Scrub Euphonia vs Thick-billed Euphonia

In short

The most reliable field mark between Scrub Euphonia and Thick-billed Euphonia is the extent and brightness of the yellow on the male's forehead: Scrub Euphonia males have a small yellow forecrown that stops at the eye, while Thick-billed Euphonia males show a larger, brighter yellow patch that extends slightly past the eye. Females are trickier, but Scrub Euphonia females are generally grayer and duller, while Thick-billed Euphonia females are more olive-green above and yellowish below. Structurally, Thick-billed Euphonia is noticeably heavier-bodied with a distinctly thicker bill, though both are similar in length. In the field, bill size and forehead color are the quickest checks, but subtle differences in female and immature plumage require careful attention. Both species overlap in parts of Central America, so range alone is not always diagnostic.

Identification tips

1. Male forehead patch (adult): Scrub Euphonia males show a small yellow forecrown that stops at the eye; Thick-billed Euphonia males have a larger, brighter yellow patch that extends slightly past the eye. This is the fastest field check for adult males.
2. Bill thickness (all ages/sexes): Thick-billed Euphonia has a noticeably thicker, heavier bill than Scrub Euphonia. In side view, the bill appears deeper and more robust. Use this when plumage is ambiguous or in poor light.
3. Male upperparts (adult): Both species have glossy blue-black upperparts, but Scrub Euphonia males may show a purplish sheen. This is subtle and not always reliable.
4. Tail pattern (adult): Both species have dark tails with white on the inner webs of the outer tail feathers; Thick-billed Euphonia may show white on three pairs (on the underside), Scrub on two. This is a minor and often hidden mark.
5. Female upperparts (adult): Scrub Euphonia females are olive-yellow on the forehead, grayish on the crown/nape, and olive with a gray tinge above. Thick-billed Euphonia females are more uniformly yellowish olive-green above. Look for the grayer crown and back in Scrub Euphonia.
6. Female underparts (adult): Scrub Euphonia females have mostly olive-yellow underparts with a yellow belly; Thick-billed Euphonia females are greenish yellow below, palest on the undertail. Scrub is generally duller and grayer.
7. Subspecies variation (male Thick-billed): Some Thick-billed Euphonia subspecies (e.g., crassirostris, hypoxantha) show even more extensive yellow on the crown or brighter yellow below. If you see a male with a full yellow crown, it's Thick-billed, not Scrub.
8. Body mass and jizz (all ages/sexes): Thick-billed Euphonia is heavier and bulkier (15 g vs. 10 g); Scrub Euphonia is lighter and more delicate. This is subtle but can support other marks.
9. Range (all ages/sexes): Scrub Euphonia is limited to Mexico–Costa Rica; Thick-billed Euphonia is widespread in Central and South America. In Costa Rica, both occur; elsewhere, range can be a strong clue.
10. Habitat (all ages/sexes): Both species use open and edge habitats, but Scrub Euphonia is more tied to drier, scrubby areas in its range. This is not diagnostic in overlap zones.
11. Vocalizations: No diagnostic differences in voice are provided; do not rely on song or call for separation in the field between these two.
12. Immature/juvenile plumage: No distinctive age-specific upperwing or flight marks are described for either species. Separation of young birds relies on overall structure and subtle plumage cues as above.
13. Iris, leg, and bill color: Both species have dark brown irises, blackish bills with bluish or grayish base to the mandible, and dark gray legs and feet. These are not useful for separation.
14. Pitfalls: Female and immature euphonias are often misidentified due to their dull, olive/yellowish plumage. Focus on bill thickness and subtle differences in head and underpart color for best results.
15. When in doubt: In areas of overlap, prioritize bill size and the extent of yellow on the male's forehead. For females, look for the grayer crown and back (Scrub) vs. more uniform olive (Thick-billed), and use structure as a backup.

Size

Both species are about 10 cm (3.9 in) in length, but Thick-billed Euphonia is substantially heavier (average 15.0 g vs. 10.0 g for Scrub Euphonia). This translates to a chunkier, more robust impression in Thick-billed, especially in the head and bill. Scrub Euphonia is noticeably lighter and more delicate. Bill thickness is also a key structural difference: Thick-billed Euphonia has one of the thickest bills among euphonias, while Scrub Euphonia's bill is short and stubby but not as deep or broad.

Plumage

No distinctive age-specific upperwing or flight pattern marks are described for either species, so separation relies on perched views and overall coloration.

Adult males: Scrub Euphonia males have a small yellow forecrown (just to the eye), with the rest of the head, nape, and upperparts glossy blue-black with a purplish sheen. Thick-billed Euphonia males have a larger, brighter yellow forehead patch that extends slightly past the eye, and the rest of the crown and upperparts are glossy blue-black. Tail and wing patterns are similar, with both showing dark tails and wings, but Thick-billed Euphonia may show white on the underside of the outer three tail feathers (vs. two in Scrub).

Adult females: Scrub Euphonia females are olive-yellow on the forehead, grayish on the crown and nape, and olive with a gray tinge above; underparts are mostly olive-yellow with a yellow belly. Thick-billed Euphonia females are more uniformly yellowish olive-green above and greenish yellow below, with the lightest yellow on the undertail coverts. Scrub Euphonia females are generally grayer and duller, while Thick-billed females are brighter and more olive.

Subspecific variation in Thick-billed Euphonia can affect the extent of yellow on the crown and the brightness of underparts, but the main contrasts above hold for the nominate forms and most field encounters.

Behaviour

No strong behavioral differences are described in the sources. Both species are typical euphonias, often seen in pairs or small groups, foraging in the canopy or edge of trees. Flight and foraging jizz are not noted as diagnostic.

Habitat

Both species occur in somewhat open, edge, or disturbed habitats, but their ranges only overlap in Costa Rica and parts of Central America. Scrub Euphonia is found from Mexico to Costa Rica, favoring scrubby areas, gardens, and open woodland. Thick-billed Euphonia has a much broader range, extending from Costa Rica south through much of South America, and uses a wide variety of open and semi-open habitats, including forest edges, clearings, cultivated areas, and gardens. In areas of overlap, habitat preference is not a strong separator.

Voice

Scrub Euphonia's calls are described as 1–3 pure thin whistles (dee-dee-dee), a shorter 'whit-it-it-it', and a longer 'wheeee'. Its song is a weak, slow twittering and chirping medley. Thick-billed Euphonia's vocalizations are not detailed here, but the Scrub Euphonia's song is noted as slower than that of the West Mexican Euphonia, not Thick-billed. Thus, vocalizations are not reliably diagnostic between these two species based on the provided sources.