A Silver Swallow

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Here's a comparison with an adult male Violet-green Swallow. Note the band of gray across the breast and how the bank broadens out in the middle and extends a bit farther down the breast. 

While birding east of Portland, Oregon on 4 August 2013, Shawneen Finnegan, Rich Hoyer and I found an odd-plumaged pale swallow in a large mixed group of Tree, Violet-green, Barn, Cliff, Northern Rough-winged and Bank swallows. It was mostly white below and the areas of the upperparts that would typically be dark, were instead pale silvery gray. 

I took several photos of the bird and we watched it on and off for nearly 45 minutes. Based on what I could see, I thought it was a hatch-year Bank Swallow. It was smaller than most of the other swallows and by direct comparison it appeared slightly smaller and shorter-winged than a Violet-green Swallow when perched. It spent most of the time perched on the wires with about 200 other swallows, so we got a good feel for its size and proportion. We did see it fly a couple of times. In flight, the shape and proportion seemed to best fit Bank Swallow in my opinion. Rich thought that it might be a Tree Swallow.

I've seen a number of leucistic or at least partially leucistic swallows over the years. They have been either white/or dappled white, or in a couple of cases creamy-tan overall. None displayed this sort of silvery-gray quality. In fact, until recently, I had never seen any species of bird that showed this sort of gray plumage anomaly. Just recently, Cathy Sheeter found and photographed a mostly gray Swainson's Hawk (I believe in Colorado), which had essentially no brown coloration. She shared the images with Jerry Ligouri, who posted them on his blog. http://jerryliguori.blogspot.com/ 

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In the harsh afternoon light, the pale gray band across the breast was often lost in the glare off of the otherwise white breast. In both of these images (above and below) you can see a dark area on the side of the upper breast right below the shoulder. In several images, the darker auriculars appear to extend below the imaginary line extending back from the gape and then there is a paler comma mark behind the auriculars. This pattern is suggestive of Bank Swallow. 

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Here's the best of two flight shots that I captured. 

I've spent quite a bit of time online trying to find references to similarly plumaged swallows and found none. I also couldn't find any references to plumage anomalies that involved darker feathering being replaced by gray feathers. If anyone is aware of a description/explanation of this plumage aberration, I would like to learn more about it. 

1

Didn’t find gray or silver but a couple articles on albino swallows show some pictures where they are not white but gray. Just thought you might not have thought to look at albino articles.

http://www.naturephoto-cz.com/barn-swallow-albino-photo-1520.html

http://www.birdphotographers.net/forums/showthread.php/45162-Albino-barn-Swallow

http://www.starhq.com/feathered-friends-albino-swallow-stands-out-in-a-flock/

2

What about R r diluta subspecies rather than R r riparia. Perhaps an anomaly would be a more likely answer than this asian subspecies?

3

The metabolic pathway for producing brown melanins (Phaeomelanins) is different than the one for black melanins (Eumelanins) and they are usually distributed to different places in the feathers. My guess is that this bird is not producing phaeomelanins (presumably because of a genetic defect), but is producing eumelanins. Structural interference parts of the feather which would normally be brown are muting the underlying black pigments producing silver.

But that’s just an hypothesis and I’m willing to be talked out of it…

4

While waiting here for a client to arrive I did a quick search of the web for Bank Swallows…Sand Martins…from Asia, I saw a few grayer birds but not as apparently light gray as yours. I assume most of the pics I found were of our typical subsp.. so not much help.

5

Thanks for all the great comments and for posting them directly to the site. Links to articles on these topics are always appreciated. Thanks Marlowe, I will have to check out the links that you shared.

6

I like Mike’s explanation. Here is an essay.

http://www.sibleyguides.com/2011/08/abnormal-coloration-in-birds-melanin-reduction/

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