Joyce Herd of Dunedin asks :-

Friends of mine in Germany paint bird sillouettes on their large windows. Is there any evidence that this deters birds from flying into the glass?

Christine Thomson, of Massey University's Institute of Veterinary, Animal and Biomedical Sciences, responded.

The simple answer is yes, but it may not matter what shape the silhouette is.

To understand this, we have to think about why birds fly into windows. Birds fly into windows primarily because they don’t see them. This may be especially so if there are plants inside the window, or trees reflected in the glass, or another window opposite to the first, which the bird perceives as fly-through space. Birds also fly into windows for territorial reasons. This is particularly true in the breeding season when birds may see their own reflection and, thinking it is an intruder, fly at it to chase it away.

The most commonly described way to decrease the number of birds flying into windows is to decrease the transparency of the window. There are a number of ways to do this, but attaching objects/images to the window to break up the space can be helpful. It probably makes little difference whether the object attached to the window is shaped like a bird, or is abstract, or any other sort of shape.

However, you probably need to attach many shapes, placed fairly close together to the window, to really limit collisions. Otherwise the birds could still fail to see an area of window glass which does not have a shape attached to it. Moving shapes should be more effective than stationary shapes. So if objects (bird-shaped or otherwise) can be suspended 10-20cm from the outside of the glass, they will blow in the wind and be a more effective deterrent.

For more information on this and other ways to decrease window transparency, see the following websites: http://www.birds.cornell.edu/AllAboutBirds/attracting/challenges/window_collisions or http://www.rspb.org.uk/advice/watchingbirds/behaviour/birdwindows/index.asp or http://www.flap.org