Appearance

Thick-billed Vireos are a compact passerine (13.5 cm) of the eye-ringed vireo clade. They have black lores, a dark eye, two white wing bars, and yellow spectacles that are broken around the eye.  Underparts are uniform and vary in color from yellow to grayish.

Thick-billed Vireos exhibit variation in the degree of yellow in and around supraloral stripe (all individuals are adults):

During the nonbreeding season, Thick-billed Vireo co-occurs with the morphologically and vocally similar White-eyed Vireo (Vireo griseus). White-eyed Vireos have a grayish white throat and breast, yellow flanks and a white belly; Thick-billed Vireo has uniform yellow/gray underparts. White-eyed Vireo is also greener on its back and grayer on its nape compared to the more beige green Thick-billed Vireo. The bill of Thick-billed Vireo is grayish to horn colored, whereas White-eyed Vireo typically has a black bill (Smith et al 1990). Adult White-eyed Vireos have white eyes, whereas juveniles (up until about January) have brown eyes.

There is broad overlap in wing chord, tail, and mass between the two species, and some overlap in bill measurements (Smith et al 1990). However, these species may be distinguished using a metric of overall bill size: multiplying bill length by depth by width (Smith et al 1990). All Thick-billed Vireos measured by Smith et al (1990) had values of this metric over 158, whereas all White-eyed Vireos were under 134. However, using similar methods, the account's author (KSP) obtained values for Thick-billed Vireos as low as 128. White-eyed Vireos were all under 137, except one individual with a value of 144 (unpublished data).

The songs of White-eyed Vireos are higher-pitched and more musical than Thick-billed Vireos, and the chatter calls of White-eyed Vireos are very distinct from Thick-billed Vireos. Thick-billed Vireo song has been described as "hoarser and more wiry" than White-eyed Vireo song (Smith et al 1990). Thick-billed Vireo songs have more long duration syllables than White-eyed Vireos and overall longer songs (Walker 1998). The chatter call of Thick-billed Vireo has been described as similar to the 'mewing' note of Gray Catbird (Dumetella carolinensis) (Vaurie 1953).

The slightly larger Yellow-throated Vireo (Vireo flavifrons) has yellow underparts from chin to breast, and white from belly to undertail coverts.

From Ridgway (1904: 189), describing individuals collected from the western Bahamas islands (Abaco, Eleuthera, New Providence, Andros, San Salvador or Cat Island, Green Cay, Inagua), plus Highburna Key and Pimlico Key: "Above plain olive, varying from grayish olive to deep olive-gray to brownish olive, the rump and upper tail coverts more decidedly olive, approaching dull olive-green; wings and tails dusky grayish brown, with pale olive or olive-grayish edgings (these nearly white on primaries), the tertials broadly edged with dull white; middle and greater wing-coverts broadly tipped with white, forming two conspicuous bands across wing; broad supraloral stripe and orbital ring pale yellow (varying from sulphur yellow to yellowish white), the latter interrupted on upper portion by a dusky spot on middle of upper eyelid; a dusky grayish loral streak, becoming blackish at anterior angle of eye; auricular region and sides of neck similar in color to upper parts but slightly paler; under parts varying from very pale grayish buffy or dull buffy whitish to light dull yellowish buff, sometimes slightly tinged with pale yellow; axillars and under-wing coverts pale yellow (primrose yellow); inner webs of remiges broadly edged with white".

From Ridgway (1904: 191), describing individuals collected from the eastern Bahamas islands (Conception Island, Rum Cay, San Salvador or Cat Island, Green Cay, Eleuthera, Inagua): "Much more brightly colored; upper parts yellowish olive or dull olive-green; under parts pale yellow (varying from straw yellow to naples yellow or dull canary yellow), tinged with olive laterally; supraloral stripe and orbital ring bright canary yellow".

From Ridgway (1904: 192), describing individuals collected from Grand Cayman and Cayman Brac: "Upper parts browner, white wing-bands apparantly broader, and outermost primary smaller".

The iris varies from black or dark brown (Vaurie 1953) to grayish (Bond 1936) or grayish white (Cayman Islands) (Richmomd 1917). However, at least in Bahamas specimens, gray is restricted to the outer iris, and never approaches the solid gray of a young White-eyed Vireo.

Vaurie (1953) states that "the yellowish eye-ring, lores, and frontal band, and the yellowish wash of the underparts are more strongly defined in the adult than in the young". Though there is a clear difference between juvenal and adults, there is broad overlap between first-year and adult individuals in the strength of these characters, and they should not be used for aging (personal observations).

Unless otherwise noted, all molt observations are from individuals caught on Abaco and San Salvador Islands (KSP, personal observations). There are no differences in plumage between the sexes.

Natal down

No information.

Juvenal plumage

Looks similar to adult plumage; supraloral stripe dull yellow, lores brownish. Contour feathers more loosely textured than in adults.

Prebasic molt

Adults

Molt begins in August (no data on specific start date). The last adult caught that had not yet initiated molt was Sept 4. Individuals caught at the start of September had already molted up to 5 primaries. Molt is usually initiated with median coverts followed by greater coverts. In the flight feathers, molt proceeds from p1 to p10, and from s1 to s6. s7-9 are usually molted during early primary flight feather molt. s1 molts after about p1-3 have been replaced. Primary coverts molted concurrently with matching primary flight feather. Alula is often molted last. Tail dropped around molt of p6-7, with all rectrixes dropped at about the same time. Body molt occurred concurrently with wing molt. The first individual caught that had completed wing molt was Oct 3; the last was Nov 7. The interior lining of the upper mandible ranged from light gray to very dark gray. Adults caught in September before they had initiated molt had very worn median and greater coverts, sometimes to the extent that wing bars were barely visible.

Hatch-year (HY)

Molt likely begins late August. The last individual caught that had not yet initiated molt was Sept 26. Molt sequence was similar to adults except that molt was eccentric. Typically p4 or p5-10 and s6-9 are molted. However, feather molt of primaries ranged from 9 primaries (p2-p10) to no primaries molted; molt of secondaries was always at least s8-9, and was usually s7-9, though some molted as many as s2-9. Therefore, all HY birds had a molt limit in the remiges (p1-4 and s1-5 most typically brown, contrasting with black innermost secondaries and outermost primaries); most HY birds had a molt limit in the primary coverts (outermost 2-3 primary coverts black with green edging, contrasting with brown innermost primary coverts) or a limit between the primary coverts and the greater coverts. Body molt occurred concurrently with wing molt. The last individual caught in molt was Oct 29. Hatch-year birds typically had a visible yellow gape even into the new year. The interior lining of the upper mandible was typically cream with some gray.

Prealternate molt

A few individuals had face/throat feathers molting in February; otherwise no prealternate molt has been noted.

A few individuals had white head feathers; the cause is unknown. One individual had a single white primary covert.

The only other observations on molt come from Smith et al (1990): Two individuals in Florida on September 14-16 were molting; the first individual's underparts appeared to be in heavy molt and the tail was half-grown, and the second individual was described as being in obvious molt with a longer but still incomplete tail. By September 30 the tails were fully grown.

From Ridgway (1904: 189): "Maxilla dusty horn color with paler tomia; mandible pale horn color". The tarsi and toes are described as bluish-gray (Vaurie 1953) or leaden (Cayman Islands) (Richmond 1917).

Initially, the Thick-billed Vireo was split taxonomically within The Bahamas, with V. c. crassirostris more common on the western islands ('western' form), and V. c. flavescens more common on the eastern islands ('eastern' form) (Ridgway 1904). The following tables were taken from that paper and so reflect that east/west split, but this split is no longer recognized.

*It is unclear why one of these individuals was described as having a white eye; it is possible it was a very large White-eyed Vireo, as that species was also seen and the authors admitted to not distinguishing between the two species when they were foraging.

High/low habitat comparisons were not significantly different.

See Buden (1985) for mean and range of measurements from 11 localities from The Bahamas, plus the Cayman Islands, and Providencia.