In northern Central America, a perhaps under-appreciated ID challenge among the «spectacled» vireos is the subtle difference between Central American Plumbeous Vireo and Blue-headed Vireo. The latter is a common non-breeding visitor in most of northern Central America, where Plumbeous Vireo is resident. Most Central American Plumbeous Vireos do not look all gray (i.e. ‘plumbeous’) like the northern Plumbeous Vireos, but rather look more like Cassin’s Vireos, i.e. intermediate between northern Plumbeous Vireo and Blue-headed Vireo. Since the identification challenges of vagrant Cassin’s Vireos in Blue-headed Vireo range and vice-versa is a recurrent theme in US birding, it should come as no surprise that the identification of Central American Plumbeous Vireo versus Blue-headed Vireo can also be tricky.
In 1997, the AOS split the Solitary Vireo into three species: Blue-headed Vireo (Vireo solitarius), Cassin’s Vireo (V. cassinii) and Plumbeous Vireo (V. plumbeus). The split was based on genetic work carried out by Murray et al. (1994) and Johnson (1995) on North American populations that are also phenotypically distinct. Howell & Webb (1995) anticipated the split and suggested that «studies including all Middle American populations are needed» (my emphasis). However, the aforementioned studies did not sample the southernmost two subspecies of this complex, montanus and notius, and this group was assigned to Plumbeous Vireo, just as resident Mexican subspecies. Slager et al. (2014) did sample specimens from south of the Isthmus of Tehuantepec when compiling a multilocus phylogeny of the Vireonidae and found that the individuals they sampled from southern populations of Plumbeous Vireo differed genetically as much from northern Plumbeous Vireo as the three northern species in the «Solitary Vireo» complex—i.e. Blue-headed, Cassin’s and Plumbeous—differ from each other. They found that northern and southern populations of Plumbeous Vireo are not monophyletic and likely represent two species, but since they only sampled Honduran specimens south of the Isthmus of Tehuantepec, they recommended wider sampling of this southern group. This is a doctoral study waiting to happen…
The subspecific division of these resident vireo populations into montanus and notius is weak and not universally accepted. Both are notably smaller than the nominate plumbeus, which is migratory, and notius is said to be grayer and duller than montanus, approaching plumbeus. A Mexican subspecies, gravis, is even larger than plumbeus. The two Central American subspecies were described on the basis of a handful of specimens, and individual variation may show overlap in characters in this group.
In Central America, resident Plumbeous Vireos are found in a variety of semi-open forests from foothills (600 m) to highlands (2500 m), and locally lower in the pine forests of Belize and the Honduran Moskitia. There is much habitat overlap with wintering Blue-headed Vireo in Central America. Blue-headed Vireo is one of the last winter visitors to arrive in Central America and is rare in the region before the last week of September, with the majority arriving mid-October. It is also the first migratory vireo to depart the region in March, with a few lingering into mid-April (Morton and James 2014, eBird). Central American Plumbeous Vireos sometimes join mixed insectivorous flocks, but are often seen in pairs, and don’t follow the flock far from their home territory. Blue-headed Vireos often join mixed insectivorous flocks but are usually not seen with other conspecifics (i.e. they remain «solitary»).
On average, the Central American Plumbeous Vireo looks much like a Cassin’s Vireo, i.e. intermediate between a Blue-headed and a northern Plumbeous Vireo. Be aware of individual variation, with some birds approaching the brighter, i.e. «blue-headed» end of the spectrum, and other birds the duller, i.e. «plumbeous» end of the spectrum. These differences may be sex- and age-related, with adult males on the brighter side and immature females on the duller side; they may be range-related, with Belizean birds (notius) averaging duller and interior highland birds (montanus) averaging brighter; and they may be wear-related, with freshly molted individuals in fall brighter than worn dull individuals in spring. All Central American Plumbeous Vireos are, however, smaller than Blue-headed Vireo. Not many field guides emphasize this difference, and not all birders are accustomed to judging size on small passerines, but with practice this character can be used in the field. Monroe (1968) reported differences in wing chord for Honduran «resident Solitary Vireo», i.e. Central American Plumbeous Vireo (males: 66.3 mm; females: 65.7 mm) and Honduran «migratory Solitary Vireo», i.e. Blue-headed Vireo (males: 75.8 mm; females: 74.0 mm). Blue-headed Vireo often looks bigger overall, especially bigger-headed and longer-winged, than Central American Plumbeous Vireo, with more overall contrast in the plumage.
In summary, given good views, most «solitary» vireos in northern Central America can be identified using the following characters:
|Character||Central American Plumbeous Vireo||Blue-headed Vireo|
|Hood||Medium gray||Dark gray|
|Contrast between hood and back||Subtle||Marked|
|Contrast between hood and throat||Gradual||Sudden|
|Flanks||Yellowish, often not contrasting strongly with rest of underparts||Greenish yellow or olive, usually strongly contrasting with whitish underparts|
|Underparts||Dingy whitish, with a variable yellow wash not particularly restricted to certain areas as on BHVI||Usually clean white, with greenish yellow or olive flanks and a faint yellow vent strap|
|Primary projection||Difference between longest primary and longest secondary is smaller than length of longest secondary||Difference between longest primary and longest secondary is about equal to length of longest secondary|
|Size||Smaller than Blue-headed Vireo||Larger than Central American Plumbeous Vireo|
Be prepared, however, to leave some intermediate individuals identified to genus level only.
Thankfully, Cassin’s Vireo does not occur in the range of Central American Plumbeous Vireo, although vagrant Cassin’s have been found far away from the breeding and wintering grounds, so at least the potential for vagrancy to Central America exists (Goguen and Curson 2002). A vagrant Cassin’s Vireo in Central America would be very difficult to detect indeed, even in the hand at a banding station. The migratory subspecies of Cassin’s Vireo is on average longer-winged than the resident Central American Plumbeous Vireo, but individual variation in both groups would make it difficult to rely on this character alone. The gray, colorless northern subspecies of Plumbeous Vireo does not winter in Central America either (Goguen and Curson 2012), but a vagrant northern Plumbeous Vireo would be very dull (gray back, without green tones; grayish flanks, without yellow tones) and slightly larger, with a longer primary projection, than the Central American Plumbeous Vireos. Some notius Central American Plumbeous Vireos can be quite gray though, so caution is warranted.
Goguen, B. and D. R. Curson. 2002. Cassin’s Vireo (Vireo cassinii), version 2.0. In The Birds of North America (A. F. Poole, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bna.615.
Goguen, B. and D. R. Curson. 2012. Plumbeous Vireo (Vireo plumbeus), version 2.0. In The Birds of North America (A. F. Poole, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bna.366.
Howell, S. N. G. and S. Webb. 1995. A guide to the birds of Mexico and northern Central America. Oxford University Press, Oxford, UK and New York, NY, USA.
Johnson, N. K. 1995. Speciation in vireos. I. Macrogeographic patterns of allozymic variation in the Vireo solitarius complex in the contiguous United States. Condor 97: 903–919.
Monroe, B. L. Jr. 1968. A distributional survey of the birds of Honduras. Ornithological Monographs No. 7, American Ornithologists’ Union, Allen Press, Lawrence, KA, USA.
Morton, E. S. and R. D. James. 2014. Blue-headed Vireo (Vireo solitarius), version 2.0. in The Birds of North America (A. F. Poole, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bna.379.
Murray, B. W., W. B. McGillivray, J. C. Barlow, R. N. Beech and C. Strobeck. 1994. The use of cytochrome B sequence variation in estimation of phylogeny in the Vireonidae. Condor 96: 1037–1054.
Slager, D. L., C. J. Battey, R. W. Bryson Jr., G. Voelker and J. Klicka. 2014. A multilocus phylogeny of a major New World avian radiation: The Vireonidae. Molecular Phylogenetics and Evolution 80: 95–104.