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birds to watch for in Costa Rica

The Situation with Yellow-Green Vireos and Related Species in Costa Rica

Vireos are, more or less, these small warblerish, deliberate birds with miniature shrike-like bills. The Vireonidae family also includes the tiny, active greenlets of Central and South America, the small chattering vireos of scrubby habitats (such as the White-eyed and Bell’s Vireos), and the big and hefty shrike-vireos with equally impressive hooked beaks. Lest we leave them out of the Vireo picture, this family also has some colorful representatives in montane zones of southern Asia; the aptly named “shrike-babblers“.

However, the vireos that seem to capture the heart and soul of this avian family are the ones that take it easy, that take their sweet time to forage in the foliage of trees and bushes, constantly singing as they do so. On account of that repeated carefree song, these are the vireos that tend to engender familiarity among birders of all stripes, the leader of the bunch probably being the Red-eyed Vireo.

When spring gets truly warm up north, go birding, take a walk, or just listen in a park in almost any wooded area from eastern Texas and Florida north to the Great Slave Lake and you will probably hear Red-eyed Vireos. Be as patient as the bird you are looking for, watch for the slightest movement in the leafy scene above and you will eventually espy one, a bird with white underparts, olive above, and a white eyebrow separated from a svelte gray crown by a fine line of black. That’s the Red-eyed Vireo, a plain yet clean-cut bird with a sweet June song. Always one of the first migrants I would come across as I rode my bike to experience May migration on Goat Island, I would hear them throughout the day for for the next two months.

In Costa Rica, although thousands, more likely millions, of Red-eyed Vireos that migrate through this part of Central America on a biannual basis, “our” most familiar of the Red-eyed bunch is actually the Yellow-green Vireo. The Red-eyeds don’t sing around here, they barely even act like the summer birds of the north. In Costa Rica, they don’t have any time for that happy, lazy attitude because they have a vital appointment arranged by the imperative of instinct. The destination isn’t exactly just around the corner. To reach the leafy woods of western New York, the forests of Ontario, at just the right time, these small birds have to fuel up fast and in Costa Rica, this is why we see flocks of them busily picking bugs and larvae from the leaves, eating small berries to bulk up so they can take that personal bird express to the north.

They are only here for the eating business, just passing on through until they can get back to regular vireo business, that of casually foraging while singing all day long. In Costa Rica, that vireo job, living “la dolce vireo vita” is held by the Yellow-greens.

Although they also migrate through, large numbers fly in from habitats in the Amazon to stay and breed. As with their Red-eyed cousins of the north, Yellow-green Vireos likewise come to breeding grounds to take advantage of a sudden wealth of invertebrates, in their case, brought on by the onset of the wet season. Like the Red-eyeds, their constant singing is an essential part of the summer birding scene in the Central Valley and the dry forests of the Pacific slope. Unlike the Red-eyeds, they share their breeding surroundings with the likes of Masked Tityras and Clay-colored Thrushes and have to avoid the nest depredation antics of toucans.

They act and look quite similar but with a good view, these two vireo species are pretty easy to identify. Some of the ways in which they differ:

Red-eyed Vireo

-Daintier grayer bill.

-Mostly white underparts.

-Hint of pale brown on face.

Yellow-green Vireo

-Can have somewhat diffuse face pattern.

-Largish, mostly pale bill.

-Lots of yellow below, mostly on vent and flanks.

Although these are two of the most common Vireo genus species in Costa Rica, they aren’t the only similar Red-eyed type vireos to keep in mind. Granted, the following two are very rare and one has yet to be documented for Costa Rica but they are possible. The Black-whiskered occurs as a rare vagrant and is likely overlooked among the large numbers of very similar Red-eyed and Yellow-green Vireos. Although a birder still needs a close, definitive look at the head, a Black-whiskered can also reveal its true identify when it sings its distinctive, double-noted phrases. These can be reminiscent of a House Sparrow, the only problem is that it probably keeps quiet in Costa Rica.

While watching for vireos with a black line on the lower part of the face, we can also challenge ourselves (or drive ourselves crazy) by looking for the Chivi Vireo. Although its name might make you wonder if I’m joking, save the laughs for when you see how ridiculous it would be to find one of these pseudo Red-eyed Vireos, a species that would also be a first for Costa Rica. The Chivi Vireo breeds in South America and because it looks so similar to the Red-eyed, was formerly considered to be that species. Some are resident in northern South America, others migrate from places like Brazil and Argentina to the Amazon and ever since genetic studies showed that they are more closely related to Black-whiskered Vireo than the Red-eyed, Chivi Vireos have been recognized as a distinct species.

BUT, since they look just like Red-eyed Vireos, how on Earth would we even recognize one that just happened to overfly its usual destination? In all likelihood, we wouldn’t, but given the right circumstances, finding one in Costa Rica is possible, this is what we would need:

Look for a Red-eyed Vireo from June to August, this is when a Chivi would mostly likely occur.

Take a close look at the undertail coverts. If it looks like too much yellow for a Red-eyed Vireo, the bird might be a Chivi.

Take a close look at the color of the eye– if it looks pretty dark and without the slightest hint of red, it might be a Chivi.

If it sings, record that song! Both main groups of Chivi Vireos sound different from the Red-eyed Vireo. To my ears, the migrant ones we are most likely to get have a sort of warbled or trilled note.

To sum things up, if you see a bird in Costa Rica between June and August that sort of looks like a hybrid between a Red-eyed Vireo and a Yellow-green Vireo and happens to be a singing a song with a trilled phrase, there’s a fair chance it’s a seriously lost bird new for the Costa Rica list. In the meantime, if birding up north, enjoy the cheerful summer phrasing of the Red-eyed Vireos. Birding in Costa Rica? Be happy with the rainy season songs of Yellow-green Vireos but keep listening and watching for something different, you never know what you might find even when birding from home.