The dark morph of the pomarine jaeger (Stercorarius pomarinus) is one of two distinct color variants found in this species of predatory seabird. The other morph is known as the light morph. Understanding the differences between these two morphs can help birders and ornithologists identify pomarine jaegers correctly in the field.
Overview of the Pomarine Jaeger
The pomarine jaeger is a large, powerful jaeger species that breeds in the Arctic tundra. Jaegers are part of the skua and jaeger family (Stercorariidae) and are known for their harassing, kleptoparasitic behavior towards other birds. Pomarine jaegers have a circumpolar distribution, breeding in northern Eurasia and North America and migrating south along coastlines to spend the non-breeding season at sea.
Like other jaegers, pomarine jaegers exhibit striking plumage polymorphism with two different color morphs. The morphs differ prominently in their breast, belly, and undertail coverts. This species got its common name from its shaggy twist of feathers behind the bill that resemble a pom-pom.
Details of the Dark Morph
The dark morph pomarine jaeger has entirely dark plumage except for pale markings on the underwings. Key identification features include:
- Dark brown plumage on the head, neck, chest, back, wings, and tail
- White shafts streaking on the nape and hindneck
- Pale yellowish panels on the underwings
- Dark belly and undertail coverts
The dark breast and belly contrast with the pale underwings. The tail is rounded or spoon-shaped. The feet are bluish-gray, and the bill is black with a yellowish or pinkish base.
Comparison to Light Morph
The light morph pomarine jaeger has a distinctly different plumage pattern from the dark morph. Instead of being mostly dark, the light morph has pale underparts contrasting with dark upperparts. Key features of the light morph include:
- Dark brown upperparts (back, wings, crown)
- Pale yellowish or whitish neck and underparts
- Fine barring on flanks and undertail coverts
- Pale panels on underwings
- Rounded, spoon-shaped tail
The light morph has variable amounts of white on the face, neck, breast, and belly. The undertail coverts are whitish with wavy barring. In flight, the contrast between the dark upperparts and pale underparts is striking on the light morph pomarine jaeger.
Feature | Dark Morph | Light Morph |
---|---|---|
Head | Dark brown | Dark brown |
Breast | Dark brown | Pale yellowish-white |
Belly | Dark brown | Pale yellowish-white |
Undertail Coverts | Dark brown | White with wavy barring |
Upperparts | Dark brown | Dark brown |
Underwings | Pale yellowish panels | Pale yellowish panels |
Tail Shape | Rounded | Rounded |
Identification Tips
To identify whether a pomarine jaeger is a dark or light morph, focus on the coloration of the underparts. A dark breast band contrasting with pale underwings indicates a dark morph bird. Extensive pale areas on the neck, breast, belly and undertail coverts indicate a light morph jaeger. The rounded, spoon-shaped tail and large size helps distinguish pomarine jaegers from other jaegers.
The dark morph adult pomarine jaeger resembles a dark morph parasitic jaeger but is larger with a thicker, heavier bill and more rounded wings. Juvenile and subadult jaegers have more variable plumages and can be harder to differentiate to species and morph without experience.
Geographic Variation
Both dark and light morph pomarine jaegers occur across their breeding and non-breeding ranges, but the frequency of each morph varies regionally. For example, dark morph birds tend to predominate in eastern North America along the Atlantic coast. Farther west in the interior of North America, light morph pomarine jaegers are more common.
Conclusions
In summary, the key differences between dark and light color morphs of the pomarine jaeger are:
- Dark morph has dark underparts while light morph has pale underparts
- Dark morph shows contrast between dark breast band and pale underwings
- Light morph exhibits extensive white on face, neck, breast with dark upperparts
- Other features like tail shape are similar between morphs
- Geographic variation occurs in morph frequencies
Distinguishing these two distinct plumage types aids identification and appreciation of the highly polymorphic pomarine jaeger. Understanding differences between morphs also provides insight into the species’ life history and enables accurate monitoring of populations.