I’v always found that vireos are a tricky proposition when it comes to identification, especially here in Costa Rica. They are mostly greenish birds with a bit of yellow here and there; in fact, the very word vireo is Latin for I am green. Garrigues & Dean’s field guide, the Birds of Costa Rica, lists 16 family members, with illustrations of just 15 of them, because the northern migrant Black-whiskered vireo (Vireo altiloquus) is only a casual visitor, recorded infrequently on the Caribbean side. Here’s a run-down of what I think I’ve learned so far about vireos in my area, on the Turrialba Volcano slopes.
Firstly, what seems to be a slow-moving warbler almost always turns out to be a vireo. Then, however, six of the species, including the aforementioned Black-whiskered, are definitely not to be expected, either because they are very rare migrants (White-eyed vireo, Blue-headed vireo, Warbling vireo, Black-whiskered vireo) or are not readily found in our area (Mangrove vireo, Scrub greenlet). Actually, there is an old record of White-eyed vireo for the CATIE campus at Turrialba but I have not so far been able to trace the circumstances of the sighting.
The last two species illustrated in the Costa Rica guide (p. 228 of Garrigues & Dean) are the strikingly plumaged Rufous-browed peppershrike (Cyclarhis gujanensis) and the Green shrike-vireo (Vireolanius pulchellus). Both are residents and should be around here somewhere, but they would be an easy id, and I don’t really ever expect to run across them because there’s been no sign of them in three years!
Likewise, and shown on the same page, the rather neat-looking Tawny-crowned greenlet and the Yellow-winged vireo (also residents) have managed to skilfully conceal themselves from me, despite bearing some fairly easily distinguishing field marks. Where do all these common to fairly common birds manage to hide? Do they not know I’m just dying to catch a glimpse of them?
Three years of poking around my garden have allowed me to identify with certainty only 5 of the gang of 16. In order of frequency, these are:
1. Yellow-throated vireo (Vireo flavifrons)
2. Red-eyed vireo (Vireo olivaceus)
3. Yellow-green vireo (Vireo flavoviridis)
4. Lesser greenlet (Hylophilus decurtatus)
5. Brown-capped vireo (Vireo leucophrys)
I think it’s best to split these into the migrants (the first two, and the third, the southern migrant Yellow-green) and those that nest here, because then you can exclude the migrants when they’re out of season. It’s also a good split for me because I know from experience that any vireo I see is likely to be one of these three.
Here in San Antonio, the Yellow-throated and the Red-eyed vireos are to be expected almost on any given day in fall migration, but only the former has appeared here for the trip back north in spring. The Yellow-throated is an easy id even for the novice because of the spectacles it wears, but the Red-eyed has to be separated from the Yellow-green when both birds are present in the country. I find that the Yellow-green has lots of yellow below. Garrigues warns that some Red-eyed vireos can also show yellow, but if you get a good look, the black line above the white eyebrow is a sure mark.
I did have one spring sighting (2010) of a bird that I thought might be a Philadelphia vireo (Vireo philadelphicus), but I got only a fleeting look. You would think that this should be an easy bird to find here, particularly since it is banded from time to time down in Turrialba at CATIE, but I have never seen it for sure up here at 1200 m. I imagine I will probably have difficulty with it in the field. Will it look like a slow-moving Tennessee warbler? Will it let me have a clear view of the cap to distinguish it from the Brown-capped vireo?
The three remaining species that I have seen here are breeding residents, but the only vireo that I now hear quite often in the garden is the southern migrant Yellow-green vireo. This is a bird that is in Costa Rica only between late January and late October. After nesting, it migrates south. This is the first year that I have recorded the Yellow-green here consistently in San Antonio. Its song seems to me to begin with a chirp like a House sparrow’s and then continue with a fairly measured series of afterthoughts. I first became familiar with it in San Ramón de Alajuela, but its current stay in my rainbow eucalyptus trees (presumably nesting up high) have made it easy for me now to identify the song.
The Lesser greenlet is a bird that at first I was only able to hear, never see. Finally, however, I got a really good look at one over by Guayabo National Monument, and I don’t think it can really be mistaken for anything else. I still haven’t had a clear look at one here in the garden, but this one at Guayabo looked exactly like the illustration in Garrigues & Dean.
Finally, the Brown-capped vireo was present here two years ago but hasn’t been back since, so far as I know. It should be relatively easy to find, and the brown cap distinguishes it quite well. I would be delighted should it reappear.
I’m happy to say, then, that these greeny guys are no longer as baffling as they once were, but I will be in for real trouble should any of those rare migrants turn up.