The PARC tunnel is 8 m (27 ft) long with a cross section of 1 x 1 m (3.3 x 3.3 ft) at the sample end, tapering to 0.45 x 0.45 m (1.5 x 1.5 ft). The tunnel is open at the sample end. A mounting apparatus holds two 1 x 0.5 m (3.3 x 1.65 ft) panels separated by a 100-mm (4-in.), 6-mm (0.24 in.) clear window glass control (supplemental materials A) and a test sample. A net prevents birds from hitting the glass. At the operator end, a lightproof sleeve permits birds to be released in a dark interior, with brightly lit “exits” viewed at the far end. A video camera is mounted to record flights, and control glass permits direct comparison of samples in the same trial.
The tunnel is mounted on a pivot and is moved every one to five minutes to keep a constant orientation, with the sun directly behind the operator. Timing depends on visual assessment of shadows before each trial. Mirrors at the sides of the tunnel reflect light onto the front surfaces of the glass, and natural light falls on the back surface. Test materials are presented in a random order and in equal frequency on the left and right sides.
At the start of the testing season, trials using two clear panes are run as a control. Equal numbers of flights to the sides indicate the tunnel itself is not influencing the choice made by the birds. A tunnel score of 50 (±5) means a test glass product does not influence flight direction.
The ABC focuses on how many times birds approach the glass samples in the tunnels. Following testing, ABC provides manufacturers with a bird-friendly rating, called a threat factor. Developed in 2010, a threat factor is a way to assign scores measuring a bird’s ability to see and avoid patterned glass and other materials. This rating makes it easier for direct product comparisons but also allows governments to enact bird-friendly policies and architects to design bird-safe structures.
Unlike a traditional test lab, ABC typically only tests products which advance the science in some new way. Based on past research, there are certain parameters which are effective. Leading researchers on the topic have already identified the required spacing, length, width, opacity, colour, and orientation of elements marked on glass. These determined parameters become the basis of prescriptive or permitted products and substitutions.
The key benefit to tunnel testing is it produces rapid reproducible results. Tests can be usually completed in a day. The two North American testing facilities can issue a threat factor, and based on the score, architects design buildings using rated glass. The test also permits the evaluation of products which can be applied to existing glass (retrofits) to reduce collisions.4
A lower threat factor score means the product is more effective at reducing collisions. Glazing product manufacturers strive for a threat factor of 30 or under.
Field testing—a natural option
Field testing is offered at the Acopian Center for Ornithology. In field tests, the setup simulates how a window will behave when installed in a “human” structure. The windows in the field experiments look (to both birds and humans) exactly as they would if installed on a building—as a see-through, reflective window. Similar to actual buildings, the reflective windows cover a recessed dark interior. This is done because a perfectly clear pane will act like a mirror, reflecting the facing habitat and sky if it covers a dark interior. Most window installations in the built environment result in reflective panes.
This field testing examines the effectiveness of systems by using birds in their natural settings, where they live and move about. Birds are monitored while interacting with systems as they would at any human structure they are attracted to or encounter in their daily movements. Field testing depends on the chance of free-flying birds having a glass panel in their flight path. Feeders are used to increase the odds of the birds flying towards the glass.
Field testing is representative of the conditions during the exposure period and does not assign a threat factor to the rating. However, for manufacturers who want to take advantage of the real-world results of a field test, this may be an option to consider.
Bird-friendly glazing design parameters
Some well utilized and substantiated patterns have been derived from years of testing. For example, certain maximum distances (e.g. 100 mm [4 in.] for vertical stripes, 80 mm [3.25 in.] for dotted grids and for horizontal lines, 50 mm [2 in.]) have proven to be effective in preventing collisions. Patterns covering as little as one to five per cent of the total glass surface can deter most strikes under experimental conditions. As a rule, most birds will not attempt to fly through horizontal spaces less than 50 mm (2 in.) high or through vertical spaces 100 mm (4 in.) wide or less. This has become known as the “2 x 4” rule, and it relates to the size and shape of birds in flight.
