Update 16 November – There are a couple of points I think I should clarify. First – and I guess this may be clear already – I’m still not convinced this will work, or how well it will work, and it will probably work in some situations better than others, and never be 100% effective since the hard glass surface is still exposed. If you try it please let me know how it works, good or bad.
I should have mentioned the Bird Screen, which apparently does offer nearly 100% protection, and might be the best solution for a lot of situations. I suggest the potential of the highlighter as an easy, cheap, and possibly effective solution, but if you’re really interested you should check out the Bird Screen as a proven, reliable method of preventing window strikes.
Update 17 November – It’s definitely not 100% effective; I’ve had two bird strikes in the last few hours, but that’s still only two in a total of about 16 hours, which at the previous rate would have resulted in about 20 strikes.
Update 19 November – A potentially serious flaw in the “highlighter method” of preventing window strikes has come up – the UV color fades quickly.
Here (left) is a window just 20 minutes after applying the highlighter and (right) another window after the highlighter has been on for six days. (Both photographed at night with 1 sec exposure, same distance and position of black light). The six-day-old highlighter is not as bright and obvious as the fresh ink, especially to the right in the less intense UV light, so effectiveness for preventing bird strikes presumably decreases. In order to maintain the effectiveness one would have to reapply the highlighter frequently, maybe weekly.
I’m investigating some sunlight-stable UV pigment, but it’s said to be less bright, and it’s much more expensive and can’t be delivered by pen. More on that, hopefully, later.
In the meantime, if you need immediate bird collision prevention try the Wisconsin Humane Society online store.
Estimates of the number of birds killed in window collisions each year in North America run as high as nearly a billion birds. It’s the biggest source of direct human-caused mortality in wild birds. But a simple means to prevent birds from hitting windows on your house or office could be in your desk drawer, or at least as close as your local office supply store, costing only a couple of dollars and a few minutes of your time. This needs further testing, but it appears that an ordinary yellow highlighter can be used to draw lines on the window, and those lines may be visible to birds, warning them away from the window, but are almost invisible to people.
Windows like the one below can be deadly for birds. This is my window with a bird feeder I recently set up. Placing the feeder within one meter of the window prevents the birds from picking up much speed before they hit it, so any bird collisions are supposedly non-lethal, but this reflection was so deceptive that an average of nearly two birds every hour were hitting it in early November – incredible and distressing! I left the feeder empty after a couple of days but kept thinking it would make a good opportunity to study bird-window ‘interactions’.
photo taken automatically by a Wingscapes Birdcam
I live in a house with lots of windows, and I like to keep lots of bird feeders. This is good for me as a birder, but often bad for the birds. Over the years I have tried various methods of making the windows either more visible or less harmful, but sometimes it just doesn’t help. Birds fly very fast and at times (for example, when startled by a predator) will try to fly through things they would normally avoid. Occasionally, Cooper’s and Red-tailed hawks seem to learn that they can pop around the corner of the house, cause a panic at the bird feeder, and then have easy pickings from under the window.
I found lots of helpful info on the web. Groups like FLAP have good suggestions about how to minimize the danger of window strikes here, and New York City Audubon has a detailed guide to Bird-safe building. Some of that worked for me, but some of it was either impractical or unsatisfactory for my situation. For several years I had pretty good success with simple lengths of white string hanging in front of the worst windows. This cut down on collisions, but did not eliminate them, and the string was unsightly and distracting, tricky to install, and required some tedious maintenance. I wanted a better solution.
Hypothesis: Since birds can see ultraviolet wavelengths of light and we can’t, it must be possible to add a UV-reflective coating to windows that would make them more or less opaque to birds but still transparent to humans.
Methods: In the darkened kitchen with a black light and piece of plexiglass, my kids and I tested various household products to see what, if anything, might meet the twin requirements of being visible to birds and invisible to humans.
Results: Various juices and cleaning supplies all proved to be non-UV-reflective: Pledge, Simple-Green, Windex, Rain-X, Dawn, Orange Juice, Apples, Shampoo, Conditioner,… all no.
Olive oil, yes! Drops of it look like red curry sauce under the black light, but once it’s spread thin the color is so faint it would presumably not be obvious to birds (plus it’s messy and hard to see through).
Thinking beyond “fluids” we wondered about dry-erase markers, but no, they don’t glow under the black light either.
Then I noticed the brilliant orange light from a supermarket price tag reflecting the UV. I thought of fluorescent colors (duh) and wondered about a thin wash of diluted day-glo paint, then I thought of highlighters. My son found one in the desk drawer, and… Bingo! Under the black light we could draw a pattern of brilliant yellow lines on the plexiglass, but under normal light the lines were almost invisible. Now we needed to test it on an actual window with real birds….
Here (above) is the window and new bird feeder that had been combining for an average of nearly 2 bird strikes per hour until I left it empty. I’ve already drawn on the window with a highlighter, but the lines are invisible in this photo. Below is the same window illuminated with the black light to show the grid of highlighter lines (window shades inside are glowing blue). Presumably the birds see something like this. I filled the bird feeder and went inside to observe.
As I write this on 15 November, I have spent 11 hours monitoring steady bird activity at the feeder. After nine hours with NO window-collisions, a light rain started. The highlighter markings were getting washed away and a goldfinch hit the window. So I wiped off the outside of the glass and drew new lines on the inside. That’s working so far, with no strikes in about two more hours of bird activity. Not enough data to really say anything conclusive, but testing continues….
Markings inside vs outside the glass: The first test was done with highlighter drawn on the outside of the window. That seems to work very well to deter birds but the ink I used washes right off with water. I tested the highlighter on the inside of another window and the markings appear to be equally visible from the outside. So I assume that marking the inside – where the glass is generally easier to reach and the marks will be protected from weather – would be just as effective at stopping bird collisions. Testing that now, and so far so good.
Grid size and pattern: In this first trial I’ve drawn a rough grid with squares about 2.5 to 3″ across. I’ll try to test some other designs to see if less marking or different designs give the same benefit. You could be really creative and draw architectural patterns or write ‘secret’ messages on the glass, as long as you don’t leave big parts of the window unmarked. From other research it seems that the “openings” should be no larger than 4 inches high by 2 inches wide, so maybe my grid is about right.
Other inks: It would also be interesting to test other colors of highlighters or other kinds of fluorescent ink or paint. I notice that there are commercially available “invisible” fluorescent inks, the kind that are used for admission stamps at concerts, etc. Those could be even better than the highlighter, being clear in normal light, and there’s probably a formulation that would be water-resistant for outdoor use. I’ll try to check that out too. But there’s almost no disadvantage to the highlighter, and it’s so simple and readily available.
Disclaimers: These are preliminary results, and it’s possible that further trials won’t be quite as successful, but these early results are so promising (and it’s so easy and low-risk) I wanted to get this information out there so that other people could try it and hopefully save some birds. The highlighter that I used seems to wash off easily with water, and does not stain the window frames here, but I make no warranty against staining or other damage to windows or adjacent materials that might be caused by following the above instructions.
Let me know how it works for you.