The following photo has been the subject of a discussion on the ID-Frontiers listserve recently. It’s a Swainson’s Thrush, but is it the first-ever record of a melanistic Swainson’s Thrush, or just a normal bird seen in shadow?
It certainly looks dark, but it is also in the shade. Points in favor of the “shadow” hypothesis include: the pale eyering and pale breast are visible and look normally-colored, the dark gray color of the back is matched by shaded parts of a normal Swainson’s Thrush in the same photo (see below). Points in favor of the “melanistic” hypothesis include: it looks really dark, the flanks (especially below the bend of the wing) seem darker than one would expect, and Brush Freeman (the photographer) says it looked very dark in his brief views in the field.
With another Swainson’s Thrush in the photo for direct comparison, the impression of darkness becomes even stronger, but two things are happening to reinforce that impression. First, the normal-looking thrush is in dappled sunlight, so parts of it’s body show the typical brownish color and we automatically interpret the darker parts as shadow. The dark-looking bird is entirely in shadow and gives us no other reference point to judge its color. Second, the normal-looking bird is turned so we see more of the pale breast and belly, while the dark bird is turned in a way that hides almost all of the pale underside, reinforcing the impression of darkness. I think it probably is a melanistic Swainson’s Thrush, but it’s remarkable that the photos are ambiguous, and I suspect we will never know for sure.
We do not perceive color absolutely, we see relative colors, and we are constantly making mental adjustments so that we can interpret the “true color” of a thing even as it is moves from shadow into sunlight, or when viewing the world through yellow or blue or rose-tinted glasses. A striking example of how we can misinterpret colors is shown by this example from the Lotto Lab.
On this cube, the two brown tiles marked with white arrows appear to be very different colors, but that is only because we expect the tile on the shaded side of the cube to be darker. We interpret the relative colors of these tiles automatically, making allowances for the brightly-lit and shaded sides so that we see the red tiles, for example, as the “same” color on the top and side of the cube, just in different lighting. When confronted by a tile that is actually the same color, it is impossible for us to overrule our brain’s interpretation of shadows, and impossible to see those two brown tiles as the same color, even when the proof is shown.
In the field, with birds constantly moving from one setting to another, it would be easy to misinterpret the conditions of light and shadow at any particular moment, and automatically “adjust” our color expectations to give a misleading result. The overall impression of a bird’s color is something we develop over multiple observations under many different conditions.
Another example from the Lotto Lab shows two different cubes, one suffused with blue light, the other with yellow light. Compare the tiles marked with white arrows.
In this case we judge the color of the plain gray tiles relative to the colors around them, and they take on a color cast opposite that of their surroundings. This is an example of the “contrast effect”, which makes cold water feel warm when our hands are cold. We are struck by the bright yellow plumage of an Orange-crowned Warbler when we see it among sparrows and dry weeds in winter, but the same bird in a field of sunflowers in August would look positively gray.
Getting back to the Texas Swainson’s Thrush, it’s clear that our perception of color is subject to all kinds of illusions. What appears to be a very dark bird may be just a normal Swainson’s Thrush in shadow and turned away from the camera. Or it may be melanistic. These two photos are the equivalent of a momentary glimpse in the field, showing the bird only under one set of conditions. Figuring out exactly what is going on would take longer views or more photos, and we may never know this bird’s true colors.