Glass Collisions

Learn More: Bird-Friendly Legislation

Vassar Bridge Science Building, N.Y. ©Christine Sheppard
Learn More About Writing Bird-Friendly Ordinances

This page is designed as a supplement for those interested in further exploration of the finer points of language used in bird-friendly building guidelines.

If you would like help working through your community's language, please contact ABC's Glass Collisions Program.

Common Ways to Restrict the Number of Buildings

Often, existing legislation does not apply to all buildings but instead to a subset of new buildings. The subset is frequently defined by, for example, building height, floor area, amount/size of glass, proximity to parks, zoning status (e.g., commercial/residential), or other characteristics or classifications.

Exempted buildings can still cause a lot of collisions because glass is frequently concentrated on the lower floors, where it is most dangerous for birds, and all glass on low-rise buildings is in the primary collisions zone. In fact, buildings under 12 stories tall account for over 99% of collisions, so effective guidelines should not be designed to exempt all but the largest buildings.

Here we take a brief look at examples of common thresholds used to limit the number of buildings included in bird-friendly legislation.

Some guidelines only apply to buildings that exceed thresholds for building size or the amount of glass — for example, buildings with a gross (i.e., total) floor area over a certain number of square feet (e.g., 10,000 ft2) or percentage of glass on a façade (e.g., 50%). Such limits might sound reasonable, but they essentially serve to exempt small- to medium-sized buildings and buildings that are not entirely glass, no matter their size.

Consider two examples:

First, a typical three-story (~30'-tall) building with a 50 ft x 50 ft footprint. This building has a gross floor area of 7,500 ft2 and the entire façade is in the primary collisions zone. Under a 10,000 ft2 gross floor area threshold, this building would be exempt from bird-friendly building guidelines even if built of 100% reflective glass.

Second, a 10-story (~100 feet) building with a 150 ft x 150 ft footprint. The total façade area (i.e., building envelope) of this building is 60,000 ft2. If legislation applies only to buildings with over 50% glass, this building would be exempt from design requirements as long as it had less than 30,000 ft2 of glass. However, even guidelines that only encourage designers to use less can be positive for birds because they will reduce the total amount of glass in the environment.

These types of exemptions are not what most people have in mind when they advocate for bird-friendly building design requirements. Restrictions like these falsely imply that only large, all-glass buildings are a threat to birds. Bird-friendly building guidelines will be most effective if they focus on the lowest floors of all building.

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How “Major” is a Major Renovation?

Legislation that applies to new construction (which should include additions), may also include “major renovations.” Predictably, and not unreasonably, there will be concern that a small project, or simply replacing a single window, might trigger the need for a building to comply with the legislation. This should not be the case, and clarity is needed: It is therefore important to specify that in existing building renovations, only major glass replacements fall under bird-friendly guidelines.

Including renovation requirements might not seem worth the trouble, but glass and glass assemblies fail and eventually need to be replaced. Communities are also adopting more stringent greenhouse gas emission limits for buildings, targets that might not be able to be met without new glass. So, there is significant potential to address collisions at existing buildings over time through a major renovation clause. This is important, as legislation in general does not apply to existing glass.

When defining what constitutes a “major renovation,” keep in mind that renovation requirements will only be triggered for buildings that meet your legislation's other thresholds for inclusion. Consider using language such as: “Renovations that add new glass or replace >25% of existing glass.” Historical and landmarked buildings might be exempted, but most of these do not have much glass.

How to Keep Birds from Hitting Your Windows

Are birds colliding with your home or building? Use our guide to find solutions and protect birds!

Proximity to Bird Habitat

Some existing legislation only applies to buildings within a fixed distance of a green space of a certain size, or some other measure of bird habitat.

These restrictions are problematic because birds are highly mobile. They use street trees and small patches of vegetation in the built environment, spending time in close proximity to buildings. They also fly from one habitat patch to another, passing buildings along the way. These movements in built environments create collision risks throughout urban areas, as has been documented by window collision monitoring groups in the United States and Canada.

The best choice here is not to restrict bird-friendly design only to areas in close proximity to what is perceived as prime habitat because birds do not restrict themselves to these areas.

How High up the Building Façade Should Bird-Friendly Materials be Required?

While we have documentation of collisions several floors above the 10th floor (i.e., about 100 feet above grade), research shows that collisions tend to be most frequent in areas where glass reflects vegetation. This suggests that the priority area for bird-friendly building guidelines should be the average height of the local tree canopy, perhaps adding as much as 50% – 100% to account for birds flying above trees. This zone is where birds spend most of their time.

Guidelines generally range from 40 feet to 75 feet. ABC's model ordinance suggests 100 feet. Basically, the higher up the building you go, the more birds you will save!

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Which Auxiliary Structures and Features Should Always be Bird-Friendly?

There are many auxiliary structures and features that are notorious for causing collisions and should never be built out of anything but bird-friendly materials.

In other words, these structures and features should be bird friendly no matter where they occur within a building or as free-standing structures. To view a full list of these features, visit ABC's Model Ordinance.

High-Risk Auxiliary Structures: Structures that pose significant collision risks to birds wherever they are found. For example:

  1. Transparent or highly-reflective:
    a. Railings, including balconies
    b. Noise barriers
    c. Wind barriers (including parking structures)
    d. Transportation (e.g., bus stops) or weather shelters
  2. Small, stand-alone buildings that present conditions that can be both transparent and reflective and are often located in bird flight paths:
    a. Gazebos
    b. External ticket booths
    c. Any other free-standing glass, plexiglass, or other clear, transparent, or highly reflective free-standing structure, including decorative objects..

High-Risk Building Features: Certain building features are frequently collision hotspots and should always receive special attention, including:

  1. Skyways/skywalks
  2. Building connectors, no matter the number of floors
  3. All outside corners where a bird can see in one side of the building and out the other within 30 feet of the corner.
  4. All interior corners within 30 feet of the corner
  5. Parallel glass walls ≤50 feet apart
  6. Courtyards, including internal atria,
  7. Atria
  8. Three floors of glazing adjacent to and above green roofs.
Quality View Requirements

Some municipalities have quality view rules that require transparent ground-level windows. The good news is that most types of bird-friendly vision glass provide a quality view, with patterns that prevent collisions covering less than 7% of the glass. There are also a handful of bird-friendly glass products that create patterns in ultraviolet light that some (but not all) birds can see and people cannot, so quality view requirements need not conflict with bird-friendly glass.

Visit our photo gallery to see how a variety of materials and techniques can be used to make buildings safe for birds and great for people.

Should Green Roofs be Included?

Glass adjacent to green roofs is frequently included in bird-friendly glass requirements, regardless of a building's height or size. These roofs attract birds, even in the most urban settings, so including green roofs in legislation is a good idea.

There are a variety of green roof types, from those with short plants like sedges and grasses to a few that can even have trees up to 40 feet tall, although 10- to 15-foot-tall trees are most common. As with vegetation on the ground floor, any glass that is next to one of these roofs will be a collision threat. Green roofs should be included in legislation, even if they are not common yet. All glass adjacent to green roofs should be bird friendly to 30' above the roof surface.

Vision and Non-Vision Glass

Vision glass is generally thought of as glass through which people can see the outside world from inside a building. However, this is not the only type of glass that poses a threat to birds. Building exteriors frequently consist of 50% or more of non-vision glass, which is referred to as spandrel glass. For any of a number of reasons, people are not able to see through spandrel glass.

Think of an all-glass building — there are wires, spaces between rooms and floors, and many other building materials that are hidden by spandrel glass so that they cannot be seen from outside. This glass is just as deadly to birds as vision glass, so it should always be included in bird-friendly building guidelines by simply using the word “glass” or “glazing” without specifying a type.

Why Birds Hit Glass

Birds, unlike humans, are unable to understand or learn the concept of ‘glass' as an invisible barrier that can also be a mirror. Birds take what they see literally – and glass can appear to be habitat they can fly into, whether the habitat is reflected, or seen through a pane of glass.

Photo of hummingbird

News, Updates, and More

Bird Collisions on ABC's Blog

Check out BirdCalls Blog for frequent updates and insights into birds and window collisions…. Read more >>