The species's ecology is poorly-known, and it has been recorded in a wide range of habitats, including gallery forest, monte (chaco-type woodland) and transitional woodlands (Pearman and Abadie 1995), savannas and grasslands, marshland, swamps, campo cerrado, lagoon edges with spiny scrub, and along streams, pools and flooded palm groves (Straneck and Viñas 1994, Cleere and Nurney 1998, Kirwan et al. 1999, Accordi 2002). It was previously thought to be most strongly associated with forest habitats (Pearman and Abadie 1995), but observations at key breeding sites in Argentina and Paraguay suggest that it is a grassland specialist and may be associated with flooded grasslands in parts of its distribution (Fariña et al. 2018, Bodrati et al. 2019). It has been suggested that it may be a migrant in the southern part of its range, with individuals breeding in Argentina migrating north in winter (Pearman and Abadie 1995), but at least some individuals are resident year-round in Corrientes (Argentina; Bodrati et al. 2019), and more recent evidence suggests that individuals may disperse over long distances outside the breeding season (Guest et al. 2020). It is a nocturnal aerial insectivore. It appears to have a lek- or lek-like mating system, with males gathering to display at arenas from August to December (Smith 2013, Fariña et al. 2018).

Eleothreptus anomalus has been recorded at few fragmented sites in eastern Paraguay (Hayes 1995, Lowen et al. 1996, Capper et al. 2001), north Argentina, and central and south-east Brazil (Bornschein et al. 1996, Kirwan et al. 1999). In Paraguay, the only reliable known breeding area is on Isla Yacyretá in the Paraná River (Smith 2013). In 2010, it was reported that an individual was recorded in Tocantins, Brazil (Pacheco and Olmos 2010), but there have been no further records to confirm the species's presence in the state. In Argentina, the species's remaining stronghold is in Corrientes, although it is only encountered regularly at a few sites (eBird 2021, A. Bodrati, N. Fariña and K. Cockle in litt. 2021). There are few confirmed localities in Misiones, and little remaining well-preserved grassland habitat in the province (A. Bodrati, N. Fariña and K. Cockle in litt. 2021).

The species formerly had a much larger range. It has almost certainly been extirpated from Buenos Aires, and almost all habitat in Entre Rios has been destroyed, although a small number of individuals may persist (A. Bodrati, N. Fariña and K. Cockle in litt. 2021). In Santiago del Estero and north of Cordoba there is a small area of remaining habitat where a population could persist, but there are no recent records (A. Bodrati, N. Fariña and K. Cockle in litt. 2021). Single birds recorded in Santiago del Estero (M. Rumboll in litt. 1986), El Palmar National Park, Entre Rios, in 1985 (M. Nares and D. Yzurieta in litt. 1986), and Colon, Entre Rios in 1979 (M. Nares and D. Yzurieta in litt. 1986), and in the 1990s, four records were made at three sites in Sao Paulo State, Brazil (Kirwan et al. 1999). Despite ornithological surveys (López Lanus et al. 2013), there have been no recent records from these areas, and so the species is now presumed to be locally extinct.

Conservation Actions Underway
It is listed nationally as Endangered in Argentina (MAyDS and Aves Argentinas 2017) and Paraguay (Ministerio del Ambiente y Desarollo Sostenibile 2019) and Near Threatened in Brazil (MMA 2014). In Brazil, it occurs in Brasília and Serra da Canastra National Parks and Cambuí Biological Reserve. It has been recorded from Rincón de Santa María and Urutaú Nature Reserves, Argentina (del Hoyo et al. 1999, Fariña et al. 2018, Bodrati et al. 2019). In Paraguay, Isla Yacyretá, which has the only reliable known breeding population in Paraguay, has been designated as a reserve (Smith 2013), and there are records from the San Rafael National Park (Smith and Clay 2015, Esquivel et al. 2019), Ybytyruzu Management Reserve and Tapyta Private Nature Reserve (Esquivel et al. 2019).

The Southern Cone Grasslands Alliance was established in 2006 to promote sustainable ranching practices across the grasslands of southern Brazil, Uruguay, southeast Paraguay and northeast Argentina, and a certification scheme has been developed to provide a financial incentive for sustainably-produced beef (Azpiroz 2012). At Rincón de Santa María Provincial Reserve, park rangers have been monitoring the population since 2009 and are working to restore grassland habitat (Fariña et al. 2018).

Conservation Actions Proposed
Study its precise ecological requirements, tolerance of grazing, and seasonal movements. Survey historical and potential sites to determine its current range.

Effectively protect core areas of habitat within its range and discourage tree plantations on native grasslands. Effectively enforce protected areas where the species occurs and designate further protected areas. Enforce existing laws against habitat conversion. Promote best practices in tree plantations to prevent trees from encroaching into remnant grasslands. Restore grassland habitat throughout its range. Remove exotic pines from protected areas.