Tuesday, November 20, 2007

More on windows and birds

After a week of pretty good success on the windows, today was cold with light snow, and the bird feeder was very active. I realized last week that one of many variables I need to consider when I record bird-strikes-per-hour is bird activity, but I haven't tried to record that yet other than excluding long periods when no birds were present. Obviously, if there are few birds visiting, there are fewer chances for collisions. At the other extreme, like today, when the feeder gets crowded there seem to be birds in the air all the time - hovering and jostling for a perch. In these conditions there are many more chances for collisions.

So today I started observing and recorded 6 window strikes in 1 hour 40 minutes - about one every 16 minutes. Thinking that this was happening because of the faded highlighter I reapplied the lines, and in the next 3 hours 30 minutes there were 8 strikes, or about one every 33 minutes - about half as frequent but still pretty distressing and not really a significant improvement. However (one can always find a hopeful angle) five times during that period birds flew in and landed scratching at the windowsill. Maybe these were birds that would have crashed into the glass, but recognized the danger at the last second and fluttered harmlessly down to the windowsill.

A physics lesson
Some of my assumptions about UV light and what birds perceive were wrong, and I've been educated by some posts on the NikonGear UV photography forum, and some follow-up research. What we see under the black light is fluorescence. Certain compounds are "excited" as they absorb UV energy and then - when electrons return to their normal state - they fluoresce: releasing energy as light in the visible range. So this is why day-glo colors appear intensely bright in sunlight - UV energy from the sun causes fluorescence, and the colors actually glow.

Therefore my experiment with the black light has very little to do with UV other than as the light source. The black light is good for finding fluorescent things, and fluorescence begins with UV absorption, but many things absorb or reflect UV without fluorescing and we simply can't see it. A search for UV-visible substances to mark windows would require much more specialized equipment. You can see some really cool examples of UV photography here.

The fluorescent highlighter should still be visible to birds, because the fluorescent ink will simultaneously absorb UV and release visible light. But the visible fluorescence in daylight may be about as obvious to the birds as it is to us (i.e. not very). So if the highlighter works it may work largely because it absorbs UV, or maybe the combination of absorbing UV and releasing visible light is more obvious to the birds. Either way, that process requires a UV light source, and another issue that I think might explain today's poor results (still searching for that hopeful angle) is that UV wavelengths are blocked by clouds. Apparently "thin" clouds allow about 60-80% of UVA (the longest wavelengths of UV and the range that birds can see) to pass through, but thicker clouds block most UVA. So with little or no UV, such as during today's snowstorm, the highlighter marks would neither absorb UV nor release fluorescence, and the birds' vision would rely on the same visible spectrum that we see. If this is true then all efforts to use UV-related markings on glass will only have limited effectiveness (but don't take my word for it). So I'll have to note weather conditions in the future as I record window strikes to see if the highlighter is more effective on sunny days.

By the way, window glass blocks some shorter UV wavelengths, but at least 90% of UVA apparently does pass through ordinary glass.

So what does this mean?
Well, clearly the highlighter is not the "magic bullet" that everyone is hoping for. I'm still anxious to hear from others who have tried it. So far I have two responses indicating that it works, and I'd like to hear more, but if anyone has tried it without success I'm especially interested to hear about that.

If it does work, it will presumably work best in sunlight, and worst in low light or on overcast days, and unfortunately bird feeders are generally most active at those times.

As I said previously, we can keep trying to find ways to make the reflection in the glass look unattractive to the birds, but there will always be things (like hawks) that look even less attractive and cause birds to try to escape through the window. To truly prevent window strikes will require a barrier such as t
he BirdScreen.


cyberthrush said...

not directly related to the UV discussion, but thought this post from a Carolina listserv may be of general interest to folks, re: window strikes (sorry for the length):
> Just as an aside to this discussion, I can say pretty confidently that birds NEVER break their necks when they hit a window.
I have dissected dozens of window kills. I have worked in or curated bird collections at several different universities, and these days, a large proportion of specimens come from donated window kills. None that I've seen have ever had broken necks. They all have brain injuries (usually quite visible as pooled blood inside the thin skull). Bird necks are so strong, long and flexible, that breaking one would be kind of like trying to break a rubber band by throwing it at a wall.
Though this might seem like trivia, it's actually useful knowledge to
have when trying to figure out the broader impacts (sorry!) of bird
strikes. If you assume what kills a bird that hits a window is a broken neck, you would tend to be optimistic that if a bird survives
the initial strike, it's OK. On the other hand, if you've seen as much
bleeding on the brain as I have, you tend to think that even those
that hit and fly away may have bad prospects - just as with human brain trauma, a slow bleed may lead to death hours or days later. And
even if the bird has only "moderate," survivable brain
damage, the prospects of long term survival with brain damage are
probably next to nil in the wild.
I'm sure some birds kept warm until they can fly do recover from the
concussion and do fine. That's worth doing. But I'm equally sure
others go on to certain death, which only emphasizes the importance of the efforts people are taking to prevent strikes in the first place. Keep it up!
> Chris
> Christopher E. Hill
> Biology Department
> Coastal Carolina University
> Conway, SC 29528-1954
> chill AT coastal.edu

Anonymous said...

Your statement on the birds "scratching" at the window made me wonder if the highlighter grid looks to them like a wire grid fence and that they might be trying to land on it for a nice perch right by the feeder.

sitta said...

As per cyberthrush's comment above-
There have been at least two papers published on why birds die from collisions, both by Daniel Klem of Muhlenberg College, both of which support your crossposted comments: that intracranial hemorrhaging was the suggested cause for nearly every fatality, and that there can be long-lasting effects ranging from short-term disability to paralysis and eventual degeneration into death...

Klem, D. 1990. Bird injuries, cause of death, and recuperation from collisions with windows. J. Field Ornithol., 61(1):115-119


Veltri, C.J. and D. Klem. 2005. Comparison of fatal bird injuries from collisions with towers and windows. J. FIeld Ornithol. 76(2):127-133.

Bisbee Border Birder Bloggers said...

Sorry to hear that the UV/fluorescence angle doesn't seem to work consistently. I got so excited reading the original post that I jumped up and tried a dry fluorescent highlighter on the nearest pane of glass (thought it might be wax-based and more weather resistant, but it washed right off).

I've been wondering about the effectiveness of narrow strips of CollidEscape. This micro-perforated film, which can be purchased direct from the manufacturer with a donation back to FLAP, is transparent from the inside but opaque-looking whitish from the outside. It's costly to cover large areas of glass with it as a continuous film, but a closely spaced grid of narrow strips could minimize bird strikes with limited degradation of the view. If only it were available as a roll of tape... See http://www.flap.org/new/film.htm for more info.

I also noticed at Lowe's home center the other day that there's a new window screen material that's more transparent than traditional wire or plastic mesh. My first thought was whether it's as good at preventing strikes as traditional screens.

We've had good success with light-colored blinds both at home and at the Southeastern Arizona Bird Observatory's field station. There's still the occasional strike on the large field station windows when the blinds are open, but we keep them closed when the viewing room isn't in use.

Laura Erickson said...

I've posted photos of several successful ways people are preventing bird strikes at such buildings as the Rowe Audubon Sanctuary on the Platte River in Nebraska and the Cornell Lab of Ornithology on my own website, at

It's a very important issue.

Lee Ann said...

Although I haven't conducted a scientific study of the before-and-after effects of using fluorescent markers on our sunroom windows, I have noticed a marked reduction in thumps since scribbling a rough grid on the window interiors. I first did two of the windows, and even at that, there were fewer hits. After we removed the screens for the winter and I was able to access and mark two other windows, I haven't heard any strikes at all. I'm not here all the time, but I usually heard possibly 1-3 strikes a day; now it's down to 0 (as far as I know) in the week and a half since trying this.
Six of our sunroom windows have built-in venetian blinds and those work well to deter the birds. I didn't use the markers on them. I'm optimistic that this might work, even though fluorescence is different from UV.

Christine Okon said...

The potential of "black light" material is intriguing. I checked out http://science.howstuffworks.com/black-light.htm to learn a bit more. Maybe that old psychedelic Hendrix poster would deter birds. How about Invisible Ink, visible only with black light (and maybe to birds)..it may last longer. http://www.globright.com/invisibleink.html

Lee Ann said...

While I was at one of the windows yesterday, and looking out at the snowstorm, a goldfinch flew into it right in front of me. I had a chance to observe exactly what happened. It took off from a feeder, flew at the window, drew up about a foot away and thumped lightly, flying away with no visible signs of injury.

grant said...

coops., sharpies, and shrikes have been documented to follow their avian prey to their deaths as well. ian newton said the european sparrowhawk's main mortality factor is window collision. the british hawk and owl trust areed and indicated roadkilling for barn owls and c. kestrels, though nthey, too, fall victim to windows (www.hawkandowl.org). also try www.birdstrikes.org. 100 million to 1 billion birds in the us alone die each year at windows. in australia, the swift parrot is critically endangered because of windows and habitat destruction, and global warming. non-native oriental storks in belize crash and die at windows. in germany, a whole barn swallow colony met its anilation at windows.

grant stevenson
http://pahawkowl.livejournal.com; to be updated soon
em pahawkowl@yahoo.com