Tuesday, September 1, 2015

Dove Impression

Final impression of a dove  -  Photograph by Nancy Kaproth
Our friend Nancy in Ithaca, NY arrived home one fall day to find this imprint on her south facing window.  She described it as "...like the Shroud of Turin - distinct features of a mourning dove who had perhaps taken a recent dust bath.  Wingtip to wingtip measurement of 18" confirmed the identification (wingspan 17-19")."

Scene of the collision  -  Nancy Kaproth
I forwarded it on to Lisa Berger who sent back this comment:
"Interesting images. All of our Mourning Dove window hit impressions have been made as the dove has taken off the ground, hitting the window almost parallel to the pane. This produces an almost full ventral mark of flight and tail feathers, so ID is easy. Some of this may be dust, but the birds have a lot of dander from feather wear and mites: Think of the stuff in our bed pillows.

This impact appears perpendicular, or head-on, so you don't get a good view of the body proportions. The bird had been airborne for a distance; could not have just taken off from the ground under the feeder. 
Did they find the body? If it was being pursued by a raptor, it may have been picked up after bouncing off the window. This is an increasingly common practice used by Cooper's Hawks. Not in the sense of intentionally driving prey into a window, so much as just not abandoning it after it hits. Flopping on the ground is probably hard to resist, too."
This is not an uncommon phenomenon.  There is actually a Flickr collection of 292 Bird Imprints on Glass.  Some dramatic images are seen here.  The cause of the image hasn't been studied but some speculate that it is powder protecting the newer feathers.  According to one source:
"The imprints are caused by the bird’s powder down, a special type of down which helps feathers to grow. In some species, the tips of the barbules on powder down feathers disintegrate, forming fine particles of keratin, which appear as a powder, or ‘feather dust’. When a bird strikes a glass pane, the powder is shaken lose and adheres to the glass."
A further description from Wikipedia:
"Some birds have a supply of powder down feathers which grow continuously, with small particles regularly breaking off from the ends of the barbules. These particles produce a powder that sifts through the feathers on the bird's body and acts as a waterproofing agent and a feather conditioner. Powder down has evolved independently in several taxa and can be found in down as well as in pennaceous feathers. They may be scattered in plumage as in the pigeons and parrots."
Bird window collisions are a common cause of bird mortality, estimated by the U.S. Fish and Wildlife Service to cause up to up to one billion bird deaths a year.  Some are merely annoying as when a cardinal or robin repeatedly attacks a window to drive an intruder (its own reflection) away from a nesting area.  Much more serious is when a bird sees a reflection of the landscape and flies directly into the glass at full speed.  The highest bird mortality occurs windows in tall building windows where light distracts the bird as described in Allaboutbirds.com.

Bonus Points
Now go back to the top picture and look for the image of a different bird, an apparent owl above the "dove's" head.  Several of us have been wrestling with this for weeks with no answer.  All of Nancy's pictures taken at different angles are at this link for you to study.  I will discuss our theories and answer some naturally occurring questions in a blog a week from now.

On the other hand you may want to skip this as it may drive you crazy also!

2 comments:

  1. This is a real mystery. Photographers probably have the key to solving it; there must be a rational explanation! This inquiring mind won't rest until it has closure. HELP.

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  2. I remain staunch in my impression that the second bird face is that of the pursuing raptor. A small hawk? Perhaps a Sparrow Hawk, Pigeon Hawk or a Sharp-Shinned Hawk? Even possible is a Cooper's Hawk has been sighted hunting in my backyard (many bird-feeders are out year round).

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