Sickle-winged Nightjar (Eleothreptus anomalus)

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Image of a Sickle-winged Nightjar in its natural habitat
Birds Nightjar

Introduction

The Sickle-winged Nightjar (Eleothreptus anomalus) is a poorly known nightjar of central and south-eastern South America. It belongs to the family Caprimulgidae and is one of two recognised species in the genus Eleothreptus, the other being the White-winged Nightjar (Eleothreptus candicans). Its unusual male wing feathers, secretive habits and patchy distribution make it a specialised subject for field observation and conservation research.

Physical Description

The Sickle-winged Nightjar is a small nightjar, measuring approximately 18.2–19.4 cm in length in the limited specimens measured. Its plumage is mainly pale greyish-brown, with darker spotting and barring that provide effective camouflage when the bird rests on the ground or among low vegetation. The underparts are typically buff to brownish, with dark bars and streaks.

Adult males have the species’ most distinctive feature. Their primary feathers are modified and their secondaries are extremely short, producing a paddle-like or sickle-shaped wing outline when the wing is folded. The flight feathers also include contrasting pale or white markings, although these details are not always easy to see in the field. The modified feathers can affect the bird’s flight, which has been described as rapid and fluttering rather than strongly gliding.

Females are generally larger than males and have darker, more typically patterned nightjar plumage. They lack the male’s strongly modified wing feathers. Juveniles of both sexes also resemble females and do not show the adult male wing structure. This means that the name "sickle-winged" is principally a reference to adult males rather than to the appearance of the species as a whole.

Range and Habitat

The documented range extends from central and south-eastern Brazil through eastern Paraguay to northern and north-eastern Argentina. Occasional records farther north in Brazil may represent a broader distribution or seasonal movements. The species has sometimes been reported from Uruguay, but documented evidence remains insufficient to confirm a regular occurrence there.

The Sickle-winged Nightjar has been recorded in several habitat types, including grasslands, flooded grasslands, marsh and wetland margins, gallery forest, dry woodland, monte and transitional forest. Earlier accounts emphasised wetland edges, but more recent field work indicates that the species may also use wooded edges and open grassland habitats, particularly where suitable display and foraging areas occur nearby.

The species is probably resident in at least parts of Brazil. In the southern portion of its range, especially in Argentina, some evidence suggests austral seasonal movements, with birds moving northwards after the breeding season. The extent and regularity of these movements remain incompletely understood.

Behaviour and Diet

The Sickle-winged Nightjar is crepuscular and nocturnal. It may forage by making short aerial sallies from the ground, roads, tracks or low perches to catch insects in flight. Beetles, moths and other flying insects are reported among its prey, although detailed dietary studies are lacking.

Individuals may sit quietly on roads, trails, low branches or fence wires between feeding flights. The species is difficult to detect because it is well camouflaged by day and appears to occur at low density. Its flight can be fluttering and somewhat uneven, particularly in males with highly modified wing feathers.

Breeding information remains limited. Observations in north-eastern Argentina indicate male display activity from approximately August to December, with eggs recorded in October and November and young appearing later in the season. Like other nightjars, the species lays its eggs directly on the ground or on sparse vegetation rather than building a conventional nest. Recent research indicates a lek-associated mating system and female-only parental care, but many details of courtship, nesting and chick development remain unknown.

Identification Tips

  • Look for a small, pale grey-brown nightjar with a large-looking head, heavily barred underparts and a short tail.
  • Adult males may show paddle-shaped or sickle-like wings caused by modified primaries and very short secondaries.
  • The folded primaries can project in a gently curved arc towards the middle of the tail.
  • In flight, the male’s wing shape may be difficult to judge. Rapid, fluttering wingbeats are often more useful than trying to see a clear sickle outline.
  • A pale grey-brown overall appearance, dark barring on the underparts and contrasting markings in the male’s wings can help separate it from darker or more strongly patterned nightjars.
  • Females and juveniles lack the obvious male wing modifications and may be especially difficult to identify without a combination of habitat, size, plumage and vocal or behavioural evidence.

Conservation Status

The Sickle-winged Nightjar is currently assessed as Vulnerable on the IUCN Red List. The population is considered to be declining, although its total population size has not been reliably quantified.

The principal threats are the loss and degradation of grassland, wetland-edge and woodland habitats through agricultural conversion, drainage, vegetation clearance and commercial tree plantations. Because the species appears to occur in small, scattered concentrations and remains difficult to survey, habitat destruction may affect local populations before their status is fully understood.

Important conservation measures include protecting known breeding and display sites, retaining native grassland and gallery vegetation, avoiding the drainage of wetlands, managing plantation expansion and improving surveys during the breeding season. Standardised monitoring is also needed to clarify population size, seasonal movements, habitat requirements and breeding success.

Frequently Asked Questions

Where does the Sickle-winged Nightjar live?

It occurs in central and south-eastern Brazil, eastern Paraguay and northern to north-eastern Argentina. Reports from Uruguay remain unconfirmed or insufficiently documented for a regular range assessment.

Why is it called the Sickle-winged Nightjar?

Adult males have unusually modified primary feathers and very short secondaries, giving the folded wing a curved, sickle-like or paddle-like appearance. Females and juveniles do not show this feature clearly.

Is the Sickle-winged Nightjar migratory?

The species is probably resident in much of Brazil, but birds in southern Argentina may undertake austral seasonal movements, including movement northwards after breeding. The full migration pattern is not yet established.

What does the Sickle-winged Nightjar eat?

It is an aerial insectivore. Its reported prey includes moths, beetles and other flying insects, but the details of its diet are still poorly documented.

How does the Sickle-winged Nightjar breed?

Breeding activity has been recorded mainly from late winter through summer in the southern part of its range. The species lays its eggs on the ground, and available evidence indicates female-only parental care and a lek-associated mating system.

Is the Sickle-winged Nightjar endangered?

It is globally listed as Vulnerable, a threatened IUCN category. Habitat loss and degradation, particularly from agriculture, wetland drainage and commercial plantations, are the main recognised concerns.

References

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