Swainson’s hawks (Buteo swainsoni) are a medium-sized hawk native to western North America. They are one of the most variable raptor species in terms of plumage coloration and patterning, with several distinct color morphs occurring within the population.
Light Morph
The light morph Swainson’s hawk is the most common and widespread color variant. Light morph birds have a brownish upperparts and white underparts with variable amounts of dark spotting and streaking. The head can range from almost entirely white to showing variable dark streaking and speckling on the crown and behind the eyes.
Some key features of the light morph include:
- Brownish-gray upperparts
- White underparts with variable dark streaking and speckling
- White throat
- White “bib” on the upper breast
- Reddish-brown patches in the bend of the wings (“wing windows”)
There is much variation within the light morph, with some individuals being almost entirely white below with minimal streaking while others can show extensive dark markings on the breast and belly. Intermediates exhibit every combination in between. However, the white bib, wing windows, and generally light coloration distinguish this morph.
Dark Morph
The dark morph Swainson’s hawk represents the opposite end of the plumage spectrum from the light morph. Dark morph birds have very dark brown plumage overall, often appearing almost black in flight.
Some features of the dark morph include:
- Very dark brown upperparts
- Dark brown underparts with little to no white showing
- Dark brown head, often with white throat
- Lacks white bib
- Prominent rufous wing windows
The dark morph occurs at a much lower frequency than the light morph, representing less than 5% of the overall Swainson’s hawk population. It occurs primarily in the Great Plains region east of the Rocky Mountains.
Intermediate Morphs
In addition to the classic light and dark morphs, Swainson’s hawks exhibit a wide range of intermediate plumage types that blend features of both morphs. These intermediates demonstrate the full spectrum of variability in this species.
Some characteristics of intermediate morphs can include:
- Grayish-brown upperparts
- Mottled and streaked underparts with a mix of dark and white markings
- Partial or mottled white bib
- Gray, brown, or rufous patches on the head
- Wing windows range from reddish to dark brown
The amount of white, brown, and rufous on an intermediate bird can vary considerably, with no two intermediate birds looking exactly alike. The most common intermediate morph type shows a brownish wash on the underparts but maintains some white patches like a partial bib.
Rufous Morph
The rufous morph is one of the rarest color variants of Swainson’s hawks. These birds are a distinctive cinnamon-rufous color overall. Some features include:
- Bright rufous-cinnamon plumage on head, body, wings, and tail
- Pale buff or whitish underparts with variable rufous streaking
- Lacks white bib
- Bold rufous wing panels
The rufous morph occurs in fewer than 1% of the Swainson’s hawk population, predominantly in the southwestern United States and northern Mexico. It remains one of the most unique and striking color variants in this highly polymorphic species.
Melanistic Morph
Melanistic Swainson’s hawks are extremely rare, with only a handful of definitive records existing. As in other raptor species, the melanistic morph displays an excess of black pigmentation.
Traits of melanistic Swainson’s include:
- Entire plumage is dark blackish-brown
- Lacks rufous tones seen in classic dark morph
- Yellow cere, feet, and eye ring provide only color contrast
Due to its rarity, relatively little is known about the specific genetics and inheritance patterns of melanism in Swainson’s hawks. More research is needed to understand this unique morph.
Albinistic Morph
Albino hawks lack all pigmentation and are exceedingly rare in the wild. There are only a few documented cases of albino Swainson’s hawks, identifiable by their complete white plumage and bright yellow eyes, beak, and feet.
Characteristics of albino Swainson’s include:
- Entire plumage is pure white
- Bright yellow eyes, cere, beak, and feet
- Pink or reddish skin visible beneath feathers
Albino raptors rarely survive long in the wild due to poor eyesight and lack of camouflage from their white plumage. Nonetheless, the few records of albino Swainson’s provide valuable information about the upper limits of plumage variation in this species.
Geographic Variation
Beyond individual color morphs, Swainson’s hawks also demonstrate geographic differences across their range in North America. For example:
- Dark morphs occur at highest frequency in the Great Plains
- Rufous morphs occur more frequently in the Southwest
- Alaska birds are darker with more streaking on average
Some researchers suggest these geographic shifts in morph ratios hint at local adaptations to environments or conditions. However, more research is needed to understand the evolutionary forces driving these patterns.
Sexual Dimorphism
There are no major differences in plumage between male and female Swainson’s hawks. Both sexes demonstrate the full spectrum of color morphs. However, females average slightly larger in size than males, with somewhat broader wings and longer tails. Experienced observers can try to distinguish males and females in the field based on subtle size differences.
Morph Frequencies
The relative frequency of different Swainson’s hawk color morphs across their full population is summarized below:
Morph Type | Frequency |
---|---|
Light Morph | 90-95% |
Intermediate Morph | 5-10% |
Dark Morph | Less than 5% |
Rufous Morph | Less than 1% |
Melanistic | Extremely rare |
Albino | Extremely rare |
Causes of Polymorphism
The exceptional color variability of Swainson’s hawks is believed to be driven by multiple factors:
- High genetic diversity maintaining multiple plumage alleles
- Possible local adaptations to different environments
- Random chance and genetic drift
- Relaxed selection allowing multiple morphs to persist
Researchers continue to study Swainson’s hawk morphs to better understand the evolutionary mechanisms and advantages behind polymorphic species. The plumage diversity of this species provides a unique window into these processes.
Identification Tips
Despite the confusing array of color morphs, Swainson’s hawks can still be identified in the field by the following traits:
- Medium-sized buteo shape with long, pointed wings
- Long tail with wide dark terminal band
- Rufous patches in wings (“wing windows”)
- Soaring flight on flattened wings
- Smaller and more slender than most other buteos
The key is not to rely on any single plumage trait and instead focus on size, proportions, flight style, and any rufous markings that can help distinguish Swainson’s from similar species like rough-legged hawks or ferruginous hawks.
Conclusions
In conclusion, Swainson’s hawks display an incredible array of plumage morphs unmatched by any other North American raptor. Scientists continue working to unravel the genetics, environmental factors, and evolutionary forces that drive this remarkable polymorphism. The variability of Swainson’s hawks provides a unique system to study plumage evolution and adaptation in birds of prey. Though identification can prove tricky at times, experienced observers can distinguish Swainson’s from similar species through a combination of structural traits, flight style, and plumage characters like rufous markings. The striking diversity of Swainson’s hawk morphs will continue providing insights into raptor biology and evolution for years to come.