Breeding behavior, distribution, and conservation of the Sharp-tailed Tyrant Culicivora caudacuta (Vieillot, 1818) (Aves: Tyrannidae), a South American grassland specialist

Culicivora caudacuta occurs in the Cerrado, Pampa and Chaco grasslands of Bolivia, Brazil, Paraguay, Argentina and Uruguay. Its breeding biology is poorly known. Here, I present a summary of the published information and new data gathered between 2003 and 2009 in southeast Brazil at Tapira, Minas Gerais. Breeding occurred during the rainy season (October to March), clutch size being three eggs. Juveniles and immatures show a different plumage from the adults, mostly brownish orange. All nests studied at Tapira showed evidence of cooperative breeding, with one helper engaged in incubation and provisioning the young. This is the first observations of this behavior for the species. The species has a wider range than currently understood and its presence in protected areas is similarly more common.

Culicivora caudacuta occurs in central South America in habitats dominated by tall grasses and bushes in the Cerrado, Chaco and Pampa of Bolivia, Brazil, Paraguay, Argentina and Uruguay (Azpiroz, 1998;Fitzpatrick, 2004;Lopes et al., 2009).
Here I summarize all the available information on the distribution and breeding biology of C. caudacuta, and add new data mostly based on observations carried out at Tapira, Minas Gerais state, southeast Brazil, with the first observations of cooperative breeding by this species.

Study area
The main study area is in the municipality of Tapira, southeast Minas Gerais, on the property of Fosfertil (now Mosaic Fertilizantes) known as Tapira Mining Complex (Complexo de Mineração de Tapira -CMT). This covers an area of 7,150.86 ha, including the largest phosphate mining operation in Latin America, with elevations from 950 to 1,300 m a.s.l. The area has several tailing dams (BDs) built to receive the discards from the mining process and protect the watershed downstream from the CMT (Santos et al., 2002). One of these dams, BD-5 ( Fig. 4) (Poaceae) and Brachiaria decumbens (Poaceae), and native ones as Echinolaena inflexa (Poaceae), with scattered bushes, such as Baccharis dracunculifolia (Asteraceae) and Solanum lycocarpum (Solanaceae), among others.
Around the BD-5, as in the vicinity of Tapira and Araxá, there are also a few remnants of Atlantic Forest fragments, especially along watercourses, and a few plantations of Eucalyptus sp. (Myrtaceae).
Besides the BD-5, Culicivora caudacuta was also found and studied in another two areas in the CMT, one in the 1,430.20 ha legal reserve upstream from the dam (19°51′44″S, 46°47′52″W, elevation 1,250 m), an area with very similar habitat, and near Ribeirão do Inferno, another similar area covering 510 ha partially included in the municipality of Araxá (19°46′22″S, 46°52′48″W, elevation 1,240 m).    Dairy ranching is the main economic activity in the region where CMT is located but grazing animals are excluded from the dam area and the legal reserve. Grazing has transformed the native grasslands in open landscapes dominated by African grasses with some remnant native plants. Fires for "pasture renovation" are frequent and have a direct impact on the avifauna, especially grassland-dependent species.
The second area, also in Minas Gerais, in the municipality of Paracatu, includes the headwaters of the Ribeirão Batalha (17°29′48″S, 47°15′48″W, elevation 880 m) at the border with Goiás state and the municipality of Catalão, about 260 km from CMT. The area has remnant veredas dominated by Mauritia palm swamps bordered by hydromorphic grasslands on undulated terrain and grasslands with mounds built by termites campos de murunduns in a landscape where most native vegetation has been replaced by soybean monoculture.

Sampling
The research done at CMT was part of a broader project to survey the avifauna in areas belonging to Fosfertil in Minas Gerais and Goiás. I made two-day visits every month totalling 158 days between June 2002 and October 2011. Observations were made ad libitum (Altmann, 1974) from sunrise to sunset over the entirety of the areas.
At Patrocínio I made monthly two-day visits between 2007 and 2012, 15 days in August 2020, and 16 days in May-June 2021 totalling 114 days, while at Ribeirão Batalha I made occasional visits between 2004 and 2009 with a total of 28 days.
Territorial groups at CMT were mist-netted in 2005, and as C. caudacuta would easily pass through the mesh I used a speaker to attract the birds while an assistant held the pole supporting one extremity of the net, closing it once a bird hit the net.
Netted birds received both metal rings from CEMAVE (Brazil's official ringing scheme) and colour rings to allow individual identification. Bill, tail, tarsus, wing chord, nests and eggs were measured with a digital Mitutoyo® caliper; birds and eggs were weighted with Pesola® spring scales. All measurements are in mm, and when necessary other units are used.
Birds and their nests were found by active search with playback in suitable habitat inside CMT, especially near BD-5, where the birds were more habituated to human presence and, when nests were located, monitoring caused little interference in their behavior. Nests were not tagged with tape or other markers to avoid attracting predators.

Breeding season
Culicivora caudacuta was first recorded at CMT on 26 November 2002 at BD-5, the same spot where nests were found, and in another two areas along the reservoir.
At CMT, the breeding season occurs from October to March (Silva e Silva, 2006), considering the time the first nests were found to the latest date fledglings were fed by their parents. The actual start of the nesting period is probably September since nest building take 10 to 15 days (Di Giacomo, 2005).
At Águas Emendadas Ecological Station, Central Brazil, the breeding season was bracketed between October and April, based on the presence of brooding patches, active nests and records of young being fed by adult birds (Sousa & Marini, 2007;Marini et al., 2012). Not far away, at Chapada dos Veadeiros National Park, also in Central Brazil, breeding runs from mid-October to late March (Braz, 2008).
In Formosa, northwestern Argentina, nests were active between October and March, with no information on the time when fledglings were attended by adults (Di Giacomo, 1996Di Giacomo et al., 2011).
Culicivora caudacuta, as most species breeding at this time of the year, starts nesting at the end of the dry season when the first rains begin in September (Marini et al., 2012), as the rainy season is associated with greater abundance of food, especially insects, to feed the young.

The nest
The first nest (Nest 1) was found on 8 December 2003 at CMT, on the left bank of 46°50′28″W) when the fledglings were being attended by the parents (see below). It was a small and delicate, deep cup made of coarser plant fibres, flowers and cotton-like material bound with spider webs and lined with soft material. It was ca. 0.7 m above ground on the branches of a Baccharis dracunculifolia (Asteraceae) bush ca. Nest 4 was found still empty on 10 October 2008 (Fig. 7). It was built on a Vernonanthura cf. polyanthes (Asteraceae), also at BD-5 but was not monitored for long as on 27 October it looked unkempt and abandoned, apparently due to predation.
At Ribeirão Batalha, on 28 November 2007, one adult was seen collecting the soft fibres (Fig. 8), likely for a nest that was not located. This strongly suggests the bird was breeding, as Di Giacomo (2005) points the adults continue to bring nest material throughout laying and incubation.
At Chapada dos Veadeiros, another nest was found while still being built on 3 September 2007, at Fazenda Chapada das Almas (13°58′28″S, 47°27′03″W, 1,380 m), near the Rio das Almas, Alto Paraíso de Goiás, Goiás. This was in a wet grassland dominated by grasses some 40 cm tall with scattered bushes ca. 1 m tall by a riverine forest. Built ca. 85 cm above ground on a bush, the nest had grass seeds lining its structure, which was held to-gether and attached to the branches with spider webs. (Dante Buzzetti pers. comm. May 2020).
Nests found at our study areas agree with descriptions from the Argentine Chaco (Hartert & Venturi, 1909), Formosa Province (Di Giacomo et al., 2005), Central Brazil (Braz, 2008;Marini et al., 2012), Minas Gerais (Lombardi et al., 2010), and Rio Grande do Sul (Rovedder et al., 2007). In all areas, nests were half-spherical, deep, solid cups made of fine plant material bound with spider silk and lined with soft material, especially fine fibres from Asteraceae seeds. They were built among the vertical branches of low bushes, mostly less than 1 m above ground.
The exception to the use of bushes seems to be the nest built close to the ground in a grass tussock found by Hass & Silva e Silva (2008) at Emas National Park, Central Brazil, suggesting some degree of plasticity. The same behavior was recorded by Lombardi et al. (2012) at Carrancas, Minas Gerais, in October 2009. Peixoto (2014 also recorded an adult carrying cotton-like soft fibres to its nest on 12 October 2013 at Andrelândia, Minas Gerais, in grassland with shrubs. The first published description of a C. caudacuta nest, from the Chaco of Santa Fe province, Argentina, was made in the early 20 th century (Hartert & Venturi, 1909). The nest, with three eggs, was built on a low spiny bush of a kind growing sparsely in the grasslands; it was solidly built with grass flowers and soft fibres neatly organised and lined with softer material. Its measurements were:    Silva e Silva, R.: Culicivora caudacuta: breeding behavior, distribution, and conservation Pap. Avulsos Zool., 2021;v.61: e20216165 4/27 height 70, diameter 50, internal depth 40, inner diameter 25-30 (Hartert & Venturi, 1909).
As in our study area, nests were supported by several vertical or nearly vertical branches or, in some cases, inflorescences as in V. cognata, E. eburneum and E. elegans. Average nest height above ground was 1 m, ranging from 0.45 m to 1.8 m. Most nests were quite visible and similar to each other, built in the shape of compact, wellbuilt half-spheres, straw or yellowish coloured with white inner lining. Nests are built with fine plant matter such as dry fibres, petioles, Poaceae and Asteraceae flowers bound with silk from spider webs or egg sacs. The same material is used to anchor the nest to supporting branches, which varied from three to eight. The nest chamber is lined with very soft material, mostly fine fibres from Asteraceae inflorescences (Di Giacomo, 2005).
Additional nests were found at Brasília National Park (15°47′S, 47°56′W), central Brazil, on 12 November 2004, and two nests were found at Chapada dos Veadeiros National Park (14°05′S, 47°40′W), Goiás state, in November 2005 and November 2006 (Braz, 2008). The nests were attached to vertical branches and shaped as deep cups. These were built with fine plant material including dried fibres, petioles, grass flowers and leaves woven with spider webs. Height above ground ranged from 64 cm to 1 m. Measurements of the nest found in November 2006 were inner diameter 20.9, outer diameter 44.4 and depth 54.8 (Braz, 2008).
Another study at Águas Emendadas Ecological Station  (Sousa & Marini, 2007). In the same area another nest was found on 4 October 2009 in a campo sujo area built 38 cm above ground and having three eggs (Marini et al., 2012).
The following additional accounts of nests of C. caudacuta found in Brazil provide more limited but important information as to localities and breeding dates for the species.
A nest found in late October and another in early November 2000 were cup-shaped and built near the ground inside grass clumps at Emas National Park (18°08′S, 52°56′W), Mineiros municipality, Goiás state (Hass & Silva e Silva, 2008).
Southeast of Serra de Carrancas (21°27′S, 44°37′W, ca. 1,250 m), Carrancas, Minas Gerais state, a nest with three eggs was found on 21 October 2008. This was on a Diospyros hispida (Ebenaceae) bush in a small patch of campo sujo by a forest. The nest had been built with grass inflorescences and fine plant fibre (silk cotton), the latter mostly lining the incubation chamber. As other nests, the material was bound by spider webs, resulting in a soft and light, but resistant, construction. Measurements were: depth of the incubation chamber 30, external height 45, inner diameter 30 and external diameter 55 (Lombardi et al., 2010).
On 9 November 2006 a nest with two eggs was found at the headwaters of the Arroio Macena (28°30′55″S, 50°47′56″W, 940 m), Vacaria, Rio Grande do Sul. This was described as a delicate cup-shaped structure covered by silk cotton, built on a bush in a small wetland near a fallow dominated by Senecio sp. (Asteraceae) (Rovedder et al., 2007).
One adult was photographed bringing nest material (silk cotton) to its nest at Itirapina Ecological Station Further nests found in different Brazilian localities have been documented with photographs. On 12 November 2019 a nest was found at Barbacena, Minas Gerais state, and on 10 January 2008 a nest with eggs was located at Indianópolis, Minas Gerais. Another was photographed at Piraí do Sul, Paraná state, on 18 January 2014 (WikiAves, 2020).
All published information puts egg size range at 14-18.7 × 10.3-12.4 mm, the eggs found at CMT being unusually short. The significance of this, if any, remains to be assessed.
Di Giacomo (2005) and Di Giacomo et al. (2011) state that incubation begins after the penultimate egg is laid and lasts 15 to 16 days. The pairs may have a second clutch during the same nesting season but this is raised in a newly built nest in a different part of the territory.

Nestlings, juveniles and parental care
Nest 1 was found on 8 December 2003 when the young had already left and were being attended by the parents. The short-tailed fledglings were orange-brown colour (Fig. 11). They remained in the nearby shrubs and were seen to be fed by three different adults (sometimes at the same time) with dragonflies, horseflies and other flying insects (Figs. 12 and 13) until the 11 December. This family group provided the first published photographs of young C. caudacuta (Silva e Silva, 2004).
Later, on 16-17 January 2004, the young already showed longer tails and foraged by themselves, but always associated with the three adult birds.
On 8 November 2004 Nest 2 had two downy nestlings only a few days old attended by three adults that fed them beetles, dragonflies and flies and removed faecal sacs (Fig. 14), which were dropped 10 m away from the nest.
On 15 December 2004 the young had already fledged and were foraging by themselves but were still followed by the three adults. A few times two adults were seen to harass the young to direct them back near to the nest.
On 7 and 8 November 2005 Nest 3 had one egg and one nestling ( Fig. 15) already covered in orange-cream feathers, lighter on the underparts, and scattered down. It was also attended by three adults that fed it with horseflies, small grasshoppers, flies and dragonflies ( Fig. 16) mostly caught 5-10 m from the nest. The nestling would produce a faecal sac soon after being fed and one of the adults would at once take it and drop five meters or so from the nest, closer than seen in the previous nest. The nestling was able to raise itself from the nest but kept still most of the time, calling when it heard the voice or wing sounds of adults nearby.
Nests were not found the following nesting season at CMT but breeding was confirmed on 29 November 2006 when two adults, one previously banded at the site on 2 December 2005, and one short-tailed fledgling were seen. The young bird already showed a long tail and was following the adults when found again on 28 December 2006 and 24 January 2007.
The following day one adult pair followed by two young was found in the same area, with two adults and three young on 7 March 2007. One of these showed a whiter supercilium, a characteristic of an older bird than    its siblings suggesting that the young from two consecutive clutches were following their parents. Additional information on youngsters was gathered at other sites. At Patrocínio, around the headwaters of Córrego Bebedouro, two adults and one young were found on 6 February 2009, with one adult being captured and banded. Two adults and one young were also seen on 14 and 16 January 2012.
Also at Patrocínio, at the headwaters of Córrego Capoeira Grande, three adults and one young were seen on 1, 6 and 13 March 2011 (Fig. 17). The following sea-son, two adults followed by two young were found on 12 January 2012.
At Paracatu, around Ribeirão Batalha, three adults and one young (Fig. 18) were found near a Mauritia palm swamp on 27 January 2009.
The available literature provides further data on the presence of young C. caudacuta in other localities, mostly in Brazil. Although most mentions are just brief it is worth listing them in order to add more information.
In Brazil, the first data on young Sharp-tailed Tyrants are from 12 October 1988 at Fazenda Perdões (18°11′41″S, Figure 14. Adult removing a faecal sac from Nest 2 after feeding the nestlings. Photo: RSS.      45°25′17″W, 810-730 m), Três Marias, Minas Gerais. Two adults were seen feeding two juveniles showing a paler colour compared to the adults, lacking the black on the cap (Ribon et al., 1995). On 17 February 1991 two fledglings were recorded calling at Itirapina Ecological Station (Parker III & Willis, 1997).
In December 1996, a family group with two dependent young was found at Serra da Canastra National Park (20°15′S, 46°37′W), São Roque de Minas, Minas Gerais (Silveira, 1998). Adults and three young were seen on 12 February 2002 (Dante Buzetti pers. comm. May 2020), and on 8 February 2006 three adults and one young were seen in the same park (RSS).
At Brasília National Park, a nest was found on 12 November 2004 with three nestlings being fed by two adults. The young had emerging wing, tail and body feathers and an average weight of 4.33 g, but were predated five days later (Braz, 2008).
At Chapada dos Veadeiros National Park a nest was found in November 2006 with two nestlings still showing a mostly naked body, with just some down on the cap, back and wings. These had an average weight of 1.5 g and were taken by a predator four days later (Braz, 2008).
During a study at Águas Emendadas Ecological Station, two young in different family groups were found in November 2004 and April 2005. They showed completely cinnamon-coloured faces, short tail feathers and limited flight but, two months later, one of them had a full black mask and made several vocalizations (Sousa & Marini, 2007). In the same area, a nest was found on 4 October 2009. It had three eggs that hatched around 17 October, but the nestlings were preyed on about eight days later (Marini et al., 2012).
At Carrancas, Minas Gerais, young were found in May 2009 and January 2010, and in the same region, at Serra da Chapada das Perdizes (1,500 m), on the border of Minduri municipality, further young were recorded in January 2010 (Lombardi et al., 2012).
At Fazenda Pai João (28°10′58″S, 50°38′28″W, 1,005 m), Capão Alto, Santa Catarina, one pair of adults followed by a juvenile was found on 13 January 2013 in a grassy area with sparse flowering bushes (RSS). At the same locality, on 22 January 2012, one young and one adult were photographed together, while on 30 January 2012, one young was photographed (Wikiaves, 2020).
The young acquire full adult plumage five to six months after fledging and juvenile-plumaged birds were not recorded during the cold and dry winter months.
The juvenile plumage of C. caudacuta remained undescribed for a long time despite at least two juveniles having been collected by Johann Natterer in the early 19 th century. Among the eleven C. caudacuta collected by Natterer in Brazil, including nine from São Paulo and two from Paraná (Pelzeln, 1868(Pelzeln, -1870, two specimens, NMW 17822 and NHMUK 1888.1.13.331 (Fig. 19), both collected on 11 February 1821 at Itararé, São Paulo state, are juvenile. The specimens are held at the collections of the Naturhistorisches Museum Wien (NMW), at Vienna, Austria (Hans Berg-Martin in litt., 2020), and Natural History Museum, Tring, UK (Hein van Grow in litt., 2021).
Another specimen (ZSM 32700), which had probably just fledged judging from its short tail and colour, was collected in Paraguay on 4 December 1931 and is housed in the ornithological collections of the Zoologische Staatssammlung München (ZSM), Munich, Germany (Fig. 20), remained unknown until now (Markus Unsöld in litt., 2020). This is one of four specimens of C. caudacuta collected in Paraguay during the 1931 expedition (Laubmann, 1940).
Despite this, the juvenile plumage of C. caudacuta was first shown in a field guide by Sigrist (2004Sigrist ( , 2007, based on photographs taken at CMT by RSS. As described in this guide, the juvenile plumage of C. caudacuta, from the time they fledge until acquiring adult plumage, is quite distinctive and unlikely to be confused with the non-breeding plumage or female of the Bearded Tachuri Polystictus pectoralis, as shown by Smith (2017).
The female and young male of P. pectoralis show most of the head blackish, contrary to C. caudacuta which, after fledging and while still showing a short tail, has practically no black on the head.
As the tail grows and becomes as long as the head and body, the young start showing the first black streaks on the cap and show a very different jizz from the short, broad-tailed P. pectoralis.
Of 1,698 photos of C. caudacuta available at Wikiaves (2020), up to 16 May 2020, only 47 show juvenile birds. These were taken at Tocantins, Goiás, Distrito Federal, Bahia, Minas Gerais, Paraná, Santa Catarina and Rio Grande do Sul, showing the confirmed breeding range includes most of the species' range in Brazil. The presence of young in the months of October, November, December, January, February, March, April, and May agrees with the observations in the literature and the ones presented above, and suggest more than one brood can be raised per season.

Cooperative breeding and helpers
A common feature of family groups of C. caudacuta was the presence of three adult birds sharing the duties of incubation and provisioning of the young (Silva e Silva, 2006).
When Nest 1 and its fledglings were found in December 2003, three adults were found caring for the two young that remained next to the nest. Even when the young were already foraging by themselves, on 16 January 2004 and 2 February 2004, the three adults stayed near them, forming a family group, but only two of the adults would react aggressively to playback.
At Nest 2, found with two nestlings, three adults took turns feeding them and removing faecal sacs. The young continued to be attended by the three adults after they left the nest but, again, only two of the adult birds would approach in response to playback, the third remaining some distance away uttering softer and lower calls. Nest 3 also had three adults attending its single young from incubation until well after it had fledged.
The same behavior was also recorded in other sites we visited in Minas Gerais. At Ribeirão Batalha, Paracatu, one young bird attended by three adults was recorded on 9 November 2005 and 27 January 2009. The same grouping was seen on 1, 6 and 13 March 2011 at Patrocínio, near Córrego Capoeira Grande, when three adults and one young bird were attracted with playback.
The same grouping was found during a study on C. caudacuta at Águas Emendadas Ecological Station, where one young bird was attended by three adults, one of these providing food with higher frequency (Sousa & Marini, 2007). Cockburn (2006) states that among the 1,097 New World suboscines, cooperative breeding is consistently rare, found in just 1% of the species. By contrast, a sig-nificantly larger proportion of all oscines are cooperative breeders (13%).
Although Fitzpatrick (2004) states no tyrant flycatcher is known to exhibit cooperative breeding on a regular basis, Griesser & Suzuki (2016) show that among the Tyrannidae, six species show cooperative breeding.
According to Griesser & Suzuki, (2016), most (93%) helpers are offspring that remain associated with their parents until the next breeding season and provide alloparental care at the nest of their parents or close relatives.
Following this pattern, it is likely the helpers among C. caudacuta are young from the previous nesting season that remained in the natal territory through the following season.
During the fieldwork recently carried out in Patrocínio, Minas Gerais, in August 2020, three groups of three individuals each of C. caudacuta were found, where it was noted that only two of these individuals responded promptly to playback, while the third individual was more distant and responded with a much softer and weaker vocalization, being possibly a young bird accompanying its parents.
Detailed studies, including genetic profiling, are necessary to further knowlege on this behavior in C. caudacuta and the few tyrant flycatchers also showing helper behavior like Streamer-tailed Tyrant Gubernetes yetapa (Wagener et al., 2019).

Birds' measurements
Eleven birds were captured for ringing in the study sites: nine, including one young, at Tapira between 1 and 2 December 2005, one at Paracatu on 28 December 2005, and another at Patrocínio on 6 February 2009. The measurements are in Table 1. These fall in the range of the 13 birds captured at Águas Emendadas Ecological Station (Sousa & Marini, 2007).

Distribution
The first information on the distribution of C. caudacuta was supplied in the 18 th century by Félix de   Azara and mentioned in the species' description, where Paraguay was assumed as the type locality (Vieillot, 1818). Nevertheless, Azara's observations also covered nearby parts of Argentina and Brazil (Beddall, 1975) where C. caudacuta is known to occur, the presence of the species in Brazil having already been noted in the early 19 th century (Temminck, 1822). The assignment of Paraguay as type locality is accordingly best seen as tentative.
It took much longer for the actual distribution of C. caudacuta to be better understood as including parts of Brazil, Bolivia, Paraguay, and Argentina (Lopes et al., 2009). However, coverage was limited and the gaps that were shown may not be real.
Technological progress, especially the greater availability of digital cameras and sound recorders, with the rise of birdwatching and on-line platforms for ornithological data, such as Wikiaves, xeno-canto and eBird has increased coverage in a dramatic way and resulted in a jump in the known localities for the species.
On the other hand, greater coverage and the resulting increase in the number of localities mean that caution is required in considering claims that C. caudacuta may be expanding its range.
The recent data show that the area of occupancy of C. caudacuta has been underestimated. Using Minas Gerais state (58,652,800 ha), in Brazil, as an example, the species was previously known from just three localities (Lins, 1998), later jumping to 20 (Lopes et al., 2009), and with the compilation presented here this jumps to 134 localities (Appendix 1; Fig. 21).  Table 1. Measurements (mm) and body mass (g) of Culicivora caudacuta from Tapira (n = 9), Paracatu (n = 1) and Patrocínio (n = 1). Culicivora caudacuta is mostly associated with open grassland and savanna of the Pampa and Cerrado biomes characterized by a dense ground cover of grasses and scattered, well-spaced low bushes, occasionally near watercourses, but it also occurs in areas where the original vegetation has already been contaminated by exotic grasses.
Records show it occurs from elevations of 60 m in the Chaco of Argentina and Paraguay to 1,550 m at Serra da Chapada das Perdizes, in Brazil (Lombardi et al., 2012).
The species has most of its range in Brazil, occurring in all major regions. Its presence in the states of Amazonas, Tocantins, Bahia, Santa Catarina and Rio Grande do Sul was confirmed only in the past two decades, showing the imperfect knowledge on its distribution. Regarding the last two states, Fontana et al. (2008) point out that the lack of previous records is best considered a result of a lack of studies rather than a colonization event.
Among the new localities where the species has been found are savanna enclaves in the region of Humaitá and Canutama (MZUSP; Wikiaves, 2020), in southern Amazonia (now being converted for soybean crops), and similar habitats at Campos Amazônicos National Park (961,317.77 ha) further east, where other grassland specialists have also been found (Cândido-Jr. & Dal'Maso, 2016). The Amazonian savannas are of great ecological interest and, in the case of the national park, may protect significant populations of the species.
The species has also been cited as occurring in Amapá state (BirdLife International, 2020b), another Amazonian region with extensive savannas, but the source of this record could not be traced and this locality was excluded here.
The current information shows that C. caudacuta has a much broader range than first thought, and besides at least 13 Brazilian states where it has been found, in Bolivia, in the departments of El Beni, La Paz and Santa Cruz; in Paraguay in the departments San Pedro, Presidente Hayes, Cordillera, Paraguarí, Concepción, Canindeyú, Caaguazú, Caazapá, Itapúa and Misiones; in Argentina in the provinces of Misiones, Chaco, Corrientes, Entre Ríos, Formosa and Santa Fe; and In Uruguay it is known from a single record from the department of Rivera.
Its presence is probable but still undocumented in southeast Peru, at Madre de Dios in the Pampas de Heath (12°53′S, 68°54′W), a seasonally flooded grassland on the Peru-Bolivia border making a northwestern extension of the Gran Chaco Boliviano-Paraguayo, where several grassland specialists such as Rhynchotus rufescens, Micropygia schomburgkii, Ammodramus humeralis, Sporophila plumbea and Coryphaspiza melanotis have been found (Graham et al., 1980). A record of C. caudacuta from Pampa Moscoso was made just 13 km from Pampas de Heath, adding to the likelihood of the species occurring there.

Conservation
Culicivora caudacuta has long figured in threatened species lists: at global level it was considered as  Most countries in its range consider it to be threatened, as Argentina, where it was considered as Vulnerable (Fraga, 1996), and now as Threatened (Ministerio de Ambiente y Desarrollo Sustentable & Aves Argentina, 2017).
On the other hand it has been dropped from the national list since 2014 (ICMBio, 2018), where it was previously listed as Vulnerable (Hass & Silva e Silva, 2008).
In Uruguay, where it is known from a single locality, it is considered as Vulnerable (Azpiroz et al., 2012), while in Bolivia it is not included in red lists (Balderrama, 2009).
The main reason for C. caudacuta to be considered threatened is the widespread conversion of native grasslands all over South America into cultivation, mostly for soybeans, and pastures dominated by exotic grasses where the birds cannot persist (Codesido & Fraga, 2009;BirdLife International, 2020b).
In the study area the expansion of Eucalyptus monocultures for pulp and charcoal over the natural grasslands continues unchecked and is the main driver of habitat loss (Fig. 22), a trend also in southern Brazil, Uruguay and Argentina also (Modernel et al., 2016).
There are little data on population densities, and all come from Brazil. At Chapada dos Veadeiros National Park the estimated population is 1,475-4,022 individuals, with an average of 2,433 individuals, while at Brasília National Park there are estimated to be 440 to 2,374 individuals, with an average of 1,021 (Braz, 2008 (Reinert et al., 1998), (Braz, 2008)