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Anatomy of an avian collision

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These impact patterns, which have been enhanced, tell a story. Feather details seen on the glass indicate that the hawk, on the right, was on a slight descent,but the dove, coming from the left, had executed a midair U-turn, an escape maneuver. Even if there were no glass, it is unlikely that the hawk could have seized the dove. (Submitted Photo)

Ka-WHAAANNG! Except that what I heard was a double “ka-whang.” Looking up quickly from my desk, I saw nothing.

Hurrying to the south picture window, I saw a rare Short-Tailed Hawk (small relative of the Red-Tailed Hawk), sitting on its haunches and dazed. I was going to dash outside — not to pick it up — but to give it a few drops of water to help it recover.

Lacking a fear reflex, a disoriented hawk will usually tolerate the approach of another creature without alarm. However, the bird was beginning to steady itself, so I opted not to disturb it, but instead get a photo of it. When I turned to get my camera, the raptor flew off in a slightly wavering flight down-slope heading southwest. However, and curiously, this did not explain the sound of a double impact.

An hour later, as I walked around the south side of my house, I surprised a Ringed Turtle-Dove that quickly lifted off from a cactus bed, headed in a southeast direction. Because it was only a few feet from where the hawk had been, I assumed it was the target of the hawk. It had three missing tail feathers, but there were no spots of blood to suggest that the hawk had struck the dove. That explained the second “ka-whang.”

Doves are fast fliers and only the fastest raptors stand a chance of catching them, such as high-speed falcons like our native Harriers. However, if a slower-flying hawk sees an opportunity to grab a dove, it will do so, as this Short-Tailed Hawk (rare in New Mexico) tried to do.

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A few hours later, thanks to the angle of the sun, I was surprised to see two clear imprints of the nearly simultaneous collision of the birds on the window. The prints are created from feather dust or “down” powder shaken loose (like powdered sugar) at the moment of impact, looking somewhat like an X-ray.

The impact patterns were unusually clear, revealing even individual flight and tail feathers, indicating the directions of flight by the two birds.

The dove, one of several living nearby, frequently flies past the house about six feet above the ground and heads west to feed on seed spread for the birds every morning. Birds, when flying across the south side of the house, can avoid the wind as well as make use of a “safe-fly” zone to avoid raptors.

However, this migrating hawk, flying 18 or more feet above the ground and speeding in the opposite direction, apparently spotted the dove, quickly sized up the opportunity and swung into a tight turn to bring the dove into its visual crosshairs. It would go into power mode to intercept its meal. Suddenly, through all of its “go, go, go” signals, there would be a warning: “Abort! Abort! Abort!” as the raptor, quickly closing in on the dove, would have seen a reflection in the glass of another hawk rushing toward ii on a collision course! With no time to redirect its flight, the hawk would have desperately pulled its head back, extended its legs and spread its wings and tail for impact.

We see the feathers clearly outlined where its sternum (breast) and individual feathers of both wings and tail took the shock. Lacking dust, the head and feet would not leave any print. However, its feet, probably already extended would help absorb the shock of impact thanks to a precious half-second alert which might have saved the hawk’s life.

The dove, on the other hand, probably saw the hawk when it was in full swoop and fast approaching. Flying a few feet above the ground, the dove would have had few alternatives except to attempt a desperate midair U-turn to starboard. From the print, we do see part of the bird’s head with the wings extended, ready for its frantic downstroke. The dove probably never saw the window or its image coming at it and struck the glass about 45 degrees into its turn.

Both birds were lucky, striking the window at an angle, deflecting them by ricochet. About the only setback, once they had regained their senses, would be nothing more than to simply grow a new crop of feather dust.

Next to feral cats, windows are quite likely the biggest hazard to bird populations. Windows have only been around for 500 years or so, a man-made invention that birds have previously never had to contend with. What’s more, an invisible window would typically appear to be not a barrier, but a portal to more sky. Yet, an overhead shadow can send the savviest of birds dashing every direction, including into a window.

Only a few unusually smart birds, like vultures, appear to understand the hazards of collisions with glass. In fact, some have learned how to turn an impending collision with a moving car’s windshield into a flight advantage. From a roadside lift-off, and by flying in the same direction of travel as the car, a vulture may veer in front of the windshield. Just prior to collision, the rushing airstream will toss the bird upward as an assist in gaining altitude.

Birds are not dumb and, for every casualty, there will be a lot of other birds taking mental notes. Interestingly, although birds in zoos quickly become accustomed to glass-windowed enclosures, for some slow-learners the glass must be covered with strips of tape for the first two or three days.

Silhouettes of any kind attached to the inside surface of window glass can help but they won’t always prevent a collision in an avian panic. Skyscrapers in large cities often turn off their lights at night to avoid creating large-scale bird-collision slaughters due to glass reflections during spring and fall migrations.

Ray Pawley, previously with Chicago’s Lincoln Park and Brookfield zoos and the Field Museum of Natural History, consults for zoos and museums. He resides in Arabela, where his research on animal behavior and physiology is ongoing. He can be reached at raypawley@pvtnetworks.net.