that support the current inclusion of Burrowing Owls in Santa Cruz province, but only for a few locations. We recommend caution when considering the number of records presented here; they should not be taken as absolute indicators of the numbers/abundance of Burrowing Owls pres- ent in this province. Instead, they provide repeated evidence of their presence and their resident status for these years in or nearby the city of Caleta Olivia (Deseado Department, northeastern Santa Cruz). Some records, although separated by several weeks, at the same location (ca. Caleta Olivia’s cemetery and Barrio Costa del Sol, and Cañadón Seco) probably rep- resent the same individuals or populations residing there (Table 2). Nevertheless, these records indicate that the species seems to be thriving in areas within or around the city of Caleta Olivia.
The only previously published and validated ref- erence for areas outside northeastern Santa Cruz was the observation made over a hundred years ago by Scott and Sharpe (1915) of two birds (collected) in Rio Chico Department (Table 1). Whether the ob- servations made at the settlement named Río Bote, near Calafate city (southwestern Santa Cruz) in 2017, and in Río Gallegos city, southeastern Santa Cruz in 2019 represent unique findings or whether they indi- cate additional population nucleus developing there needs to be further determined. Furthermore, raptor surveys (although not specific for Burrowing Owls) in several areas of this province between 2008 and 2011 (Saggese M, Ellis D, Quaglia AI, and Nelson RW, unpublished data, see Saggese et al. 2020 for routes and transects covered) and more than 10 years of bird watching in southern Santa Cruz (Amorós C and Amorós M, pers. comm.) did not register this species.
The reasons why the Burrowing Owl appears to have such a limited distribution in Santa Cruz prov- ince are not known. Food and nesting sites and nest substrates are important limiting factors for raptors worldwide (Newton 1979). The Patagonian Steppe ecoregion of Santa Cruz is considered as of low wild- life diversity when compared with the Monte ecore- gion of central and northern Patagonia (Cueto et al. 2008, Pardiñas et al. 2011, Lessa et al. 2012, Bre- itman et al. 2014, Roesler et al. 2014, Minoli et al. 2015), where Burrowing Owls are more commonly found. Therefore, it is possible that low prey richness and availability and/or its abundance may be limiting the geographical spread of this species.
Burrows are also a critical limiting resource for this species as nesting sites; some Burrowing Owl populations rely on burrows dug by other animals while other populations are known to dig their own (Conway 2018). In central Argentina, Burrowing Owls dig their own burrows, but they can also use those made by mammals and reptiles, and even use human- made structures (Hudson 1920, Bellocq 1987, 1993, Machicote et al. 2004, Borsellino 2017, Baladrón et al. 2020). Whether in southern continental Patagonia the species do the same or they depend on burrows dug by local wildlife is unknown. The diversity of digging mammals is much lower in the Patagonian Steppe than in the Monte or Pampas ecoregions, and soil type can also influence Burrowing Owls’ home-range selection (Stevensen et al. 2011). Oil digging machin- ery and bulldozers loosens densely compacted soil in some areas around Caleta Olivia which could be facilitating Burrowing Owl’s use of these areas (L. M. Sosa, pers. comm.).
Predators may also be limiting the presence of Burrowing Owls in Santa Cruz. Low richness and abundance of predators in urban areas may explain changes in the habitat selection pattern of Burrowing Owls toward them (Rebolo-Ifrán et al. 2017). The fact that most of our records come from the city of Caleta Olivia and its surrounding areas suggests that preda- tion could limit their expansion in Santa Cruz. Sever- al species of diurnal and nocturnal raptors prey upon Burrowing Owl, sometimes even decimating local populations (Poulin et al. 2020). Recent observations of a Harris Hawk (Parabuteo unicinctus), a recently ar- rived raptor in Cañadón Seco, preying upon Burrowing Owls (M. Avalos, pers. comm.), suggests that predators could have a limiting role in the establishment of early nucleus populations.
The extinction of this species in Tierra del Fue go is allegedly caused by sheep ranching practices (Reynolds 1935, Iriarte et al. 2019). In other areas of Argentina, Burrowing Owls seem to coexist with agricultural practices, including cattle ranching (Di Giacomo 2005, de Tommaso et al. 2009, Saggese MD pers. obs). However, the real impact of current sheep ranching practices on this species in southern Pata- gonia is unknown.
Some Burrowing Owl’s populations can exploit human-made habitats, sometimes with a prefer- ence for urban habitat over natural grasslands and low-intensity agropastoral lands (Conway et al. 2006, Baladrón et al. 2016, Borsellino 2017, Martínez et al. 2017, Rebolo-Ifrán et al. 2017). Burrowing Owls ap- pear to be using both urban and natural habitats in Caleta Olivia and its surroundings. This city, near the Atlantic coast, is one of the largest and most rapid- ly growing cities in southern continental Patagonia, with an estimated population of 52 612 habitants by 2010 (INDEC 2012). Burrowing Owls can use urban- ized landscapes along the Atlantic coast in central Argentina (Baladrón et al. 2016, Martínez et al. 2017), without being adversely impacted when nesting in ar- eas with moderate levels of urban development (Bal adrón et al. 2020). While this association with urban habitats can result in positive outcomes for raptors, urban areas can also potentially act as an ecologi- cal trap (Isaac et al. 2014, Franco and Margal-Júnior 2018, Mannan and Steidl 2018).
Climate may be another limiting factor for this species at these southern latitudes. Winters may be too cold to support a population in central and south- ern Santa Cruz province. However, the species used to be present in Tierra del Fuego, at much southern latitudes (Reynolds 1935, Iriarte et al. 2019), suggest- ing that low temperatures were not restrictive for the species. At least, it seems possible that climate may determine winter presence in Caleta Olivia and sur- rounding areas, as the number of records appear to decrease in winter (Table 1). This can either be re- sult of a decrease in the number of observers and time dedicated to birdwatching due to harsh weath- er conditions or be a consequence of Burrowing Owl population dispersal and/or migration to more favorable latitudes (Holroyd et al. 2010, Poulin et al. 2020). However, studies conducted in the Pampean ecoregion of Argentina, at milder latitudes, indicate that Burrowing Owls usually disperse only on short distances and breeding pairs show nest site fidelity along the years (Luna et al. 2019, 2020).
The presence of juveniles in the months of No- vember, March, April, May, and July, and reports of pairs breeding locally confirms northeastern Santa Cruz as the southernmost latitude where this spe- cies breeds along its Pan-American distribution. The presence of juveniles in spring, summer and winter may indicate the presence of family units due to de- layed dispersal of some individuals, as reported in the Pampean ecoregion for both urban and rural popula tions (Luna et al. 2021). This can indicate either a very prolonged breeding season or that double brooding may occur in this area, as reported elsewhere (Gervais and Rosenberg 1999, Borsellino 2017, Poulin et al. 2020).
Current threats to the species in Santa Cruz are unknown. In the past, open petroleum pits were a serious threat to many Patagonian birds (Tubaro et al. 2000), including Burrowing Owls (Navas and Bó 1997). Collision with vehicles, predation by feral dogs, use of anticoagulant rodenticides, land disturbance, habitat loss and targeted persecution due to super- stition are common threats to this species along its extensive range of distribution (Trejo and Bó 2015, Cavalli et al. 2016, Franco & Marpal-Junior 2018, Pau- lin et al. 2020).
The collaboration between ornithologists, wildlife photographers, and citizen scientists combined with information gathered from social media platforms has provided an important benefit to our understand- ing of the natural history, ecology, and conservation of avian species, including Burrowing Owls and other raptors (Cavalli et al. 2014, Saggese et al. 2014). As a result of this study, we have expanded the current known range of the species at the southernmost limit of its distribution, confirmed its breeding and year- long resident status, and the colonization of an urban area in the Patagonian Steppe and coastal sand dunes in the Mar Argentino ecoregion.
Results of this study do not mean that we know in detail the overall distribution of Burrowing Owls in Santa Cruz, a province that covers a surface of 4459 km2 and has one of the lowest human population sizes, mostly restricted to a few cities and towns. We can assume that most sightings of Burrowing Owls reported here belong to areas of higher human devel- opment, such as some cities and surrounding areas, whereas vast areas of the province may be neglected. Citizen science is a useful tool in ornithological stud- ies but it also offers some potential challenges, such as spatial bias and variation in effort (Johnston et al. 2021). This can be solved by future geographically extensive and focused Burrowing Owl surveys that follow specific methodologies recommended for the study of this species (Conway and Johnson 2003).
The southernmost known population of Burrow- ing Owl in the world provides a unique opportunity to monitor the progress of its nucleus, not only in the city of Caleta Olivia but also in the rest of Santa Cruz province. Furthermore, this population could repre- sent a suitable model to understand several aspects of their ecology at the southern limit of its distribu- tion including: (a) How breeding phenology and ecol ogy compares to northern populations? (b) What are the advantages and disadvantages of Burrowing Owl’s association with the city of Caleta Olivia? (c) How the factors that contribute to Burrowing Owl’s urban col- onization in other ecoregions support their presence in this city and in the Patagonian Steppe? (d) What is the pattern of juvenile dispersal? (e) Are Burrowing Owls migratory at these latitudes? (f) What factors are limiting its distribution in Santa Cruz, and (g) Are there potential threats for the successful stability of the species in this province? Further research in the following years will help us to answer these and other questions regarding the presence of Burrowing Owl in Santa Cruz province.
acknowledgements
We are grateful to all the citizen scientists, bird watchers and ornithologists (included in the tables) whose observations have contributed to expand our knowledge on the Burrowing Owls inhabiting Santa Cruz. Special thanks to E. Tiberi for his invaluable help with the identification of mismatched and new records and for his assistance with gathering data. M. Barrientos Vidal, P. Águila, M. Avalos, R. Taggias- co, and S. Herrera supplied us with photographs of this species to illustrate this article. We are grateful to F. Gorleri from Aves Argentinas for his assistance and help to access records from eBird. S. Imberti, M. Amorós, R. Montoya Assan, L. Martin, A. Di Giaco- mo, L.M. Sosa, N. Olejnik, P. Águila, J.V. Llanos and M. Ávalos helped us to clarify the information avail- able for some records. We also thank the Secretaría de Fauna Silvestre-Consejo Agrario of Santa Cruz province (disposition DFS-CAP NQ20/09) and the Administración de Parques Nacionales (C. Chehebar, E. Ramilo and H. Pastore) for allowing us to study rap- tors under their jurisdiction. S. Henricksen and D. Griffon (Western University of Health Sciences), J.M. Blanco (Eagle Conservation Alliance), W. Voelker (Co- manche-Sia Nation), L. Joyner (Lafeber Company), the Field Museum of Natural History, I. Tizard (the Schubot Exotic Bird Center at Texas A&M University), and coinvestigators D. Ellis, R.W. Nelson, J. Morrison, I. Caballero, C. Ellis, and A.I.E. Quaglia, contributed in different ways to our studies. J. Álvarez, G. Rost, and E. Rost accompanied R. Montero in recent birding trips in Santa Cruz. M.S. Liebana, M.R. de la Peña, and M.S. Bó kindly assisted with literature on the species. H. Smodlaka kindly reviewed our manuscript and assisted with English grammar. We thank the three anonymous reviewers and the editors of El Hornero who made very helpful comments and suggestions on our manuscript. Finally, we dedicate this paper to Nelly A. Bó, Jorge R. Navas, Maria I. Bellocq and Elio Massoia, who pioneered studies on the distribution, breeding and feeding ecology of Strigiformes in Argentina.
