Sumatran Ground-Cuckoo vs Bornean Ground-Cuckoo

In short

The most reliable way to separate Sumatran and Bornean Ground-Cuckoos is by location: Sumatran is strictly found in Sumatra, while Bornean is endemic to Borneo. In the field, both are large, terrestrial cuckoos with similar overall structure and habits, and their plumages are quite close, but subtle differences in bare facial skin color and underpart tones may help. Both species have glossy greenish-black wings and tail, dull green upperparts, and a black crown, but Sumatran shows more cinnamon-buff below and a more colorful bare facial skin. Vocalizations differ, but are rarely heard. Unless you are on the correct island, confusion is unlikely; if you are, focus on subtle plumage and facial skin details.

Identification tips

1. Location is key: Sumatran Ground-Cuckoo only occurs in Sumatra, Bornean only in Borneo. If you are not on the correct island, you can rule out the other species immediately.
2. Underpart color: Sumatran shows cinnamon-buff underparts below the upper breast, which may appear warmer and richer than Bornean's typically paler or less rufous underparts. This is best seen in good light and at close range.
3. Bare facial skin: Sumatran has bare skin around the eye with green, lilac, and blue hues. Bornean's bare skin is less colorful or differently toned (exact colors not fully detailed), so look for a more multicolored facial patch on Sumatran.
4. Upperparts and wings: Both species have glossy greenish-black wings and tail, dull green mantle and upper back, and a black crown. These features are not useful for separation.
5. Juvenile/immature plumage: No distinctive juvenile or immature wing or flight patterns are described for either species. Do not rely on age-related wing markings for separation.
6. Bill and legs: Both species have green bills and legs; this is not a distinguishing feature.
7. Size and structure: Both are large, long-tailed, robust cuckoos with similar proportions. Size and jizz are not reliable for separation.
8. Behavior: Both species are secretive, terrestrial, and move through dense undergrowth. Behavior does not help separate them.
9. Vocalizations: If heard, Sumatran gives repeated low whistles with a distinctive falling and rising pattern. Bornean's voice is not described here, so use this only if you hear a match to Sumatran's call.
10. Habitat context: Both are found in humid, dense forest, so habitat alone (without location) does not separate them. However, if you are in a disturbed or open area, neither species is likely.
11. Pitfall – former conspecific status: These species were once considered the same. Old field guides or references may not separate them; always check for recent updates.
12. Pitfall – specimen scarcity: Both are rarely seen and poorly known in life. Be cautious with any field mark not clearly visible or supported by recent photographs.
13. Pitfall – misidentification risk: If you are unsure of your island location or see a bird in captivity or a collection, double-check provenance before assigning species.
14. No seasonal or sex differences: No seasonal or sex-based plumage differences are described for either species; all ages and sexes appear similar within each species.
15. Summary: In practice, rely on island location first, then check for cinnamon-buff underparts and multicolored bare facial skin for Sumatran. If these are absent and you are on Borneo, you likely have Bornean Ground-Cuckoo.

Size

Both species are very similar in size, with Sumatran Ground-Cuckoo reported at about 55 cm (22 in) in length. No direct comparative measurements are available, but both are among the largest Asian cuckoos and appear nearly identical in bulk and proportions. Size is not a reliable field mark for separation.

Plumage

Plumage differences are subtle and best seen at close range. Both have glossy greenish-black wings and long tails, dull green upperparts, and black crowns. The Sumatran Ground-Cuckoo is distinctive for its cinnamon-buff underparts below the upper breast, while the Bornean's underparts are less clearly described but are typically paler or less rufous. The Sumatran also has bare skin around the eyes in green, lilac, and blue hues, contrasting with the Bornean, which is usually described as having less colorful or differently toned bare skin (precise color differences are not fully detailed in the available sources). No age-specific or immature upperwing patterns are described for either species, so juvenile and adult plumages are not reliably separable in the field based on current knowledge.

Behaviour

Both species are terrestrial, ground-dwelling cuckoos that move through dense forest undergrowth. Their behavior is similar: they walk or run on the forest floor, rarely fly, and are secretive and difficult to observe. There is no behavioral trait that reliably separates the two species in the field.

Habitat

Habitat is a strong clue: Sumatran Ground-Cuckoo is restricted to the forests of Sumatra, while Bornean Ground-Cuckoo is found only in the forests of Borneo (including Brunei, Malaysia, and Indonesian Borneo). Both prefer humid, dense lowland or hill forest. If you know your location, this is the single most useful separation mark.

Voice

Sumatran Ground-Cuckoo gives repeated low whistles, described as falling then rising in tone ("WE-ow-WE"), and sometimes a rising series of phrases, each slightly higher than the last. There is no detailed vocal description for Bornean Ground-Cuckoo in the provided sources. Voice is rarely heard and not a practical field mark unless you have a recording or direct comparison.