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South American Harris’s Hawk Form Reached Definitive Adult Plumage After Four Annual Molts

Harris's hawk perched in Buenos Aires, Argentina
Research · Evolution & taxonomy

A study of the South American nominate form of Harris’s hawk (Parabuteo unicinctus unicinctus), referred to by the author as the bay-winged hawk, found that the birds pass through five recognizable age-related plumages before reaching definitive adult plumage after four annual molt cycles. The study was published on 20 March 2026 in Ornithology Research and examined natural-history traits that are important for reassessing the taxonomy of the Parabuteo unicinctus complex.

Matías A. Juhant analysed the plumage of 139 photographed individuals of the South American form across the Austral Temperate Region and combined these records with field observations of breeding and social behaviour. The photographs allowed individuals to be placed into age classes according to their molt stage and plumage.

Definitive adult plumage followed four annual molts

Five age classes were identified. Basic I–V describe successive plumages associated with the annual molt cycles. Birds in Basic I were up to eight months old, Basic II covered roughly 9–20 months, Basic III 21–32 months and Basic IV 32–44 months. Definitive Basic Plumage, Basic V, was reached from about 45 months of age, after four annual molt cycles. The sequence differs from previous descriptions of plumage development in the South American form and gives observers a more detailed basis for ageing birds in the field.

Three nests suggested breeding may occur year-round

The field observations also added information about breeding. Three active nests were documented, two during the austral winter and one in spring. Juhant interprets these records as evidence that breeding may occur throughout the year rather than being confined to a single season.

Mixed-age groups and a breeding trio were observed

Social behaviour was another prominent feature. Mixed-age groups of five to eight individuals were repeatedly observed soaring or perching together, and one active nest was associated with a trio. These observations add further evidence of complex social behaviour in the southern form, which belongs to a raptor complex already known for group living and cooperative behaviour.

The study places these natural-history differences in a taxonomic context. The South American form is currently treated as P. u. unicinctus, while the northern form is generally treated as P. u. harrisi. Juhant argues that better field information on plumage maturation, breeding phenology and social behaviour is important when assessing relationships within the complex. He particularly highlights tropical South America, where the two forms meet, as a key region for further fieldwork.

Sources and image credit

  1. Juhant, M. A. (2026). Framing the natural history of the bay-winged hawk (Parabuteo unicinctus unicinctus) for accurate taxonomic assessment. Ornithology Research, 34, 14. https://doi.org/10.1007/s43388-026-00282-3Primary peer-reviewed study published 20 March 2026. Analysed plumage in 139 photographed individuals of the South American form and field observations of breeding and social behaviour; identified five age classes, definitive adult plumage after four annual molt cycles, three active nests and mixed-age social groups.

Image: Sebastián Fornés (seba_fornes) — Harris's hawk (Parabuteo unicinctus) photographed in Buenos Aires, Argentina, 29 April 2024; iNaturalist / Wikimedia Commons; cropped to 16:9 for Wildlife Vagabond; representative South American image, not one of the study individuals · CC BY 4.0 · Source

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