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When it comes to identifying small olive-green birds with wingbars and eye rings, birders are often left befuddled. Separating flycatchers, vireos, and kinglets can certainly be daunting. Distinctive feeding behaviors and upright perching postures usually yield enough clues for the average birder to differentiate Empidonax flycatchers from vireos and kinglets, as the latter two groups tend to be gleaners rather than flycatching for their prey. After that, many birders will encounter serious roadblocks. One of the most problematic species pairs is Hutton's Vireo and Ruby-crowned Kinglet.
Field guides will tell you that Hutton's Vireo has a larger, thicker bill than a kinglet and that its feet are bluish-gray. Conversely, Ruby-crowned Kinglets are described as having a smaller bill and more blackish legs with yellow feet. In the grand avifaunal scheme, either of these species might be described as having a small bill, thus bill size difference is more subjective than objective and is highly dependent on the observer's contextual baseline. Simply stated, if you don't have much experience with these birds, or you haven't confidently identified a Hutton's Vireo, you have no basis for comparison. As for foot and leg color, seeing these differences takes a fairly close-up view with good optics. Additionally, both species tend to be pretty active, moving in and out of the foliage and showing a preference for more shaded areas. Getting a good look at the leg and foot color will occur with about every tenth bird you see. So, how do you identify the other nine?
Over the years, I've found a couple things that I look for as I separate these two species. First, I think that bill color is far more useful than bill size. Hutton's Vireos have a paler, horn-colored bill, which is indeed thicker and larger overall than the bill of a Ruby-crowned Kinglet, while the kinglet's bill is all-black. Although a Hutton's Vireo's bill may look dark in some light conditions, I can't recall seeing one with a truly black bill.
Another thing that I look for is the contrast in the wing pattern. While both species have two white or off-white wing bars and dusky olive-green wings, I find that the wings of Hutton's Vireos are less glossy looking and show less contrast overall. While their primary, secondary, and tertial edges are paler yellow, yellow-green, and whitish (tertials only), the contrast in this aspect of their plumage is not as obvious as it is on Ruby-crowned Kinglet, which have brighter yellow whitish to yellow margins on the outer webs of the folded flight feathers. The folded wings and greater coverts of Hutton's seem to be fairly similar in color to the mantle and scapulars. Also, the tertials of Hutton's are dusky colored and less black looking than the tertials of a Ruby-crowned Kinglet.
To my eye, wingbars of a Ruby-crowned Kinglet contrast more with the rest of the wing. Until I started really studying photos in preparation for writing this piece, I didn't fully realize why. When I went online and looked at a bunch of Ruby-crowned Kinglet photos, I hadn't realized that the bases of the secondaries (the exposed flight feathers right below the lower wingbar) are black or nearly so. This field mark is mentioned in the National Geographic Guide and illustrated quite well in the Sibley Guides. I also noticed that inner webs of greater coverts, the whitish tips of which create the lower wingbar, are also black, thus darker than the rest of the upperparts. These features, particularly the black base to the secondaries can be seen well in the in-hand spread wing shots of Ruby-crowned Kinglets at this link: http://www.powdermillarc.org/highlights/2009/latefall.aspx (you'll need to scroll down once you reach the page). I also noticed that the tertials (the inner most folded flight feathers) are near black with crisp white margins on Ruby-crowned Kinglets.
When I compared the features described in the paragraph above to the same aspects of a Hutton's Vireo's plumage, I noted that neither the greater coverts nor the base of the secondaries are as dark on Hutton's. These feathers are more dusky and generally look greenish rather than black. Additionally, the outer webs of the greater coverts (again right above the lower wingbar) lack the bright yellow edges that can be seen on the same feathers of a Ruby-crowned Kinglet. Collectively, these differences combine to make the wing pattern of a Ruby-crowned more crisp and contrasty than the wing of a Hutton's Vireo.
The one new thing that I noticed as I studied photos of these species is that Hutton's Vireos consistently look more scruffy faced than Ruby-crowned Kinglets. The feathering on the face and throat area of a Ruby-crowned seems to lay more neatly, giving this species a very smooth, mousy look to the head and face. However, on Hutton's the facial and throat feathering, particularly the area in front of and below the eye and down into the malar region, is often quite scruffy or ruffled-up looking.
There are two final things that I usually see when I compare these species. First, the head of a Hutton's Vireo almost always looks disproportionately large compared to the overall body size, while the head of a Ruby-crowned Kinglet seems to fit the body size better. Also–and this is mentioned in many field guides–Hutton's have a broad pale yellow loral stripe that connects with the front of the eye ring to create a slightly spectacled look. Ruby-crowned Kinglets are also fairly pale in the lores, but they generally don't show a loral stripe connecting from the base of the bill to the front of the eye ring, thus their face is quite plain.
Like most birds that are superficially similar in appearance, there is no substitute for repeated study. However, if you can commit these basic elements to memory, I think you'll quickly develop a better search image for each species. As these search images become ingrained, you'll begin to more readily see what is different and be less inclined to get hung up on the similarities. You will also start to associate differences in their appearance with the differences in their movements.
Like other vireos, Hutton's tend to feed in a hop, stop, head swivel sequence. They are more inclined to sit in one spot for several seconds at a time, all the while turning their heads back and forth looking for non-flying prey (mostly grubs and larvae). As a general rule, they don't flick their wings much and rarely go after prey on the wing. That said, Hutton's Vireos tend to flick their wings and flit around more than other vireo species, which are even more deliberate and lethargic in their movements. Compared to Hutton's Vireos, Ruby-crowned Kinglets are perpetual motion. They flick their wings frequently as they move about and often jump or flit into the air after prey (including flying insects). They also hover as they glean prey from the undersides of leaves.
For most birders, encounters with Ruby-crowned Kinglet far outnumber their encounters Hutton's Vireo, which has a more limited range and is more habitat specific. I can't over-emphasize the importance of studying common birds. You'll be surprised by what you notice when you take a closer look at birds that you already easily recognize. If you don't occasionally take the time to linger on a Ruby-crowned Kinglet and come to know it well, your chances of recognizing a Hutton's Vireo when one is before you will be greatly diminished. Hopefully, these hints will get you started down the path of readily identifying and recognizing these two species.