Photo by D. Sillman
(Click on the link to listen to an audio version of this blog … Bird and windows part 2
Last week we started a discussion of why birds fly into window glass. We outlined the scope of the problem, talked about some bird ocular biology and began to enumerate some of the aspects of building locations and features of windows that contribute to bird/glass collisions. This week we’ll continue to discuss the features of windows that contribute to increased collisions and then talk about some possible solutions to this very significant problem!
The nature of the window glass itself has an impact on bird/glass collisions. Reflective glass is much more likely to lead to bird strikes than clear glass especially if there is vegetation around the window that is being reflected in the glass panes. Interestingly, very few bird collisions with windows are attributed to birds attempting to “fly through” the window glass, although visible house plants on the inside of a clear window can lead to increased numbers of bird/glass collisions.
Photo by Teipangshan102, Wikimedia Commons
Heights of the windows influences the probability of bird/glass collisions. Large windows at ground level and large windows ten feet or more above the ground have increased numbers of bird strikes compared to smaller windows or windows located at intermediate heights from the ground.
The time of year affects the number of and the nature of bird strikes: in winter, birds feeding at bird feeders near windows are most likely to strike the window panes. In spring and fall, non-resident (migratory) birds are most likely to strike windows. Interestingly, in the summer window strikes are relatively uncommon possibly due to the decreased ranges over which most birds move during their nesting and breeding season.
The time of day affects the number of bird/glass collisions. Most bird strikes occur in the morning unless other factors (like bird flocks flying from feeder to feeder or yard to yard) influence the movement of the birds.
There seems to be no particular selection process occurring when birds strike window glass or any particular type of learning going on. In most window strike studies there is no pattern of age or gender in the strike (although one study did report that in a particular area, juvenal individuals of the most abundant bird species were involved in most of the window strikes while in the less abundant bird species, adults were involved in most of the window strikes). Since the damage caused by the window strike is typically severe and overwhelmingly fatal, there is no opportunity for learning to occur that might lead to the birds modifying their flight behaviors around the glass panes.
Photo by Jiuguang Wang, Wikimedia Commons
Light may or may not influence window strikes. Few bird strikes occur in lighted windows at night. The most likely explanation for this, though, is that very few birds are out flying around at night. Migrating flocks of birds, though, do fly at night and can be attracted to the bright lights of cities. Many migrating flocks fly at such high altitudes that ground lights do not influence them, but at dusk and dawn when the flocks are taking off or landin or on nights with a low cloud cover or adverse weather conditions, migrating flocks fly at lower altitudes. These lower flying birds may be able to see the lights of cities and the illuminated windows of urban buildings. Under these conditions the urban light sources may be both an unwanted attractant and a bewildering and disorienting force for the birds that may lead to their exhaustion or to fatal window and/or building collisions. A number of North American cities have established “Lights Out” programs during peak bird migration periods. These programs seem to have been effective in reducing the influence of urban lights on the passing, migratory flocks.
Photo by Window Gem, ETSY
Silhouettes affixed to windows (often silhouettes of large birds like hawks) are not effective in preventing bird/window collisions. Putting the silhouettes on the inside panes of the windows, in particular, completely hides the image from the visual field of the birds (they are only able to see the outermost side of a pane of glass in a window). Even placing large images on the outside pane of the glass windows does very little to prevent window collisions unless the pattern density of the images is sufficiently close together to guarantee perception by the birds.
Some very elegant experiments have shown that birds can see a window pane only if the outer glass surface of the window has a dense pattern of lines or shapes covering all of its surface area. A rule has been coined to describe these patterns: the “2 X 4 Rule.”
The “2 X 4 Rule” basically states that the outer glass pane of a window must be completely covered with either thin (a minimum of 2 mm wide) horizontal lines that are two inches apart or similarly thin (again a minimum of 2 mm wide) vertical lines that are four inches apart. These patterns generate a grid that birds can see and can act to prevent 90% of bird/glass collisions.
Photo by Duncraft
The lines or dots in the 2 X 4 grid can be painted on the window, or they can be threads or strings or they can be special plastic strips that are UV reflective! These UV reflective strips have a great advantage over the other types of window pattern generators in that they are not visible by humans! Thin painted or threaded lines in either of the 2 X 4 grids only block 7% of the windows surface, but the UV reflective strips block 0%! Several companies are currently manufacturing windows that have built in UV strips, and one plastic manufacturer has developed a process to make clear sheets of plastic with UV reflective designs that can be affixed to the outer glass pane of a window.
There are other ways to make an existing window bird collision proof. A window can be covered with an outside sunshade or louvre to prevent birds from striking the invisible glass. A window could also be covered by a layer of screening or fine netting. This covering would both satisfy the 2 X 4 Rule and make the window visible to birds and also, if it was set up to have a space between the screen and the window glass, would allow the birds to strike a soft, giving surface if they did fly into it and not experience the catastrophic damage from a hard surface collision. Advocates of these screens describe them as “bird trampolines!”
Photo by D. Sillman
Several years ago, I built a sunroom on the west side of my house back in Pennsylvania. The sunroom was built over the site of a former wooden deck. There had been before the construction of the sunroom and continued to be after the construction of the sunroom, bird feeders approximately 15 feet away from the western-most edge of the deck and sunroom. “New glass,” as I indicated before, is often quite deadly to birds who have established flight patterns through an area. The glass of the sunroom, though, was UV reflective, and I hoped that it would be visible to the surrounding birds. Over the two years we lived in the PA house with the sunroom we only had two or three bird/window collisions, a surprisingly low number considering the nearby attractants (both bird feeders and bird baths) and the size and height from ground level of the windows. My hypothesis is that the UV reflective nature of the glass helped to make the glass panes visible to the birds! There have been a number of experiments that have tested the hypothesis that UV reflective glass would reduce bird/glass collisions, but the results of these studies have not been consistent. My experience, though, indicates that it might be another type of solution to this very serious problem.