This issue of appropriate noise barriers to minimise bird mortality is something I've been looking at recently. Barriers are problematic if they are a) transparent and appear to allow a clear route through to habitat on the other side; or b) are reflective of nearby habitats causing birds to mistake the glass surface for the real thing.
A particular difficulty for birds arises from the fact that they are unable to fly slowly. If a bird does that, just like an aircraft, it will stall and drop out of the sky. For this reason birds are no good at braking mid-air. Their instincts prevent them doing this, due to the danger of falling out of the sky. Consequently, if, as a bird approaches a barrier, it notices that what previously appeared to be a clear passage through to suitable habitat is actually a glass surface, it cannot stop. So, unless there is sufficient distance to avoid a barrier by flying over or around it, collision is inevitable. For those who subscribe to Ibis, there is an interesting paper on this topic:
Martin, G.R. 2011. Understanding bird collisionswith man-made objects: a sensory ecology approach. Ibis 153: 239-254
Consequently, surfaces that appear to be solid only at close range (for example, where a pattern such as hatching is not visible at longer range) may still cause significant mortality. I have noticed this hatched pattern on noise barriers along the Tolo Highway near Tai Po, where it is being upgraded. Fortunately, the area generally is so highly urbanised, it is unlikely to be a serious issue.
The second issue is that birds have evolved to fly through trees at speed. As a result, falcon silhouettes or other similar measures do not reduce mortality by a statistically significant level (Klem, D. 1990. Collisions between birds andwindows: mortality and prevention. JournalField Orn. 61(1): 120-128). Such objects have to be placed densely across the surface so as to break the glass into clear areas of less than either 4” vertically or 2” horizontally. Such a density may be OK for roadside noise barriers, but probably not for buildings. The effectiveness of such things would appear to come from their ensuring that a bird knows there is a solid surface, rather than any sense of fear posed by the silhouette of a falcon (which may not be an issue for certain species). Threats are only perceived as threats if they are backed up by action, so to speak.
There is plenty of information on the dangers posed by urban environments to birds in the following:
San Francisco PlanningDept. 2011. Standards for Bird-SafeBuildings. San Francisco Planning Dept., San Francisco
which can be downloaded at:
http://www.sf-planning.org/ftp/files/publications_reports/bird_safe_bldgs/Standards_for_Bird-Safe_Buildings_8-11-11.pdf
GeoffC
