Unlike most members of the family Parulidae, Orange-crowned Warbler (Vermivora celata) is a species that is essentially devoid of field marks. It offers a study in the differences between greenish yellow and yellowish green, with some occasional olive and gray tossed in. It may or may not show wingbars, and may or may not show very diffuse streaking on the underparts. 

This Orange-crowned Warbler, photographed in low light on Skinner Butte in Eugene, Oregon 29 April 2010, presents an appearance consistent with the subspecies lutescens, which accounts for most of the Orange-crowned Warblers that pass through western Oregon each spring. Note the yellow eye arcs, uniform yellow underparts with diffuse dusky greenish streaking, and the general lack of gray tones in its plumage.

For all their drabness, I find Orange-crowned Warblers intriguing, in part because they are the first "migrant" warblers to reach Oregon each spring; Yellow-rumped Warblers winter here in abundance. Orange-crowneds are conspicuous migrants along the Pacific Coast of North America, second only to Yellow-rumped (among warbler species) in terms of the numbers one is likely encounter. From early April through early May they will generally be the most numerous species at many migrant traps, particularly on the cool overcast days that create fallouts.

This Orange-crowned Warbler, photographed in bright sun, also shows characteristics consistent with the lutescens subspecies. Note the fairly uniform yellow coloration below, yellow eye arcs, and lack of gray tones on the head. It does not show any obvious streaking below. Photo by Paul Rentz

What I find most interesting about this species is the variation it displays. The local breeding subspecies (V. c. lutescens) is the brightest of the three primary subspecies of Orange-crowned Warbler.  They can be almost lemon yellow below and rich leafy green above. They have yellow eye arcs that nearly connect to create a broken eyering and they often look a bit streaked below. Most of the Orange-crowneds that I see each spring are of this subspecies. They can be quite variable in appearance, with presumed females tending to look less yellow below. 

This lutescens was on Skinner Butte, Eugene, Oregon 29 April 2010. Its duller overall coloration suggests and lack of obvious orange tinge on the crown suggest that it is a female. Note that it is lacking in gray tones, has yellow eye arcs, and shows weak streaking below. Photo by Dave Irons

The duller and more northerly V. c. celata breeds across boreal Canada and Alaska and is presumed to be a low-density spring and fall migrant along the west coast. This subspecies displays a lot of gray tones and dull olive in its plumage. The yellow undertail coverts and vent often stand out as the brightest feathers on the bird. They have white eye arcs that form a near complete eyering. They tend to look unstreaked below. The bulk of this population is presumed to migrate east of the Rocky Mountains. The degree to which this subspecies migrates north along the Pacific Coast is at best poorly understood. Dunn and Garrett (1997) discuss celata as a fall migrant along the Pacific Coast, but do not describe its status as a spring migrant through that region.

A third subspecies (V. c. orestera) breeds in the Intermountain West northward along the Canadian Rockies to northern British Columbia. This subspecies is generally described as being more brightly colored than celata. Their most conspicuous feature is a very gray head that contrasts noticeably with the yellow underparts and green back. They also have white eye arcs. Fall immatures suggest a dull immature McGillivray's Warbler. They are somewhat regular fall migrants along the West Coast (Dunn and Garrett 1997) and I've seen a handful of spring migrant Orange-crowneds in western Oregon that showed characteristics consistent with this subspecies. 

This Orange-crowned, photographed at the Chico Basin Ranch, Peublo Co., Colorado on 17 May 2008, is presumed to be of the orestera subspecies, based on range and the combination of uniform bright yellow underparts contrasting with a grayer head (creates a hooded look). It also shows white eye arcs. Photo by Larry Semo.

In Oregon, we get two distinct waves of migrant Orange-crowneds. The early birds, which arrive in late March and peak during the first two weeks of April, appear to be universally lutescens. They show no obvious gray and only minimal olive tones in their mostly bright yellowish-green plumage. Then, from late April until mid-May, we get a secondary wave of Orange-crowneds that are much duller overall. They often have distinctly gray heads and lots of gray tones on their backs. Their underparts are pale greenish yellow and usually look a bit washed out. The gray tones make them look mostly dull olive-green. In many cases their overall appearance looks intermediate between lutescens and photos of celata taken within their known range. Surely intergrades occur and it may be that many of the duller, grayer spring birds in western Oregon are intergrades. 

This bird, photographed on Skinner Butte in Eugene, Oregon 29 April 2010, is a classic example of the duller-colored Orange-crowned Warblers that pass through western Oregon during late April and early May. Its combination of paler underparts, significant gray on the head and upper back, and white eye arcs are suggestive of the subspecies celata, but this bird is greener than most of the photos I've seen of known celata. Photo by Dave Irons

My presumption, given the appearance of these birds and the timing of their passage, is that they are celata or at least celata/lutescens intergrades. However, to date there is no authoritative work on how to separate the three subspecies discussed here in the field. While I think that most of the duller Orange-crowned I see from late April on are probably celata, I have (as mentioned above) seen a few birds that were suggestive of orestera. 

I can't offer any definitive answers to the questions posed above, but I will continue be to take pictures of as many spring Orange-crowneds as I can in an effort to better quantify which subspecies are passing through Oregon in the Spring. A couple of long-time Oregon banders tell me that they have captured individuals that were keyed out as celata (Mike Patterson and Dennis Vroman pers. comm.). The photo below shows a presumed celata banded in Josephine Co. Oregon. 

This bird, photographed in Josephine Co., Oregon on 10 May 1992, was presumed to be a celata based on the overall dull plumage, pale underparts, extensive gray on head and nape, white eye arcs, and brighter yellow undertail (can't be seen in this image). Photo by Dennis Vroman

Hopefully, this discussion inspires others to take a closer look at the Orange-crowned Warblers and to photo-document celata-like birds along the West Coast. At this point, there are more questions than answers. 

They say that a picture is worth a thousand words. Sometimes I'm not sure a thousand will do. When it comes to the magic of refracted light, it's probably best to sit back and enjoy the view without getting too wrapped up in the mental gymnastics of trying to understand how we see what we see. It is important, however, that birders have a basic understanding of how the colors we think we are seeing are affected by a number of factors.

Perhaps no other group of birds captures our imaginations like hummingbirds. The iridescent flash of a hummingbird's gorget (bright throat feathers) can be almost blinding  at just the right angle. Then, with a quick turn of the head, it's gone! These two images of the same male Ruby-throated Hummingbird were taken moments apart. In the bottom image, incoming light is being absorbed and not reflected back to the camera lens and our eyes. When this happens, feathers generally look black. (Photo by Mark Szantyr)

In general, I'm a person of modest curiosity, the kind who needs to know what time it is, not how the watch was built. Yet, on occasion some topic catches my attention and I can't resist drilling down into the minutiae. My initial interest in the variability of feather iridescence got started when I noticed that a Eurasian Green-winged Teal photographed on Adak Island, Alaska appeared to show wing patches that were different colors (see "Can We Trust the Colors We See? published in this journal in March 2010).

One of the first things that I learned as I started researching the color variation in the wings of the teal was that blue coloration in birds is not the result of pigment. Instead, it is produced when light is refracted by the uneven surface of the feather. In effect, the structural characteristics of the feather bend (refract) the light like a prism, resulting in a spectrum of color that is not produced by pigment. Since feather structure plays a role in this process, feather condition (wear) will affect how much or how little color we perceive (see photos below).

I have to admit that since learning about how blue appearance occurs in birds, my appreciation for Western Scrub Jays has increased dramatically. This is a species that I see many times a day, yet, until recently, I have rarely stopped to take a closer look. A few days after posting the teal discussion, I found a roadkill Western Scrub-Jay. I took advantage of the opportunity to capture some close-up images of one of its tail feathers, which normally appear to be a rich azure blue on a live bird. 

The pictures were taken from a variety of angles with light reflecting off the surface of the feather back to my eye. Then I took some pictures holding the feather up so the light took a direct path through the feather to my eye. As I learned in my earlier research, when reflected, the incoming light waves are altered by the prismatic effect of the feather, producing the blue coloration one expects to see on a Western Scrub-Jay. Conversely, when the light passes directly from its source (the sun in this case) through the feather to my eye, the feather appears to be a dull medium gray.  The images below show a single scrub-jay tail feather (rectrix) from a variety of angles. As you can see, light angle has much to do with the intensity of color that we perceive.

This image of a Western Scrub-Jay tail feather was taken with the light source directly behind the feather. At this angle, little if any refraction occurs, thus the feather shows only the color that is produced by pigments. Under such conditions, a scrub-jay tail feather looks gray. It is important to note that the outer half of wider outer web of this feather is getting quite worn. Notice how it shows less structural pattern and is paler gray. (Photo by Dave Irons)

In this picture, the incoming light source is from above, thus it hits the feather and reflects back to the camera lens (and my eyes). In this low-light situation, the blue is darker and fairly rich because it is not being washed out by glare. (Photo by Dave Irons)

This image, taken in much brighter sunlight, shows the effect of glare. Notice how the blue on the inner web (the narrow section below the feather shaft) looks much paler than it does on the photo above. Also, note how feather wear affects the amount of refraction. The outer third of the outer web (the wide section above the feather shaft) is quite worn, reducing its prismatic qualities. Thus it looks flat gray rather than blue like the unworn inner two-thirds of the outer web. (Photo by Dave Irons)

Compare the amount of structural pattern showing in the less worn inner section of the outer web (above) to the heavily worn outer section of the outer web on the same feather (below). Feather condition plays a significant role in the amount of color we see in birds, particularly when it comes to colors like blues and greens, which are produced by refracted light and not pigment.

The lesson to be learned from this discussion is that colors we see are far from absolute. In fact, they are quite plastic. The combined effects of light angle, light intensity, feather wear, and the vagaries of our individual eyesight should be taken into consideration as we assess the colors we think we are seeing. Whether a bird appears blue-green or greenish blue, greenish yellow, or yellow-green is a matter of perception.

In a recent post to Oregon's statewide birding listserv, veteran observer Wayne Hoffman shared that on two different days during the second week of April 2010, he had seen alternate-plumaged Common Loons (Gavia immer) at Newport, Lincoln County, Oregon that showed green iridescence on their lower neck collars. This observer went on to note that he had not ever seen a Common Loon showing bright green on the lower neck collar. His subsequent search of field guides and other references found very little mention of green iridescence in connection with this species. 

Wayne's post reminded me of a similarly plumaged loon that I photographed at the same location in April 2009. The alternate-plumaged bird that I observed (see images below) showed the same green sheen he was describing on both its head and on its lower neck collar, with the lower collar being particularly colorful. I can remember being a bit taken aback by how green it looked at the time and how I was completely unaware that a Common Loon might show such coloration. Then it occurred to me that I'd never before been so close to an alternate-plumaged loon in crisp sunlight.

In just the right light, the green iridescence on an alternate-plumaged Common Loon can be quite striking, as was the case with this bird photographed at Newport, Lincoln County, Oregon on 26 April 2009. (Photo by Dave Irons)

A quick check of the most popular North American field guides revealed that nearly all of them illustrate alternate-plumaged Common Loons with a black head and black lower neck collar that are separated by an incomplete upper collar that is mostly white with narrow, dark vertical lines. The accompanying text in these volumes generally describe the head as black, which is how we perceive it in most light conditions.

Here's another shot of the same bird at Newport, Oregon on 26 April 2009. (Photo by Dave Irons)

Of the guides that I own, the 4th edition of National Geographic's Field Guide to the Birds of North America seems to best depict this green-headed appearance, but offers no description of the summer plumage in the text. The next best illustration that I found was in my 3rd edition of Peterson's Western Guide (1990). I don't own the most recent editions in the Peterson series, but I was a little surprised to see that the Common Loon in the 5th edition of Peterson's Eastern Guide (2002) showed a more purplish-blue gloss to the head and none of the green tones that are apparent in the earlier Western Guide. Maybe this was just a matter of printing, or perhaps the result of an editorial decision to show this species as we most often see it. In the Sibley Guides, which to my eye offer the best overall set of illustrations, alternate-plumaged Common Loons are shown to have black heads, which is pointed out with arrows and labeling. Perhaps, like others, Sibley made the editorial choice to show his Common Loons with black heads because that is how they appear to us most of the time.

After reading this piece, USFWS biologist Roy Lowe passed along this photo of a Common Loon that he saw on Yaquina Bay at Newport, Lincoln County, Oregon on 10 April, 2010, the day of Wayne Hoffman's original observation. It shows the green iridescence on the head and neck collar more clearly than the two photos above. (Photo by Roy Lowe). 

One point that is important to remember, iridescent colors are not produced by feather pigmentation. Instead, the glossy blue, green, and purple hues that we perceive are the result of light refracting off of the intricate structural characteristics of the feather. That being the case, whether we actually see these colors and how intense they appear is highly dependent on lighting conditions. Generally speaking, the brighter the sunlight, the brighter the apparent coloration. 

At the very least, we hope this discussion inspires you to take a closer look the next time you see an alternate-plumaged Common Loon and to better understand how lighting affects the colors our brains are telling us we are seeing.