To prevent collisions, both reflective and clear glass should feature a visible pattern that meets the following criteria:
Maximum 5 x 5 cm (2 x 2 inch) spacing between pattern elements. Some standards recommend a maximum of 10 cm x 10 cm (4 x 4 inch), which is known to be less effective at preventing collisions of smaller birds.
Pattern elements should be at least 5mm (3/16″ inch) wide.
Pattern must be on the outside of the glass. On the inside, it will not be visible enough and will thus fail to break up dangerous reflections.
Pattern must be visible in all light conditions.
The U.S. Green Building Council’s LEED Pilot Credit 55 represents a good understanding of what constitutes a bird-friendly building. Briefly, a bird-friendly building is one where:
At least 90% of the material in the exposed façade from ground level to 40 feet (the primary bird collision zone) has a threat score of 30 or less, derived from controlled experiments.
At least 60% of material in the exposed façade above the collision zone meets the above standard.
All glass surrounding atria or courtyards meets the above standard.
There are no “see through” passageways or corners.
Outside lighting is appropriately shielded and directed to minimize attraction to night migrating or nocturnal birds.
Interior lighting is turned off at night if not in use and designed to minimize light escaping through windows during night operation.
Landscaping is designed without features known to increase collisions.
Actual bird mortality is monitored and compensated for (for example, in the form of habitat preserved or created elsewhere, mortality from other sources reduced, etc.).
Source: American Bird Conservancy